Two different truebreeding lines of rats both with recessive

Two different true-breeding lines of rats, both with recessive mutations for a bone disorder, are crossed and they give rise to F1 rats without the disorder. If an F1 individual is then crossed to one of the parents, what proportion of the progeny is expected to have the disorder?

Solution

Answer: The proportion of the progeny is expected to have the disorder = 7/16

Duplicate recessive gene (9: 7):

If both gene loci have homozygous recessive alleles and both of them produce identical phenotype the F2 ratio 9:3:3:1 would be 9:7. The genotype aaBB, aaBb, AAbb, Aabb and aabb produce same phenotype. Both dominant alleles when are present together only then they can complement each other. This is known as complementary gene.

Complete dominance at both gene pairs, but either recessive homozygote is epistatic to the effect of the other gene.

AAbb x aaBB-------------------P1

AaBb (without disorder)

AaBb x AaBb

F2: Disorder : No disorder = 9:7