Cytoplasmic Linkages and Mutation
Booklet No. 276
Agricultural Biotechnology: BTS
Linkage, Crossing Over and Chromosomal M
Chromosome theory of linkage
Types of linkage
Linkage in maize
Mutation due to chromosomal aberrations
Application of Genetics to Plant a
nd Animal Breeding
Problems of the commercial breeder
Selection for desirable qualities
Modern genetic techniques
The discovery of genetic inheritance has enabled man to enter into the modern genetic
engineering which now is a branch of biotec
hnology. In spite of having the largest number of
plant and animal species, people in our country did not have any idea to begin the genetic
engineering or breeding of plants and animals by themselves.
In addition to the information given in the booklet N
o.274,this booklet describes the basic
ideas about cytoplasmic inheritance, linkages and crossing over of
genes and finally the
application of genetic techniques in crop and animal production.
Dr. K. T. Chandy, Agricultural & Environmental Education
The present booklet deals with the science of genetic inheritance and its related
concepts scientifically. Genetics is the science of heredity including the study of its chemical
foundation, its development, expression and also its bearings on
variation, selection, adaptation,
evolution, breeding and the activities of man. Whereas, heredity is the transmission of
characteristics of parent forms to their offsprings. Genetic inheritance can be defined as the
reception or acquisition of characters
or qualities by the transmission from parent to off
through cell nuclei.
It is normally assumed that the fundamental biological function performed by sex is
reproduction. However, another important function of sex consists of providing within fami
population and species, that genetic variety without which long evolutionary success is
improbable, as the chance of securing favourable recombinations of genes is much greater
under sexual reproduction. Hence, sexual reproduction has become establis
hed in the great
majority of the forms of life which exist. on the earth, including bacteria.
Sexual reproduction, does not necessarily imply a clear cut distinction into male and
female sexes. Any situation, in which there is a genetic mingling of nuclea
r material from two
different cells will come under the heading of sexual reproduction. For example, in
Paramoecium J (one celled protozoan) there is a form of sexual union called conjugation, in
which two organisms unite and exchange nuclear material. Mor
phologically, we can not
distinguish between the two conjugants, but experimental evidence shows that they are different
physiologically, and has a number of different mating types (types of sexes, not only male and
female, found in lower forms). Such mati
ng types are possibly due to variations in the alleles of
genes related to the physiological distinctions of sex, and would be determined on the same
genetic basis to other characteristics. Among the more advanced forms of life, the mating types
d to two, called sexes, possibly through an evolutionary adoption because of the
greater efficiency of having only two types rather than as many as could be produced by all the
combinations of all the mutant alleles involved.
In higher animals, the two ty
pes i.e. male and female, exhibit a greater variety of
differences from one another, differences for too expensive to be account for merely by the
simple principles of inheritance. Let us consider two children or the same parents
one a boy
and the other
a girl. These two persons will obviously show a great variety of distinctions, right
from the characteristics associated with reproduction to every part of their body including the
skin, muscles, blood, hair, bones etc. How can two persons who are so diffe
rent be the off
spring of the same parents? We certainly cannot explain these distinctions on the basis of a
difference in a single gene such as would result in variation in the colour of the eyes among
children of the same parents. The differences are far
too extensive for that. Neither could we
find a satisfactory possible explanation in the differences or a number of individual genes, such
as those involved in differences of body height. Such multiple gene variations result in
quantitative rather than qu
alitative effects. People are not either tall or short, there are countless
gradations between the extremes. With regard to sex, however, such gradations do not normally
exist. People are either male or female, with no gradations from one to the other, suc
h as would
be expected if we should seek to explain sex determination on the basis of multiple gene
inheritance. Thus, distinctive mechanism of inheritance will be the subject for consideration in
The study of genetic inheritance is divided
into two booklets: 274 & 276. In this booklet
No. 276 we discussed about the cytoplasmic inheritance, linkages, crossing over of genes,
chromosome mappings, polyploidy, mutation and application of genetics to plant and animal
II. Cytoplasmic Inh
Most of the heredity traits in an organism are determined by genes residing in the
chromosomes and the bulk of heredity is transmitted through the chromosomes. The nucleus, of
which chromosomes and genes are the part, lies in the cytoplasm and ta
kes its nourishment
from there. The question arises whether the nucleus independently governs the heredity or the
cytoplasm also plays some role? It is now clear that some inheritable characters are beyond the
control of nucleus and are related with certai
n self reproducing cytoplasmic particles. This type
of inheritance is called cytoplasmic inheritance.
The evidences in favour of cytoplasmic inheritance are briefly explained here.
1. Reciprocal cross
In cases of chromosomal heredity, the res
ults of reciprocal crosses are ordinarily
identical but in the cases of cytoplasmic inheritance the results of reciprocal cross are different.
2. Maternal influence
When reciprocal crosses for a certain character are found to exhibit different results, it
has been noticed that progeny exhibits the characteristics of their female parent. If
chromosomal difference can be ruled out, maternal inheritance usually implies transmission
When a heritable phenotype is transmitted wit
hout nuclear transmission, it suggests that
some particles from the cytoplasm of the parent/parents have been transplitted to the offsprings.
4. Indifference to nuclear substitution
When a particular genotype exhibiting a specific character is replaced by
having alternative genotype, it does not change the phenotype and suggests the possibility of
segregation and non
Segregation is characteristic of Mendelian and chromosomal inheritance. Fail
show segregation may indicate extrachromosomal heredity. Even if segregation occurs but in a
fashion inconsistent with the segregation of chromosomes, the results may suggest the
possibility of non
B. In animals
es of cytoplasmic inheritance in animals have been studied. However, the
best example is that of shell coiling in snails. In some species of snails (e.g. Limmaea peregra)
two types of coiling have been observed
dextral (when the coiling is towards right),
(where the coiling is towards left). The coiling is determined by a pair of alleles showing
the dominant allele '0' controls dextral coiling while the recessive 'd'
determined sinistral coiling. Here, the pattern of co
iling is determined in not by the genotype of
an individual but by the genotype of its mother.
