Fundamental Principles of Genetics (Chapter 8)

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Fundamental Principles of Genetics
(Chapter 8)


Genetics


study of heredity


Gene


determinant of heredity


Chapter is outline of basic principles of genetics

www.sardi.sa.gov.au/
livestock
/meat
_wool/breeding_
genetics

Cell Theory of Inheritance



Plants and animals composed of cells



Plant cell


surrounded by cell wall



Animal cell


surrounded by cell membrane



Nucleus


location of chromosomes in cell



Chromosomes


carrier of genes

Chromosomes



Carriers of genetic material


genes



Occur in pairs



Homologous chromosomes


members of a
pair



Number of chromosomes constant among
normal members of a species

Chromosome Number


Diploid (2N) numbers

donkey

62

horse

64

mule

63

swine

38

sheep

54

cattle

60

human

46

mink

30

dog

78

cat

38

chicken

78



Normal body cells have diploid (2N) number


Gametes (sperm and egg) have haploid (1N)
number

Chromosomes


Two major types



Sex chromosomes


one pair which influences
sex of organism



Autosomes


all pairs other than sex
chromosomes


http://learn.genetics.utah.edu/content/begin/traits/karyotyp
e/index.html




Sex Determination


Mammals


Sex chromosomes are:


female


XX

homogametic


male


XY

heterogametic



Chickens and turkeys


Sex chromosomes are:


female


ZW

heterogametic


male


ZZ

homogametic

Chromosomal Abnormalities


Changes in number of chromosomes
(nondisjunction of sex cell)


Aneuploidy


extra or missing chromosome(s)



Polyploidy


extra sets of chromosomes



Usually lethal in animals


Except aneuploidy of very small chromosomes
(eg Down Syndrome in humans is extra #21)

Chromosomal Abnormalities


Changes in chromosome structure



(see fig. 8.3, p. 143 illustration)


Deletion


piece of chromosome removed



Duplication


piece of homologous chromosome
duplicated



Translocation


exchange of material between
different (nonhomologous) chromosomes



Inversion


segment of chromosome reversed




Chromosomal Abnormalities


Changes in chromosome structure



Cause loss of fertility



Amount of loss dependent on type of change and
importance of the affected segment of the
chromosome


http://learn.genetics.utah.edu/content/begin/traits/karyotyp
e/index.html


Cell Division


Two types



Mitosis


division of normal body cells



Meiosis


division of germinal cells to produce
gametes (sperm and egg)

Cell Division


Mitosis



Cell division of normal body cells



Each daughter cell has same genetic complement
as original cell

Cell Division


Mitosis



Four phases



Prophase


Metaphase


Anaphase


Telophase



Interphase


period between cell divisions

Cell Division


Meiosis


Division in germinal cells to produce sperm and
egg



Each cell that results has 1N number of
chromosomes (half of number of chromosomes
in body cells

Cell Division


Meiosis


Two cycles of:


Prophase


Metaphase


Anaphase


Telophase



Results in four cells with 1N number

Cell Division


Meiosis


During late prophase I


homologous chromosomes wrap together (synapsis)



Crossing over


exchange of genetic material
between homologous chromosomes

Cell Division


Meiosis


Spermatogenesis


meiotic production of sperm
cells



Oogenesis


meiotic production of egg cells


Check It Out!


http://
www.dnaftb.org
/

The Gene


Component of DNA


deoxyribonucleic
acid



Large double stranded polymer of units
called nucleotides



Nucleotide


Sugar


deoxyribose


Phosphate group


Nitrogeneous base


adenine


guanine


cytosine


thymine

Transcription of RNA


Bases in DNA code for transcription of
RNA



RNA


ribonucleic acid


Also nucleic acid


Single stranded


Uracil instead of thymine


Ribose instead of deoxyribose

RNA


Three primary types of RNA



Messenger RNA


Transfers code from DNA to protein synthesis



Transfer RNA


Carries amino acids to the ribosome



Ribosomal RNA


Forms part of the structure of the ribosome

Translation of RNA


RNA binds with ribosome



Ribosome is location of protein synthesis



Information on RNA used to generate amino
acid sequence in proteins

Genetic Code


Each amino acid coded by a three base
sequence (codon)



Most amino acids have more than one codon



One codon specifies beginning of a protein



Three codons specify the end of a protein


Control of Gene Function


Complete gene is actually a complex of
different types of genes (complex called an
operon)



Structural genes


Code for actual protein sequence



Regulatory genes


Affect function of the structural genes


Interactions Between Genes


Epistasis


Two or more gene pairs (not alleles) in which
one gene pair influences expression of another
gene pair


may or may not be on homologs



Example:


Horned, polled, scurred


Two gene pairs (horned vs polled) (smooth vs
scurred)


