Genetics Practice Problems #7 DIHYBRID CROSSES
KEY
1. In pepper plants, green (G) fruit color is dominant to red (g) and round (R)
fruit shape is dominant to square (r) fruit shape. These two genes are located on
different chromosomes.
a. What gamete types will be produced by a heterozygous green, round plant?
the green round plant will produce GR, Gr, gR, and gr gametes in equal
proportion since the genes are unlinked
b. If two such heterozygous plants are crossed, what genotypes and
phenotypes will be seen in the offspring and in what proportions?
this will give 9/16 green round, 3/16 green square, 3/16 red round, and
1/16 red square phenotypes;
GR
Gr
gR
gr
GR
GGRR
GGRr
GgRR
GgRr
Gr
GGRr
GGrr
GgRr
Ggrr
gR
GgRR
GrRr
ggRR
ggRr
gr
GgRr
Ggrr
ggRr
ggrr
G_R_ = green, round
G_rr = green, square
Proportion
Genotype
ggR_ = red, round
ggrr = red, square
Phenotype
1
GGRR
Green-round
2
GGRr
Green-round
2
GgRR
Green-round
4
GgRr
Green-round
1
GGrr
Green-square
2
Ggrr
Green-square
1
ggRR
Red-round
2
ggRr
Red-round
3
1
ggrr
Red-square
1
9
3
1. About 70% of Americans perceive a bitter taste from the chemical phenylthiocarbamide
(PTC). The ability to taste the chemical results from a dominant allele (T) and not being able to
taste PTC is the result of having two recessive alleles (t). Albinism is also a single locus trait
with normal pigment being dominant (A) and the lack of pigment being recessive (a).
A normally pigmented woman who cannot taste PTC has a father who is an albino taster. She
marries a homozygous, normally pigmented man who is a taster but who has a mother that does
not taste PTC.
a. What are the genotypes of the possible children?
the parents.
Hint: first determine the genotypes of
We know that the woman has normal pigment which means she must have at least one A.
Her father is albino and because the albino allele is recessive, his genotype is aa. What
does this make her skin genotype?
We also know that the woman cannot taste PTC. Because the ability to taste PTC is
dominant, what does this make her genotype for the tasting trait?
Putting both traits together, we see that the woman's overall genotype is Aatt.
Now, what about her husband? You have been told that he is homozygous for normal
pigment. What is his genotype for skin color?
He is a taster and so must have at least one T. However, we also know that his mother can
not taste PTC so she must be homozygous recessive. With this information, what is his
genotype for taste?
Putting both traits together, you see that his overall genotype is AATt.
At
At
at
at
AT
AATt
AATt
AaTt
AaTt
AT
AATt
AATt
AaTt
AaTt
At
AAtt
AAtt
Aatt
Aatt
At
AAtt
AAtt
Aatt
Aatt
What percentage of the children will be albinos?
aa genotype necessary for the albino trait = 0%
What percentage of the children will be non-tasters of PTC?
T allele necessary to be a taster = 8/16 = 50%
2. Wolves are sometimes observed to have black coats and
blue eyes. Assume that these traits are controlled by
single locus genes and are located on different
chromosomes. Assume further that normal coat color (N)
is dominant to black (n) and brown eyes (B) are dominant
to blue (b).
Suppose the alpha male and alpha female of a pack (these
are the dominant individuals who do most of the breeding)
are black with blue eyes and normal colored with brown eyes, respectively. The
female is also heterozygous for both traits.
How many of the offspring (assume 16) living in the pack will have each of the
following genotypes?
(Dad) nnbb x NnBb (mom)
nb
nb
nb
nb
NB
NnBb
NnBb
NnBb
NnBb
Nb
Nnbb
Nnbb
Nnbb
Nnbb
nB
nnBb
nnBb
nnBb
nnBb
nb
nnbb
nnbb
nnbb
nnbb
Gametes X
Possible genotypes
4 – NnBb 4 – Nnbb 4 – nnBb 4 - nnbb
What percent of the offspring will be normal colored with blue eyes?
4/16 Nn (normal) bb (blue eyes) = 25%
In the breeding season, male Anole lizards
court females by bobbing their heads up and
down while displaying a colorful throat patch.
Assume for this question that both males and
females bob their heads and have throat
patches. Assume also, that both traits are
controlled by single locus genes on separate chromosomes. Now, suppose that
anoles prefer to mate with lizards who bob their heads fast (F) and have red
throat patches (R) and that these two alleles are dominant to their
counterparts, slow bobbing and yellow throats.
A male lizard heterozygous for head bobbing and homozygous dominant for the
red throat patch mates with a female that is also heterozygous for head
bobbing but is homozygous recessive for yellow throat patches.
(Dad) FfRR x Ffrr (mom)
Gametes
X
FR
FR
fR
fR
Fr
FFRr
FFRr
FfRr
FfRr
Fr
FFRr
FFRr
FfRr
FfRr
fr
FfRr
FfRr
ffRr
ffRr
fr
FfRr
FfRr
ffRr
ffRr
Possible genotypes: 4 – FFRr
8 – FfRr
4 - ffRr
a. How many of the F1 offspring have the preferred fast bobbing / red throat
phenotype (assume 16 young)? 12 = 4 FFRr + 8 FfRr
b. What percentage of the offspring will lack mates because they have both
slow head bobbing and yellow throats? 0% will be both
c. What percentage of the offspring will have trouble finding mates because
they lack one of the dominant traits? 4/16 = 25% will be slow bobbing
4. Carrion beetles lay their eggs in dead animals and then bury them in the ground until they hatch.
Assume that the preference for fresh meat (F) is dominant to the preference for rotted meat and that
the tendency to bury the meat shallow (S) is dominant to the tendency to bury the meat deep. Suppose a
female carrion beetle homozygous dominant for both traits mates with a male homozygous recessive for
both traits. What will be the genotype of the F1 generation?
What will be the phenotype of the F1 generation?
What will be the genotypic ratio of the F2 generation (FFSS : FFSs : FFss : FfSS : FfSs : Ffss : ffSS :
ffSs : ffss)?
5. Suppose in a strain of soybeans, high oil (H) content in the seeds is dominant to low oil content and
four seeds (E) in a pod is dominant to two seeds in a pod. A farmer crosses two soybean plants, both with
high oil content and four seeds per pod. The resulting F1 offspring have a phenotypic ratio of 9:3:3:1
(High oil / four seeds : High oil / two seeds : Low oil / four seeds : Low oil / two seeds). What genotype
were the parent plants?
Suppose the farmer chooses two of the high oil / four seed plants and crosses them. The F2 generation
have all high oil / four seed phenotypes. What were the genotypes of the plants chosen by the farmer to
cross?
Which known genotypes might the farmer cross her high oil / four seed plants with to determine their
genotype?