Solving Genetics Problems I: Monohybrid Crosses

Solving Genetics Problems :
Monohybrid Crosses


Two organisms

One characteristic

Complete dominance

Solving Genetics Problems :
Monohybrid Crosses

•
Complete dominance


•

the
dominant allele

completely masks the
effect of the
recessive allele

in
heterozygous

condition.

•
Bb

Solving Genetics Problems :
Monohybrid Crosses

•
Incomplete dominance


•

dominance

occurring in
heterozygotes

in which
the
dominant

allele

is only partially expressed


–
Pp

–
Sweet peas

Solving Genetics Problems :
Monohybrid Crosses

•
The problem involved two gerbils named
Honey and Ritz.


Solving Genetics Problems :
Monohybrid Crosses

•
The gene in question is a fur color gene which
has two

alleles

•
dominant brown (
B
) and recessive black (
b
)


An
allele

is one of two or more forms of a gene

Solving Genetics Problems :
Monohybrid Crosses

•
Step One: Figure out the
genotypes of the parents.

–

address the question of all of
the possible kinds of babies
they could produce










Solving Genetics Problems :
Monohybrid Crosses

•
Step Two: Figure out what kinds of gametes
the parents can produce.

–
Now you need to determine all the possible ways
that his sperm can combine with her eggs.

–
Punnett

Square

Solving Genetics Problems :
Monohybrid Crosses

•
Step Three: Set up a
Punnett

Square .

–
You need to create a chart
with one column for each of
the female's egg types, and
one row for each of the
male's sperm types



Solving Genetics Problems :
Monohybrid Crosses

•
Step Four:


Fill in the babies inside the
table by matching the egg
allele at the top of the
column with the sperm allele
at the head of the row.

Solving Genetics Problems :
Monohybrid Crosses

Figure out the
genotypic ratio
for
your predicted babies.

•
So we have now figured out
that, if Honey and Ritz have
a lot of babies, we can
predict that:

•

25% should be
BB

•

50% should be
Bb

•
25% should be
bb

Solving Genetics Problems :
Monohybrid Crosses

Step Five: Figure out the
Phenotype
ratio for your predicted babies.

•
This is where
dominance
really
enters the picture


•
If
B

is completely dominant
to b, all gerbils with at least
one
B

will look pretty much
alike, no matter what their
second allele is

Solving Genetics Problems :
Monohybrid Crosses



Phenotype will be:


•
75% brown

fur

•
25% black fur

Solving Genetics Problems :
Monohybrid Crosses

•
Where do we come from,
as a species, and how are
we all related


•
S
ee how our species
shares its genetic history
with all other species on
the planet

•
G
enetic engineering


•
DNA fingerprinting in areas
such as drug discovery and
forensics