When crosses were made between females coiled to the right and males coiled left, the
F 1 snails were all coiled to the right. The usual 3:1 ratio was not obtai
ned in the F2, and the
expected phenotype for 'd' was not expressed. Instead the pattern determined by the mother's
genes was expressed. When 'dd' individuals were inbred, progeny that coiled to the left were
produced. However, when the 'DD' and 'Dd' snail
s were inbred, they produced offspring that
coiled to the right.
From the reciprocal cross between left
coiling females and right
coiling males, all the F
1 progeny were coiled to the left. The F 2 all coiled to the right, but when each F 2 snail was
bred, those with the genotype 'dd' produced progeny that coiled to the left.
Further investigation of coiling snails has shown that the spindle formed in the
metaphase of the first cleavage division influence the direction of coiling. The spindle of
tial sinistral snails is tipped to the left, but that of 'dextral 'snails is tipped to the right. This
difference in the arrangement of the spindle is controlled by the genes of the mother, which act
on, the developing eggs in toe ovary. They determine the
orientation of the spindle which in turn
influences further cell division, and results in the adult pattern of coiling. The actual phenotype
character, therefore, is influenced directly by the mother, with no immediate relation to the
genes in the egg or
C. In plants
There are several instances of cytoplasmic inheritance studied so far in plants. However,
only two examples are described here.
I. Male sterility in maize
Male sterility in plants occurs due to pollen failure. It is controlled in
cytoplasmic factors in the case of maize, wheat, sugar beet and some other crop plants.
Certain, maize plants develop pollen which abort in the another, thus the plants are male
sterile. The female genetic cells (eggs) of these plants are fertil
e and functional. When the eggs
of a male
sterile plant are crossed with pollen from a male
fertile strain, the progeny produce
are male fertile plants. The reciprocal cross consists of fertilization of egg from a male fertile
plant with pollen of a mal
e sterile plant (some pollen of male sterile plant are fertile and can be
used for pollination).
The difference in the character of progeny of male sterility character through the
maternal cytoplasm. The hybrids are male sterile and male
to the maternal
parent irrespective of the nuclear genes. If inheritance of male sterility character is through the
nuclear genes then the inheritance pattern should not change in reciprocal crosses. The
progeny is similar to the maternal. parent as far a
s this characteristic is concerned, irrespective
of which strain (male sterile or male fertile) is used as the female parent.
The male sterile maternal strain, if repeatedly back
crossed to pollen fertile lines until all
its chromosomes are exchanged for
those of the male
fertile live and then crossed with a male
fertile male plant, still produces hybrids which are male sterile, strongly indicating the
inheritance of the character through the egg cytoplasm.
The inheritance of male sterility in maize is
maternal irrespective of the direction in which
the cross is made. The character is inherited through cytoplasmic factors, the plasma genes.
These are transmitted through the female gametes.
Male sterility strains have been helpful in ensuring cross polli
nation and getting hybrid
seeds in many crop plants.
2. Variegation in Mirabilis jalapa (Four 0' Clock plant) When a variegated plant of Mirabilis jalapa
is crossed with a plant having completely green leaves, the offspring is always like mother. If the
emale plant has all green leaves, its offspring will also have the same and if the mother is
variegated, offspring will also bee so. A variegated plant may have some of its branches green,
some white and some variegated. If a flower on green branch is cros
sed with a flower of white
branch, the result again goes towards the mother, ie. the offspring is like that of mother branch.
A cross between the flowers of green branches produces green offspring; between the flowers
of. white branches produces white and
between the flowers of variegated branches produces
three types of plants
green, white and variegated in different proportion. Since cytoplasmic
heredity resembles the female parent only, the inheritance is also termed as maternal
e, Crossing Over and Chromosomal Mapping
If any two genes are picked at random, it is most likely that they will not be on the same
chromosome pair. This is so because each chromosome bears a large number of genes. It is
quite evident that two such genes
would not show the same independent assortment which
characterizes genes located on different pairs of chromosomes, but rather they would be linked
together in heredity by virtue of their common chromosomes. Thus all the genes on one
chromosome are said to
be linked genes and tend to move 'en bloc' during meiosis when' the
chromosomes are segregated into two haploid groups. This phenomenon in which there is a
tendency of two or more genes of the same chromosome to remain together in the process of
ce is called linkage. Therefore, we can say that cytoplasmic linkage is the association
of characters in inheritance due to the character determining genes being physically located in
the same chromosomes.
This is a deviation from the principle of Mendeli
an’s independent assortment, since
random assortment of gene does not occur in such cases. Mendel could not notice the linkage
because fortnightly the seven pairs of factors or alleles selected by him were found on different
A. Chromosome the
ory of linkage
Morgan and Castle formulated the 'chromosome theory of linkage. It has the following
1. Genes that show linkage are situated in the same chromosomes.
2. Genes are arranged in a linear fashion in the chromosome i.e. linkage
of genes is linear.
3. The distance between the linked genes is increasingly proportional to the strength of linkage.
The genes which are closely located show strong linkage, whereas those, which are widely
separated, have more chances to get separated by
4. Linked genes remain in their original combination during the course of inheritance.
B. Types of linkage
Linkage has been found to be complete or incomplete depending upon the presence or
absence of new combination of the genes.
Complete linkage is the phenomenon in which two or more characters are inherited
together and consistently original or parental combinations. The complete linkage is due to the
fact that there is no break, the gene combination in the chromosom
e. In this, crossing over does
not occur, that is why the linked genes do not separate to form new combinations.
2. Incomplete linkage
Incomplete linkage involves the accidental or occasional breakage of chromosomal
segments. This is due to crossing over
and results in parental or new combinations.
