Genetics Problems for Biology 1009: Key
1. You have found a blue male bug and a green female bug of the same species living in the
rainforest. Assume that the color difference is genetic and further that only two autosomal alleles
(at the same locus) are involved. You breed the bugs (the P1 generation) and half of their
offspring (the F1 generation) are blue and half are green. Given only the information above, how
can you tell which allele is dominant? (1 pt)
You can't tell at all. You need more information.
You inbreed two of the blue bugs produced in the cross above (from the F1 generation). Of the
1000 offspring (the F2 generation) they produce, 772 are blue and 228 are green. What is the
probable genotype of the blue F1 bugs? (2 pts: 1 for answer, 1 for reasoning/work)
Both parents must be heterozygotes (Bb, Gg, etc.. Otherwise they would breed "true";
producing only offspring with the same phenotype as themselves.
Alternatively: the 3:1 ratio is typical of a hybrid cross.
2. You are studying two true-breeding populations of white mice, one with black noses and furry
tails and the other with pink noses and bald tails. You cross a male from the first population with a
female from the second population (the P1 generation) and their offspring (the F1 generation) are
all black-nosed and furry-tailed. What are the possible genotypes of the F1 generation? (1pt)
You know that the P1 generation are homozygous and different from eachother, for both
traits, so the F1s must be double heterozygotes (BbFf, etc.).
If the two traits under consideration are linked and you inbreed two F1 mice, what will be the
phenotype distribution of their offspring (the F2 generation)? (2 pts: 1 for answer, 1 for work)
Since the genes are linked, the F1s can only make two kinds of gametes: BF and bf.
Punnett Square
| BF | bf |
| BF | BFBF | BFbf |
| bf | BFbf | bfbf |
The F2s will be 3/4 black-nosed and furry-tailed and 1/4 pink-nosed and bald-tailed.
3. Color-blindness is caused by an X-linked recessive allele. A color-blind man and a
homozygous, non-color-blind woman have a daughter. How likely is the daughter to be color
blind? (2 pts: 1 for answer, 1 for work)
The daughter has a 0% chance of being color-blind, as the trait is recessive and she
will receive one X-chromosome from her mother with a gene that will giver her normal color vision.
The daughter has a son with a non-color-blind man. How likely is the son to be color blind?
(2 pts: 1 for answer, 1 for a work)
The daughter (now mother) got her father's X-chromosome with the color-blindness gene. She is as
likely to pass this on to her son as she is the normal X-chromosome from her mother (50%). The son
gets no X-chromosome from his father, so he has a 50% chance of suffering color-blindness.
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