Chapter 3, Problem 34P

A student whose hobby was fishing pulled a very unusual carp out of Cayuga Lake: It had no scales on its body. She decided to investigate whether this strange nude phenotype had a genetic basis. She therefore obtained some inbred carp that were pure-breeding for the wild-type scale phenotype (body covered with scales in a regular pattern) and crossed them with her nude fish. To her surprise, the F₁ progeny consisted of a 1:1 ratio of wild-type fish and fish with a single linear row of scales on each side.

a.	Can a single gene with two alleles account for this result? Why or why not?
b.	To follow up on the first cross, the student allowed the linear fish from the F1 generation to mate with each other. The progeny of this cross consisted of fish with four phenotypes: linear, wild type, nude, and scattered (the latter had a few scales scattered irregularly on the body). The ratio of these phenotypes was 6:3:2:1, respectively. How many genes appear to be involved in determining these phenotypes?
c.	In parallel, the student allowed the phenotypically wild-type fish from the F1 generation to mate with each other and observed, among their progeny, wild-type and scattered carp in a ratio of 3:1. How many genes with how many alleles appear to determine the difference between wild-type and scattered carp?
d.	The student confirmed the conclusions of (c) by crossing those scattered carp with her pure breeding wild-type stock. Diagram the genotypes and phenotypes of the parental, F1, and F2 generations for this cross and indicate the ratios observed.
e.	The student attempted to generate a true-breeding nude stock of fish by inbreeding. However, she found that this was impossible. Every time she crossed two nude fish, she found nude and scattered fish in the progeny, in a 2:1 ratio. (The scattered fish from these crosses bred true.) Diagram the phenotypes and genotypes of this gene in a nude × nude cross and explain the altered Mendelian ratio.
f.	The student now felt she could explain all of her results. Diagram the genotypes in the linear × linear cross performed by the student in (b). Show the genotypes of the four phenotypes observed among the progeny and explain the 6:3:2:1 ratio.