C. Linkage in maize
Maize provides a good example for linkage. A variety of maize having coloured and full
seed is crossed with a variety having colour less and shrunken seeds. The gene 'c' (colour) is
over its colourlcss allele 'C' and the gene 'S' (full seed) is dominant over 's' (Shrunkcn
seed). A cross between CCSS and ccss, produced seeds with genotype Cscs of ‘c’ and ‘s’
assort independently. In F 2 according to Mendel's law, 4 types of gametes sh
ould be formed in
equal numbers, and when back crossed with a double recessive plant" for the characters
concerned, they should produce 4 classes of progeny in equal numbers. But a different result
was obtained in back
crossing the Fl generation. The vario
us combination obtained were as
The total number
of parental combinations are "4032 + 4()35 = 8067 or 96%; While the
newer recombinants are 149 + 152 = 301 or 3.6%. This shows that the percentage of parental
combination is higher than the new combination. The genes 'S' and 'C' are linked together on
same chromosome and do not assort independently and tend to remain together.
Sometimes it has been found that linked genes separate out and new combinations are
formed. This is due to the interchange of parts between two homologous chrom
which a term "crossing over" is used. Thus, crossing over is defined as an interchange of
corresponding chromosomal parts between chromosomesite of a homologous pair of
chromosomes resulting in a recombination of genes. One or two, or more fragm
ents may be
interchanged during crossing over. Crossing over occurs during the early prophase of the first
division of meiosis while the chromosomes are closely paired.
The mechanism of crossing over can be explained easily under the followin
During the zygotene stage of the first prophase of meiosis, the homologous maternal
and paternal chromosomes pair
off and usually come to lie closely side by side. This
phenomenon is called synapsis. Synapsis provides the mechanical b
asis of heredity and
variation. The pairing of homologous chromosomes is brought , about by the mutual attraction
between the allelic genes. The paired chromosomes are known as bivalent.
b. Duplication of chromosomes
It follows synapsis and changes the bi
valent nature of chromosome pair into tetravalent.
During this each of the homologous chromosomes in a bivalent splits longitudinally into two
sister chromatids attached to the unsplitted centromeres.
c. Crossing over
After the homologous chromosomes have
duplicated to their respective sister
chromatids, the crossing over takes place. During crossing over, the non
sister chromatids of
homologous pair twist over each other due to the action of an enzyme called endo
chromatid threads are connect
ed with each other at point known as chiasma. A segment of one
side fuses with segment of opposite side and this is accompanied by the action of another
enzyme called ligase.
The crossing over can take place at several places in one strand arid many chias
can be formed. The number of chiasmata formed is proportional to the length of chromosome
thread; greater the length greater is the number but in one species this number is constant.. The
genes at distant local may undergo crossing over but closely pl
aced genes have less chances
of crossing over due to their position.
After the crossing over is completed, the non
sister chromatids repel each other due to
lack of attraction between them. The repulsion or separation of chromatids star
ts from the
centromere towards the end just like a zipper and this separation process is named as
terminalization and thus the twisting chromatids separate so that the homologous chromosomes
are separated completely.
Crossing over may be single,
double or multiple depending upon the number of
chiasmata formed during the process. Sometimes crossing over takes place before splitting of
homologous chromosomes. Thus, all the four resultants are recombination products. This is
known as two stranded cro
ssing over. Sometimes crossing over takes place after splitting of
homologous chromosomes, out of the four normally two non
sister chromatids exchange their
segments. Thus the resultants are two recombination products and two of parental combination.
Following factors show their significant influence over crossing over
In Drosophila, crossing over is completely marked or suppressed in male and there is
tendency of reduction of crossing over in most of mammals.
: These are
the gene changes causing visible effects in the organisms. They might
reduce the crossing over.
These are intra
segmental changes in the chromosome. In a given segment of
chromosome, crossing over is suppressed due to inversions.
: Crossing over tends to be reduced or suppressed at the ends of the
chromosomes and in the vicinity of a centromere.
: When Drosophila females are subjected to high temperature or low
temperature, the percentage of the crossing
over is increased.
The irradiations by X
rays and radium incrcase crossing
The age of the individual may also affect the frequency of crossing over. It has been
noticed in drosophila that as the female becomes older, the c
rossing over tends to increase.
h. Nutritional effect:
Crossing over frequencies are affected by concentration of metallic ions,
such as calcium and magnesium.
over is an universal phenomenon, observed in all groups of organisms
viruses to man. Its study is of great significance because of the following reasons:
over affords a proof for the linear arrangement of genes.
over has led to the construction of linkage maps or genetic maps of chromosomes,
ich are condensed graphic representation of the relative distances, expressed in terms of
percentage of recombination among the gene in a linkage group.
a. As a result of crossing over, new gene combinations are produced which change the
frequency of gene
s in genetic pool of a population, and therefore play important role in the
process of evolution (recombination of genes).
E. Chromosmal mapping
Chromosomal mapping is a diagrammatic, graphical representation of relative distances
between linked genes of
a chromosome. If suffieient data about the number of genes, the
number of chromosomes present in an individual and their linkage group is available, it is
possible to prepare maps of dirferent chromosomes showing the probable loci of various genes
The chromosome map can be constructed either on the bars of percentage of linkage
groups or frequencies of crossing over. The frequency or crossing over between any two genes
depends upon the distance between them. The greater the distance between two g
higher is the percentage of crossing ovcr. Thus, the percentage of crossing
over is a measure of
the relative distance or genes from one another.
1. Determination of map distance
Thc relative distance or genes can be calculated on the basis or p
ercentage of crossing
over. If the percentage of crossing over between two linked genes is 10%, it means that the
distance between two linked genes is 10 units of map distance. This is called the map unit. The
percentage of crossing over between two linked
genes is calculated by test crosses on which a
F2 dihybrid is crossed with a double recessive parent. Such crosses involve crossing over at
two points and are catlled two point test crossing. Similarly in three point test cross, a trihybrid
test cross inv
olving three genes, gives us information regarding relative distances between
these genes, and also shows the lenearjf order in which these genes should be present on
2. Determination of gene order
It becomes easy to place genes in the proper
linear order after determining the relative
distances between the genes. Supposing there are 5 genes on a chromosome and designated
as A,B,C,D, & E and the crossing over data shows that genes 'A' and 'E' are farthest apart, due
to higher percentage of cro
over between them.
