A new phenotype of mouse has been discovered – short-tails. You have been given a short-tailed mouse and do not know anything about its genotype. You decide to cross this short-tailed mouse with a long-tailed mouse. Multiple crosses of this short-tailed mouse to long-tailed mice produces 27 long-tailed mice and 25 short-tailed mice. When you cross these short-tailed progeny with each other, you get 31 short-tailed mice and 11 long-tailed mice. Required:
a. What was the genotype of the original short-tailed mouse?
b. What is the genotype of the long-tailed mice used in this breeding experiment?

a. Ss

b. ss

Explanation:

This question involves a gene coding for tail length in mice. According to the question, a new phenotype of mouse has been discovered to have short-tails. This short-tailed mouse was crossed with a long-tailed mouse multiple times to produce 27 long-tailed mice and 25 short-tailed mice. Based on this phenotypic result, the phenotypic ratio can be deduced to be:

Long tail = 27/25 = 1.08

Short tail = 25/25 = 1

Long tail : short tail = 1:1

This phenotypic result can only be achieved if

The short tailed mice got in the F1 generation was self-crossed to get 31 short-tailed mice and 11 long-tailed mice. Based on this result as well, the phenotypic ratio can be deduced to be:

Short tailed = 31/11 = 2.8

Long tailed = 11/11 = 1

Approximately, the phenotypic result of F2 is short tail 3 : long tail 1.

Based on the above phenotypic ratios, the F1 generation with {Long tail : short tail = 1:1} can only be got from heterozygous (Ss) and homzygous recessive (ss) mice. Also, based on the F2 generation with {short tail 3 : long tail 1}, the result can only be got when the parents mice are both heterozygous i.e. Ss × Ss.

This concludes that the short tail length is dominant over the long tail length. Hence, the original short tailed mouse has a heterozygous genotype of Ss while the long tailed mouse used in the experiment has a genotype; ss.

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