Short hair (S) in rabbits is dominant over long hair (s). The following crosses

Question

Short hair (S) in rabbits is dominant over long hair (s). The following crosses are carried out, producing progeny shown. Give all the possible genotypes of the parents in each cross. In mammals, albinism is caused by an autosomal allele that interferes with skin pigment Early one morning on your way to genetics class, you observe that two normally pigmented javelina parents have an albino piglet. What are the genotypes of the parents? What is the probability that their next five offspring will be albino ? What is the probability that 3 of the next 5 offspring will be normally colored? Describe the process by which you would test these normally colored offspring to see if they carried the albinism gene. What are the expected ratios from that test? You want to develop a strain of albino javelina for commercial use (the other really white meat). Describe how you would go about developing that strain starting with the 2 parents and the one albino offspring. Joe has a white cat named Sam. When Joe crosses Sam with a black cat, he gets 1/2 white kittens and 1/2 black kittens. When the black kittens are interbred, all of the kittens they produce are black. On the basis of these results, would you conclude that white or black coat color in cats is a recessive trait? Explain your reasoning.

Explanation / Answer

1. The order doesn't matter, as long as one parent is one genotype or the other. a) Rr x Rr b) RR x RR or Rr x RR c) RR x rr d) Rr x rr e) rr x rr 2. Let A = albinism allele. AA = no albinism aa = albinism Parents are both Aa (heterozygous) P(5 albino offspring) = (1/4)^5 = 0.000977 P(3 of the next 5 normal) = (5 choose 3)(3/4)^3*(1/4)^2= 0.264 To test if the offspring carried the recessive albinism gene (a), you would breed the offspring with either of the parents (opposite gender of course). If the offspring did carry the recessive allele, then the breeding would result in a 3:1 phenotypic ratio of normal to albinos (meaning you'll get at least one in the litter). If the offspring were homozygous for the normal allele (AA), then you'd get all normal litter. To develop a strain of albinos, you simply breed the aa offspring (albino) with either parent (Aa). This would yield an expected 1:1 phenotypic ratio of normal to albino (half will be albino). You can then breed the albino progeny with the aa parent and guarantee albino babies each time. 3. Black is recessive to white allele. The parents are Aa and aa, which would give the expected 1:1 phenotypic ratio. If black were dominant (i.e. Aa), then when you breed Aa x Aa, you would expect a 3:1 ratio of black to white in the next round (i.e. at least one white). This contradicts the observation of all black kittens. However, there is the possibility that no white kittens were produced by chance, as the probability of having one starts at 1/4 and decreases by 1/4 for each kitten.

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