Let 'A' be the start
point and is placed to 0.0 on the map. New if 'A' and 'B' show 5%
crossing over between them, we can place 'B' at a distancc of 5 units from 'A' on the map. If the
gene 'c' shows 3
5%) crossing over with 'B
' and 8.5% with 'A'. In this way all the genes can be
placed on the map. As new gene are found belonging to the same linkage group, they are also
placed on the map by making suitable crosses and calculating the cross over values with other
Each species of a plant or animal is characterized by a particular number of
chromosomes forming a haploid set. A diploid organism has two haploid sets of chromosomes
in its body cells. A species is recognized by its fixed number of chromosomes and
morphology of cach of its chromosome. At cach nuclear division these regularities in
chromosome number and form are duplicated, which is an exact phenomenon. Sometimes,
irregularities occur during ceIl division or some external factor like radia
tion produce aberrant
chromosomes. Changes in the number of chromosome may be reflected in high inviability and
phenotypic anomalies in those that survive.
Polyploidy is a term that refers to the condition is which organisms have extra haploid
sets of ch
romosomes. When there are four such sets instead of two sets in the cells of somatic
tissue, the cells are said to be tetraploid. Such a condition may arise as a result of abnormal
mitosis. Mitosis will start there will be duplication of the chromosomes in
the prophase and
arrangement on the metaphase plate but then, for some reason, the process stops. This leaves
the cell with double the normal chromosomal number (tetraploid) at the time it goes into the
interphase. Later the cell may undergo normal cell
division and produce a group of cells which
have double the normal chromosome number. If this occurs in the growing tip of a plant, a shoot
may be produced which consists of cells that are exclusively tetraploid. Since the tetraploid is
often larger and mo
re vigorous than the diploid, it may be commercially valuable in the
production of better fruits of other products. E.g. tetraploid grapes of the portland and Fredonia
varieties have been produced which arc just about twice the size of the fruit borne on t
plants. Various degrees of polyploidy can also be seen in plants such as pears, cherries and
Polyploidy may also arise by abnormal meiosis. In the first division of meiosis the
chromosomes may synapse and prepare for a normal redu
ction division, but fail to pull apart, so
that one ccll receives the entire diploid number and one receives no chromosomes at all. The
latter type of cell dies in a short time, but the first type may go through a second meiotic division
and produce diploi
d gametes. When such gametes unite with a normal haploid gamete, a
zygote is produced with three complete sets of chromosomes. This is called a triploid. The
genes are in proper balance, but certain phenotypic effects result from this condition, just as
om tetroploid organisms. When the time comes for reproduction, however, the triproids are
sterile. The gamete usually receive two chromosomes of some types and only one of other
types. When such gametes are used for fertilization there is an unbalanced co
zygote. Hence triploids among animals are more or less genetic freaks which cannot be
maintained in a stable condition. In plants, however, triploids may be maintained through
grafting, and by making cuttings and rooting them, or through propag
ation by bulbs and roots.
Some triploid plants have commercial value and are maintained in this way.
Whenever two diploid gametes unite, a tetraploid individual results. If this condition has
arisen from a doubling of the chromosomes within one species (a
uto tetraploids), the
chromosomes will to a certain extent, synapse in four in meiosis. The irregular chiasma
formation among the resulting eight chromatids cause an uneven distribution of the
chromosomes to the gametes, and such tetraploids are, therefore
, highly infertile. It is a different
matter, however, when the tetraploids are produced in hybrids of different species
(allotetraploids). In such cases the chromosomes synapse in normal pairs and tend to produce
viable gametes and zygotes, whereas the di
ploid hybrid is typically sterile. Tetraploids of this
nature have undoubtedly played an important part in the evolution of new species, especially in
the plant kingdom.
Karpechenko made intergeneric crosses between the radish (genus Raphanus) and the
bage (genus Brassica). While these plants show considerable differences in their
morphology, they each have a diploid chromosome number of 18 and can be crossed to yield a
hybrid. The genes in the two are so different, however, that the hybrids are usually
sterile because there can be no regular synapsis of chromosomes in meiosis. Karpechenko
found that, out of a very large number of hybrids, there were a very few that were fertile.
Cytological examination of these fertile hybrids showed that the
y were tetraploid
they had 36
chromosomes. In these few plants there had been a doubling of chromosomes and this made it
possible for normal synapsis to take place. This new form of plant bred true and when crossed
back with either of its progenitors it ga
ve highly infertile triploids. Thus, it was an artilicially
created new genus and was callcd
A number of chemicals have been found which can induce polyploidy in plants. The
most eflicient and most widely used of these is a poisonous chem
ical known as colchicine. This
chemical, in proper concentration, will inhibit the formation of spindle fibers during mitosis, but
will not inhibit the doubling of the chromosomes. Hence, when a normal diploid cell is so treated
the chromosomes double, but
when they are ready for the metaphase there is no functional
spindle and they remain clustered as in the prophase
double chromosomesheld together by
single centromeres. The centromeres then duplicate and there is a tetraploid number of
chromosomes in the
cell. The cell does not divide, however, but instead returns to thc
interphase with its chromosomes doubled in number.
Colchicine is extremely valuable in horticulture, for plant tissues with double the normal
chromosome number often yield products which
are commercially superior. In practice, some
colchicine is applied to the growing tip of a plant either in the form of liquid or mixed with lanolin
in a salve. The treated cells at the growing tip will die, but when new tissue grows out from; the
t below the dead portion, it may contain tetraploid cells because of the effect of the
colchicine. Cells of this new type can then be propagated through cuttings or grafts. It is also
possible to treat the growing tips or plants having tissue of this new t
ype and once again obtain
a doubling of the chromosome number so as to get an octoploid, and a third treatment in some
cases yields evcn a 16
This technique is well adapted for use on plant tissues, since plants grow through the
addition of new cel
ls at the tips of their stems, but animals offcr greater difficulties since they
tend to grow in many different parts or the body at the same time.
