Jersey pigs are usually red but sandy pigs are also possible

Jersey pigs are usually red but sandy pigs are also possible. Two different varieties of true-breeding sandy pigs were crossed. The F1 were all red. An F1 X F1 cross was made and resulted in the following F2 pigs: 17 red pigs, 13 sandy pigs and 2 white pigs. What is the genotype of the two varieties of true-breeding sandy pigs? Explain this pattern of inheritance. (Diagram the cross giving geneotypes)

Solution

Answer:

A. The sandy variation may be due to a homozygous recessive allele at one of two different genes in these two varieties of sandy pigs. Let’s call them genes A and B. One variety of sandy pig could be aaBB and the other AAbb. The F1 generation in this cross would be heterozygotes for both genes and are all red. This tells us that the A and B alleles are dominant.

In the F2 generation, 6 out of 16 will be homozygous for either the aa or bb alleles and become sandy. One out of 16 will be doubly homozygous and be white. The remaining 9 will contain at least one dominant allele for both genes.

B. 2 genes control this trait. Each dominant allele shows the phenotype, when dominant allele from each gene are present, the offspring is red.

AABB, AbBB. AABb, AaBb result in red.

AAbb, Aabb, aaBB, aaBb result in sandy.

aabb result in white.

F1 progeny: AaBb

F2 progeny: AABB, AABb, AaBb, AaBB, aaBB, AAbb, Aabb, aaBb, aabb