If horned


scurred vs smooth does not matter


If polled


scurred vs smooth can express


Genes and Embryological Development


After union of sperm and egg


Cell division occurs



Early development
-

all cells are alike



Subsequent development


cells differentiate



Embryological development is genetically
controlled




Genes and Embryological Development


Lethal genes


Genes which stop development



Many must be received from both parents to
cause death



Cause of high percentage of embryonic death


Biotechnology


Biotechnology


All technologies that pertain to molecular
manipulation of living material



Very difficult word to characterize



Genetic engineering


New methods for modifying the animal genome


Microbe Engineering


Recombinant DNA Technology


Segment of DNA removed from species of
interest



Segment is inserted into a microorganism



Microorganism makes the product specified by
the DNA



DNA is “recombined” into a new location


Recombinant DNA


Used for production of:


Bovine growth hormone to increase milk
production



Vaccines



Other pharmaceuticals



Bacteria to consume oil spills or other pollutants



Transgenesis


Transgenesis


Movement of genes from one species into
another



First use


development of mouse with extra
genes for growth hormone



Genetic Engineering in Plants


Round
-
up
-
ready crops


Several crop species have been engineered to be
resistant to the herbicide Round
-
up



Bt crops


Corn and cotton have genes from a microbe that
causes them to be resistant to corn borer and boll
weevil



Flavr
-
Savr tomato


Gene altered to lengthen shelf life


Cloning


Clone


genetic identical


Identical twins are clones



Cloning by embryo splitting has been available
for several years



Cloning from an adult DNA donor not
possible until “Dolly”

Cloning


Dolly


Developed at Roslin Institute



Sheep cloned from cells from adult ewe
mammary gland



Cells had to be “started over” to remove results
of cell differentiation

Cloning


Potential uses for cloning in animals


Limited importance in routine livestock
improvement



Combine with transgenesis to produce animals
with unique genetic makeup


transgenesis to create first copy of animal with
specific gene inserted


cloning to make multiple copies of that animal

Marker Assisted Selection


Current selection


Based on actual traits measured in animals



Marker assisted selection


Based on identification of genetic markers that
are associated with performance traits



Can be applied as soon as appropriate tissue
(blood, skin etc) can be obtained



Shortens time to obtain information for choosing
superior parents


Segregation and Recombination


Genes occur in pairs (except for those on
sex chromosomes which are sex
-
linked)



The segregation of genes (in body cells) at
one locus (gene location) is normally
independent of the segregation of genes (in
gametes) at another location (Principle of
Independent Assortment)



Exception to this rule when genes are linked
(on same chromosome)


Segregation and Recombination


Linkage


Genes tend to be inherited together if close
together on same chromosome



Linkage relationships are broken by crossing
over (exchange between homologous
chromosomes) during meiosis I, prophase I



Basis for idea of Marker Assisted Selection


if
genetic marker is linked with gene for trait of
interest, can select based on the marker

Mutation


Mutation


change in the base sequence


May result in change in amino acid sequence in
protein



May result in change in the phenotype



Change is usually detrimental (selection)



Only source of new genetic material

Phenotypic Expression of Genes


Phenotype = Genotype + Environment



Gene action varies among different genes



Dominance relationships vary widely

Phenotypic Expression of Genes


Dominant and recessive



Color in Angus



Genotype

Phenotype



BB

Black



Bb

Black



bb

Red



Black is dominant to red


Red is recessive



BB or bb


homozygous


Bb
-

heterozygous

Phenotypic Expression of Genes


Lack of dominance



Color in Shorthorns






Genotype

Phenotype



RR

Red



RW

Roan



WW

White



Both red and white are expressed in
heterozygote

Phenotypic Expression of Genes


Additive



Each gene substitution (A for a, B for b)
contributes the same amount (each A or B
“adds” the same amount (absolute?)


Example: skin color in humans


Livestock examples in Ch 9



Contrast with non
-
additive types of gene
action (dominance, epistasis)



Phenotypic Expression of Genes


Genetic control of almost all traits is some
combination of additive and non
-
additive



Allows use of selection to improve herds
through increasing frequency of desirable
genes



Allows use of crossbreeding to maximize
efficient combinations of genes

Sex
-
linked Inheritance


Some genes on the sex chromosomes



Inheritance pattern affected because males
have only one X chromosome



Hemophilia in humans

Sex
-
influenced Inheritance


Inheritance that is affected by sex of
individual



eg. scurs
-

dominant in males, recessive in
females



eg. horns in sheep


dominant in males,
recessive in females

Sex
-
limited Inheritance


Some traits express in only one sex



Female


Milk production, egg production, age at first
estrus



Male


Scrotal circumference

Genome Project


Human Genome project


Initiated by National Institutes of Health and
Department of Energy



Large multi
-
year, multi
-
location project to map
the human genome



Other genome projects


Cattle, sheep, swine, horses, dogs, cats, turkeys,
chickens, mice and many other species