Chromosomc doubling might be induced in various growing somatic cells, but it is
generally not possible to
propagate entire new animals from the altered portion of the body.
Inheritance is the transmission of characters from one generation to the other. It is based
on genes which are located on chromosomes. These are duplicated and passed on to th
progeny through the gametes during sexual reproduction. Thus, the genes of children are the
copies of the genes of their parents. The copies are exact and are made by an exact and
correct method of gene duplication. Occasionally this exact and accurate
method of gene
duplication goes wrong, and a copy of the gene slightly different from the original is produced.
The gene now is a modified gene. It has undergone a structural change. This is called gene
mutation, wherein the original gene has mutated to a
modified form. Thus, the term mutation
refer both to the character or characters possessed by an individual not inherited from either
parent but capable of being transmitted to the progeny. Mutation is caused by a change in
genes and also by certain change
s in environment like exposures to x
lethal temperature and treatment with certain chemicalls. Hence, mutation helps the
breeders to propagate desirable characters.
A. Modern concept
According to modern concept, the mutation i
s considered to be a sudden change in the
genotype of an animal or a plant. Small mutations are called the micro
mutations and the large
mutations are called macro
mutations. Both the types of mutations have an important role in the
evolution they result i
nto the production of new varieties which breed true in the next
days by mutations we usually mean the point mutations which change the
internal arrangement of a DNA molecule including small deletons duplications ctc.
B. Nutrition rate
The frequency of mutation is different in different genes. Genes with rapid mutation rates
are called mutable or unstable. Genes which do not mutate so easily are known as stable
genes. In man, genes associated with muscular dystrophy are estimated to muta
te once in 105
people. Bacterial cells mutate in the order of 1 in 10
and a particular locus in Drosophila
mutates in the order 1 in 1,00,000 to 2,000,000 flies. Emerson found that the R
gene in maize
mutates much more frequently than others (at the rate
of 50 per 100,000 gametes). In some
organisms, some genes mutate so frequently that the individuals are simply mosaics of mutated
and unmutated genes. Such mutable genes are more common in plants than in animals and are
more frequent in somatic tissues tha
n in germ tissues.
Temperature and age are factors which influence the frequency of mutations. Animals
raised under a high temperature will yield more mutations than animals of the same breed
raised under a low temperature. This applies mainly to animals
that do not maintain a constant
body temperature, for the germ cells of warm
blooded animals remain at a rather constant
temperature regardless of the surroundings. Also, in Drosophila it has been shown that sex is a
factor in the mutation rate. Studies of
the mutation rate in the X
chromosome indicate that sex
linked mutations occur considerably more often in the males than in the femmes. A study of the
mutation rate of hemophilia in man indicates a similarly higher rate in males.
There are five
types of mutation. All of them are discussed here in detail.
1. Somatic mutations
Mutation may occur at any stage in cell cycle and any cell may act as the scat of
mutation. The effects of mutations are mostly determined by the type of the cell in which t
occur. A somatic cell, after the mutation has occured in it, will give rise to the mutant tissue.
That is, if it possesses the power of reproduction. The other parts of the organism will not be
affected. E.g. the 'Delicious' apple and the 'Naval' orang
e have been produced as a result of
2. Germinal mutation
If the mutations originate in the reproductive cells of the gonads, then these are termed
germinal mutation. Germinal mutation further may be gametic occuring in the gametes of
dividuals or zygotic originating in the fused diploid gamete. Various sex
linked mutations are of
these types and pass from generation to generation. If the mutation occurs in the dominant
gene of the germ cell, the effects would appear in the next progeny
. The effects on the
recessive genes would not express themselves because they may be masked by other genes.
A classical example of germinal mutation is that of Ancon , breed of sheep. These sheep
have short legs and are more advantageous than the long
gged sheep because they cannot
cross over the fence.
3. Spontaneous mutation
These gene mutations occur in nature and originate in the individuals by natural
agencies like sunlight etc. Various Mendelian traits are spontaneous gene mutations. Example
spontaneous mutation can be found in mice and rodents, where it determines coat colour
which may be black, white, brown and spotted, etc. Similarly, in men, many characters, as hair
colour eye colour, skin pigmentation and several body deformities are due
to mutant genes. An
important example of gene mutation is for hemophilia appeared in the germ cells of one of the
parents of Queen Victoria.
It has been found that some of the natural occuring mutations are caused by the natural
radiations emitted by the
radioactive substances in the rocks, soil, water or atmosphere of the
earth. The organisms are also affected by the ultra violet rays and the cosmic rays coming from
the outer space. They may undergo mutations because of such effects, but the relations
ween the dosage of natural radiations and frequency of spontaneous variations is not
4. Reverse mutations
Most mutations consist of a change from normal or wild type to a new genotypic
(recessive or dominant) such mutation events are called f
orward mutations, while if the same
mutant genotype changes back .to the wild type, it is back or reverse mutations.
Reverse mutations can in some cases be confused with suppressers. The reverse
mutation occurs at the same locus as the forward mutation, w
hereas a suppresser mutation
occurs at some other locus, but it suppresses the effect of the original mutant genotype.
Reverse mutations are easy to be identified in micro
organisms, especially the nutritional
5. Point mutation
table alteration occur in a very small segment of DNA molecule, i.e. a single
nucleotide or nucleotide pair, then this type of mutations are called point mutations. The point
mutations may occur due to following types of sub
changes in the DNA a
a. Deletion mutations
The point mutation which is caused due to loss or deletion of some portion (single
nucleotide pair) in a triplet codon of a cistron or gene is called deletion mutation. .
b. Insertion or addition mutation
The point mutations
which occur due to addition of one or more extra nucleotides to a
gene or cistron are called insertion mutations. The insertion mutation can be artificially induced
by certain chemical substances called mutagens such as acridine dye and proflavin.
