Tatuo Aida investigated the genetic basis of color variation
in the Medaka (Aplocheilus latipes), a small fish found
naturally in Japan (T. Aida. 1921. Genetics 6:554–573).
Aida found that genes at two loci (B, b and R, r) determine
the color of the fish: fish with a dominant allele at both
loci (B_R_) are brown, fish with a dominant allele at the B
locus only (B_ rr) are blue, fish with a dominant allele at
the R locus only (bb R_) are red, and fish with recessive
alleles at both loci (bb rr) are white. Aida crossed a
homozygous brown fish with a homozygous white fish. He
then backcrossed the F1 with the homozygous white parent
and obtained 228 brown fish, 230 blue fish, 237 red fish,
and 222 white fish.
a. Give the genotypes of the backcross progeny.
b. Use a chi-square test to compare the observed numbers of
backcross progeny with the number expected. What
conclusion can you make from your chi-square results?
c. What results would you expect for a cross between a
homozygous red fish and a white fish?
d. What results would you expect if you crossed a
homozygous red fish with a homozygous blue fish and then
backcrossed the F1 with a homozygous red parental fish?