Genetics Practice Problems II KEY
AP Biology
1. In Duroc-Jersey pigs, coat color is determined by two genes: R and S. The dihomozygous
recessive condition produces a white coat. The presence of at least one copy of each dominant
allele (R-S-) produces red. The presence of one or the other allele (e.g., rrSs) yields a sandy coat
color.
Mating a red Duroc-Jersey boar to sow A, a white hog, produces piglets in the ratio 1 red: 2
sandy: 1 white. Mating the same boar to sow B, a sandy hog, produces 3 red: 4 sandy: 1 white.
When the same boar was mated to sow C, a sandy hog, the litter had equal numbers of red and
sandy piglets. Determine the probable genotype of the boar and the three sows.
Cross 1: R-S- x rrss yields red, sandy, and white. If we are getting white offspring, then the white
piglets must be getting a recessive r and s from each parent. Therefore, we can deduce from this
cross that the red boar must be a dihybrid.
Cross 2: RrSs x sandy (rrS- or R-ss). Because we get white piglets, the sow must be either rrSs or
Rrss, since the white piglets must get recessive alleles from each parent. As for which allele is
dominant (R or S), we do not know.
Cross 3: RrSs x rrSS or RRss. The sow must be homozygous dominant for either allele.
2. Height and weight in animals is controlled by polygenic inheritance. By inbreeding large
animals, breeders are usually able to produce some increase in size among their stock.
However, after a few generations, increase in size stops. Explain why.
Once you select for total homozygous dominants (AABBCCDD…YYZZ), you cannot get any bigger.
3. In Jimson weed, the allele that produces violet petals is dominant over that for white petals, and
the allele that produces prickly capsules in dominant over that for smooth capsules. A plant
with white petals and prickly capsules was crossed with one that had violet petals and smooth
capsules. The results of the F1 are shown here:
White petals, prickly
capsules
White petals, smooth
capsules
Purple petals, prickly
capsules
Purple petals, smooth
capsules
47
45
50
46
What were the genotypes of the P generation?
The white, prickly parent must be ppCc, and the purple smooth parent Ppcc.
4. In a certain strain of mice, coat color is determined by five different pairs of alleles. The colors
range from near white to dark brown. Is it possible for some pairs of mice to produce offspring
that are substantially different in color from the parents?
Assume that you are mating two middle-of-the-road, average colored parents: AaBbCcDdEe x
AaBbCcDdEe. According to the law of segregation, each parent can donate either the dominant
or the recessive allele. Therefore, it is possible that these two medium-colored parents could
donate all recessive alleles to the offspring (aabbccddee), and produce very light offspring; or all
dominant alleles (AABBCCDDEE) and have very dark offspring.
5. In chickens, two pairs of alleles control comb shape. RR or Rr results in rose comb, and rr
produces a single comb. PP or Pp produce pea comb, and pp produces single comb. When R
and P occur together, they produce a new type of comb, called walnut. What would be the
genotype and phenotype of the F1 generation resulting from RRpp x rrPP? If you sibling crossed
the F1 dihybrids, what would you predict in the F2?
The F1 is RrPp, which would be walnut. When you mate them, you would see a ratio of 9 walnut
(R-P-) : 3 Pea (rrP-) : 3 rose (R-pp) : 1 single (rrpp).
6. In Ayrshire cows, AA produces a mahogany coat and aa a red coat. However, heterozygotes
vary by gender, and produces mahogany in males and red in females. A mahogany-colored
Ayrshire heifer (a female) is with her red calf. Is the calf a heifer or a bull?
The mother is mahogany, so she must be AA. The calf must be female. She must inherit an A
allele from her mother; her father could donate A or a. The calf is either AA or Aa. Regardless of
gender, AA is mahogany. An Aa male is also mahogany. The only way for a red calf is an Aa
female.