A point mutation in which a nucleotide of a triplet is replaced by another nucleolide is
called substitution mutation. This type of mutation effect only a particular triplet codon. The
substitution mutations alter the phenotype of an org
anism variously and are of great genetical
They may be of following types:
(i) Transmission: In these one purine is replace by another purine or one pyrimidine is replaced
by another pyrimidine.
(ii) Transversions: In these a purine is repl
aced by a pyrimidinc or vice
6. Induced mutation
The agents used to induce mutation are as follows:
a. Radiation induced mutation
rays constitute a very potent mutagenic agent. Mutations caused by X
rays is several
times higher over the rate of
spontaneous mutation. Similarly, ultraviolet (UV) irradiation too is
mutagenic, though not as effective as X
rays because they are not that penetrating and are
partially absorbed by the protoplasm. However UV
sterilization for food preservation and
is not practised these days due to probability of high mutation rates.
b. Mutagenic chemicals
Organic peroxides and alkaloids like colchicine, caffeine, morphine and camphor have
been successfully used to produce mutations. These reagents effect the orga
but there are also some mutagens which are prescnt within the individuals.
c. Heat shocks
High temperatures have been recorded to increase the rate of mutation by two or three
d. Mutation due to chromosomal aberrations
ional breaks in the chromosomes and fusion of the broken ends causes many
changes in the chromosome called chromosomal aberrations. Likewise chromosomal
aberrations are also the kinds of mutations produced as a result of segmental change in the
The breaks may occur either single chromosome, in a pair or in non
chromosomes. The structural changes may be of the following types.
Translocation is the transfer of a portion of one chromosome to a non~homologous
and is of following three categories:
a. Simple translocation
Simple translation involve a single break in the chromosome and the transfer of a broken
piece of this chromosome to the end of another. Not very common.
Shifts involve three breaks
and the transfer of a two break section of one chromosome
within the break produced in a nonhomologous chromosome. Shifts are more common than
c. Reciprocal translocation or interchanges
Interchanges are produced by single breaks i
n two homologous chromosomes and an
exchange of chromosome section between them takes place. These are the most frequent and
best studied translocations.
2. Deficiency or deletion
The deficiency or deletion is the loss of a chromosomal segment from any ch
Depending upon the length of the lost chromosomal segment, the loss of genes involved may
very from a single gene (minimum) to a block containing several genes. The loss of
chromosomal segment occurs when a portion of chromosome gets detached due
reasons and the lost segment does not survive, because it lacks the centromere. The portion of
the chromosome carrying the centromere function as a genetically deficient chromosome.
Deletion can be of two types, namely (a) terminal deletion, w
here there is a loss of
segment from one or the other end of the chromosome, and (b) intercalary or intestitial deletion,
where there is a loss of an intercalary segment of the chromosome with the re
union of terminal
segment or a part of the chromosome becomes repeated in the same
chromosome. These additional duplicated segments are called duplications. They arise as a
result of unequal crossing over between chromosomal segments.
Inversion is a kind of c
hromosome diserration in which a chromosomal segment exist in
reverse relationship to the rest of its chromosome. Inversions are believed to arise by
breakages at the point of intersection of a chromosome loop and reunion with new partners. An
be homozygous or heterozygous for an, inversion. During meiosis, inversion
heterozyotes synapse by forming a looped configuration. The inversion is of two types.
a. Paracentric inversion
A single crossing over or an odd number of cross
overs in the inver
ted region will result
in the formation of a dysenteric chromosome (having two centromeres) and an acentric
chromosome (with no centromere). Of the remaining two chromatids one will be normal and the
other will carry the inversion. The dicentric chromatid
and acentric chromated will be observed
at anaphase in the form of a bridge and a fragment.
b. Pericentric inversion
Here the centromere is present within the inverted segment, and two of the four
chromatids resulting after meiosis will have deficicncies
and duplications. However, unlike
paracentric inversion, no dicentric bridge or acentric fragment will be observed. Consequently,
at anaphase no bridge of fragment will be seen.
VI. Application of Genetics to Plant and Animal Breeding
Application of gen
etics to the breeding of domestic animals and cultivated plants, can
without doubt achieve remarkable improvement in the commercial species/varietics.
Unfortunately, the practical breeds of plant and animals often does not have a thorough
foundation in gen
etics, and as a result sometimes uses methods which are slower and less
reliable than would be possible through the application of our present knowledge of genetics.
On the other hand, the highly trained geneticists who has worked primarily with those spec
used experimentally in the laboratory or in the field often fails to realize all the produce which
confront the practical breeder who must produce to meet market requirements. A combination of
the knowledge and skills which have been developed in the t
wo fields is highly desirable. Some
of the problems of the commercial breeder and some ways in which they may be solved in
sound genetic principles is discussed here.
A. Problems of the commercial breeder
It should be realized that the problems of the pla
nt or animal breeder are almost always
rather complex. It is easy to tell an animal breeder how to eliminate single comb from his flock of
Wyandottes, red coat from his Angus cattle, or black from his winsleydale sheep. These are
characters which are cause
d by variations in one or two genes and, once he learns the method
of inheritance of the character, he can eliminate it. It is not so easy, however, to tell him how to
eliminate broodness susceptibility to coccidiosis, or low egg production from his poultr
and most other characteristics which have commercial application are dependent upon the
interaction of multiple gene. Hence, most programmes for the improvement of commercial types
involve the application of the principle of selection base on qua
Also, the commercial breeder is faced with changing fancies about what is desirable and
what is not, and such standards vary as much by within and as suddenly as fashions in cloths.
For instance. Coloured eggs are just as nutritious
as white ones, but in certain sections of this
country the demand favours white eggs, so that the poultry man must work not only for a flock
with high egg yield, resistance to disease, and so on, but he must also consider the colour of the
eggs his hens ar
B. Selection for desirable qualities
Selection is the most widely practiced method of maintaining and improving domestic
animals and cultivated plants. But the wise application of this type of selection is not as easy as
it may seem. Far instanc
e, one might select and breed hens on the basis of size of the eggs
they lay, and in the course of several generations might secure a flock which lays eggs distinctly
larger than the average of the original flock, only to find that fewer eggs are laid and
die of disease. Thus, selection on the basis of one factor alone often defeats its purpose by
producing organisms which are deficient in other desirable qualities. Selection, to be of the
greatest value, must take all desirable qualities into acc
1. Body form
In plants, the body form bears a close relationship to the practical aspects of cultivation.
In tobacco, plants which grow tall and straight produce a greater number or large, well formed
leaves than plants which are low and branching.
In fruit tree however, the low, branching farm is
desirable, both from the stand point of production and for ease in harvesting. 'Form is perhaps
most important in ornamental plants. In most producing animals, body form is indicative of
market value. A cer
tain body build in beef cattle is required to produce the finest quality of meat
and the largest percentage of dressed beef.
The number and size of apples, oranges grapes or other fruits the number of baskets of
corn, wheat, oats per acre,
the quantity of milk, eggs, or wool per animal these are all vital
characteristics which must be considered in any program of selection which is to have
3. Quality of product
In addition to quality, it is necessary for the breeder to co
nsider the quality of his
produce. High milk yield
with a very low butterfat content is less desirable than a lower yield with
a high butter fat content. Sweet juicy oranges are more desirable than a larger number or sour,
pithy ones. Corn various in the a
mount of oil, starch, sugar and protein which it contains. A
variety with a high sugar content is desirable for human consumption in fresh condition, while
the one with high protein content is desirable as feed for livestock.
es such things as ability to withstand extremes of temperature or
moisture, resistance to disease, and general body vigour. These characters are influenced by
For example, the establishment of rust
resistant strains of wheat, may save the farmer
from huge losses due to this infection.
5. Storage qualities of product
For most fruits and vegetables, transportation and storage qualities are very important.
No matter how high the quality of the the product, as it comes from the field or the orchard,
has little commercial value if it cannot be transported without damage of deterioration and if it
cannot be stored for a reasonable length of time. In fruit, quality must often be sacrificed for
keeping and transportation qualities.
6. Early maturity
Early fruits and vegetables usually command premium prices. Similarly, early maturity in
animals is of value also. The sooner a hen matures and begins laying, the more valuable she
7. Economy in food use
Another important factor in the selection
of animals is economy in the consumption of
food. A milking cow that converts a maximum amount of her food into milk is a valuable animal.
Records need to be kept to compare food intake with milk yield in order to select for this factor.
Hogs are selected
for maximum amount of ham and bacon produced in relation to their intake of
8. Selection for combination
A program would be of little commercial value if selection is done for only one factor ,say
number of eggs. Here, other factors would be ignored
which would produce defects in the flock
that would outweigh any advantage gained by increasing the number of eggs produced. Thus
selection should be based on the overall favourableness of all the factors viz. number and size
of eggs, egg hatchaility, per
sistency in laying, early sexual maturity, non
C. Modern genetic techniques
With the great enhancement of knowledge in genetics which has been acquired during
recent years, however an acceleration in the rate of improvement is expected in
the years to
come. However, the practical breeder of plants and animals sometimes uses methods which are
slower and less reliable than would be possible through the application of our present
knowledge of genetics.
1. Progeny test
Animal breeders often r
un into a problem in selection which can only be solved by the
progeny test. Sex
limited characteristics are inherited through both parents even though only
one parent may express the character. In many domestic animals only one sex is commercially
e in certain respects. Thus if we are selecting for egg yield and egg size in chickens, but
the rooster in the pen has just as much to do with the yield and size of the eggs as the hen. If
we ignore this male parent, we can determine a roosters genes for e
gg production when he
never produces eggs. This is done by breeding and by studying the offspring. This, technique is
called the progeny test.
If we wish to test a rooster for genes influencing egg yield, we mate him to a group of
hens of a known annual r
ate of egg production. Then we study the rate of egg production of the
progeny. If we found that the hens which descended from this rooster constantly had a higher
annual egg yield than their female parents, then we would assume that the rooster contained
genes for high egg yield. And if he were the best rooster to be found, on the basis of progeny
test, he would be selected for mating to increase the egg yield in the flock. On the other hand if
he failed to raise the egg production, it is assumed that his
genes were about the same as those
of the hens to which he was mated, and if he lowered the production, it can be concluded that
he was inferior in this respect.
The process of mating of individuals which are more closely related than the av
the population to which they belong, is called inbreeding. The self fertilization in plants as in
peas and beans is an example of inbreeding. The marriage between brothers and sisters or first
cousins is another example of inbreeding among higher
Close inbreeding must almost always accompany artificial selection for the improvement
of commercial breeds of plants and animals. Of course, in some plants asexual propagation is
possible through budding, grafting, or growth of cuttings, and onc
e a desirable type has been
obtained, it is possible to continue it indefinitely without genetic change. In plants, which must
come up from seed, and in animals, however, inbreeding is resorted to as a means of retaining
a desirable genotype.
The number o
f individuals which show the desirable characteristics in any sample
population is usually small and if such individuals have been produced by artificial control of the
breeding stock, they are probably very closely related. Hence close inbreeding becomes
necessity. Also, when through accident or intent one outstanding individual is produced, the
closest kind of inbreeding is needed in order to concentrate the desirable quantities as much as
possible by keeping together the fortuitous aggregation of genes
that has made the progenitor
It is often falsely believed that inbreeding 'creates' harmful , characteristics, since
harmful characteristics often do become evident in a program of inbreeding. A genetic
explanation can be that, most genes wh
ich produce harmful characteristics are recessive, and
any individual is likely to be heterozygous for many of these. Inbreeding promotes homozygous
for many of these. Inbreeding promotes
homozygosity of these recessive genes so that the
may be expressed. Should a race be free relatively free of such harmful
recessive genes, there would be no such dire effect.
Most animal breeders note a decrease in size, vigour, and fertility among the
descendants of inbred stock. This can be counter
lanced to some extent by combining
inbreeding with a progress of selection among the offspring. So as to eliminate the deleterious
effects of homozygous harmful recessive genes. Through such a program uniformity may be
obtained in a race without much sacri
fice of quality. The, the following effects may be seen in
Sewall Wright contributed significantly to the theoretical aspects of the problem with his
studies to the effect of inbreeding in guinea pigs. The self fertilization pr
most rapidly. After 8 generations, nearly all the genes that were heterozygous before inbreeding
would be the theoretically homozygous. Brother and sister mating are somewhat less efficient
introducing homozygosis. Under this system 95%
of the genes which were heterozygous
originally would become homozygous in 11 generations of inbreeding. In case of double first
cousins the inbreeding would field about 65% homozygosity in 14th generation. The continued
matings of second cousins would pr
oduce homozygosis from 50 to 51 percent.
b. Pure lines
A pure line can be defined as consisting of individuals descended from a single self
individual and having the similar genotype. Selection is ineffective in a pure line because
not altered by environmental factors.
c. Inbreeding depression
Vigour loss following the mating of closely related individuals in known as inbreeding
depression. This happens in cross
fertilized crops when genetically close plants are mated.
epression is the result of uncovering deleterious recessive that are marked in the
heterozygous state. The inbreeding depression is calculated by comparing the frequencies of
homozygous recessive inbred and outbreed population.
3. Outbreeding and hybrid vi
One method of introducing new genes is the artificial induction of mutation. Because of the
preponderance of harmful mutations, however and expense involved in such methods, this is
not practical for the overage commercial breeder. A much more practic
al possibility lies in the
introduction of new genes into the population through out breeding. The crosses between two
unrelated pure breeding population arc called out crosses or out breeding. The out breeding
usually results in stronger, healthier, large
r and vigorous individuals in F1. The superiority , of
the hybrid over the best parent to known as heterosis. Such a hybrid may have unproved
general fitness, resistant to diseases and may show remarkable growth and vigour. Such
improvement observed in FI
generation are also called hybrid vigour. One explanation for hybrid
vigour lies in the fact that most harmful genes are recessive and most beneficial genes are
dominant. Hybridization tends to bring out the beneficial qualities of both varieties and suppr
the harmful qualities of each.
a. Hybrid vigour in maize
Maize is a cross pollinated plant in field but can also be self
pollinated. Inbred progenies (pure
lines) for various traits of maize produce low yields blue to recessive homozygosity and are of
short stature. When two inbred lines are crossed, the hybrid shows more vigour with high yields
as it restores the vigour in the next generation following the cross.
The principle of hybridization for increased vigour and yield in maize has been carried
step further, in the form of double
cross hybridization which is diagrammed below.
Inbred A Inbred B
Inbred C Inbred D
(A x B) x (C x D)
of producing a hybrid between two different parental inbred lines.
Inbred AxB produced a single cross hybrid AB. Similarly, inbred CxD produces a single cross
hybrid CD. The double cross hybrid has the gene combinations from four separate inbred lines.
he hybrids between some inbred line give yield as high as that of the cross
varieties but some inbred lines produce hybrids which are exceptionally high yielding. These
inbred lines possess good combining ability.
b. Hybrid vigour in animals
In animals outbreeding is often practiced in order to produce some specific type which is
desirable for market purposes. A cross between the white short horn and the black Angus cattle
yields a blue bean hybrid which is noted for its vigour, rapid growth,
economical utilization of
food, and the high quality of beef which it produces. Such animals are usually bred solely for the
market, and are not used for breeding purposes because of the variability of the off spring which
would result from the segregatio
n of genes in succeeding generations.
In domestic swine sows of the bacon
producing breeds, such as the Yorkshire or Tamworth,
are often outbred to boars of the lard producing breeds, such as Duroc Jersey or Poland
This type of cross yields a lar
ger number of vigorous, early maturing offspring with a superior
market value when compared with the offspring of pure breeds.
The principle of outbreeding may be applied within the breed by establishing two strains of stock
by selection and in breeding,
which then maybe crossed together to produce market animals as
animals which produce for the market. The advantages of such a method are well illustrated by
a practice in poultry raising. Two inbred strains of Wyandottes have been developed for the
purpose of cross breeding for egg production. One strain is identified by a dominant,
linked gene which produces silver plumage, the other strain is homozygous for the
recessive allele for gold plumage. The silver hens are mated with the gold cocks to
laying pullets. In this type of cross it is possible to distinguish the sexes immediately after
hatching, for the female chicks will always be gold and the male chicks will always be silver this
makes it easy to remove the males from the group
. These hybrid pullets have the vigour which
is characteristic of hybrids, are resistant to disease, are early maturing, and are heavy layers.
C. To create new breeds
When hybrids are bred to produce a second generation, there is a segregation of their gen
and a great variety of offspring will appear. It is possible to establish a breed with the desirable
qualities which will appear in the. hybrid by selection from succeeding generations, and thus
eventually to obtain a new breed incorporating from the t
wo original breeds the characteristics
which are desired. This is a long, tedious, and expensive
process, however end in spite of the
most selection there is practically always some loss of vigour and fertility as a result of the
necessary inbreeding. Nev
ertheless, this is the manner in which many of our present day
breeds have been established
through hybridization and selection from other breeds. Most
breeders of today, however, prefer to stick to improvement, within the breed because of the
lties involved in establishing a new breed.
In some cases different species are crossed and yield a hybrid of superior vigour and
commercial value. The male is a good example of such an interspecific cross. The mule is
produced by crossing the mare (Equas
equus) is the jack ass (Equus hemionus). Mule
who consistently turn out superior work animals maintain a breeding stock of jacks and mares
which have been selected especially for their ability to produce high quality
mules. There is no
ere of the possibility of establishing a breed of hybrids, for the mule (like interspecific
hybrids in general) is sterile. A study of the seminal fluid from the male mule shows that its
sperms are non
However a female mule may give birth to a
colt after mating with a jackass. While man has been
selectively breeding domestic plants and animals for thousands of years, the new methods
made possible by recent advances in genetics hold great promise for the future.