Question

A man (Bob) is heterozygous for Aa Bb Cc, where the genes show independent assortment: He does not have a known phenotype, and neither does their mother; but their father does. (A third eye). We know it has to be associated with one of the three given loci. He has 3 three children (1 boy and 2 girls) with a woman who is also phenotypically normal, of which one girl has a third eye. Based on this information, what type of inheritance could this be? Draw a pedigree of this family. Pick the A locus as the one that gives rise to a third eye, and give the genotypes of all the people involved for the A locus as part of the pedigree. How many different gametes can Bob generate, and list them all out. Based on your knowledge of mendelian genetics, what are the odds that any child of Bob and his partner will have a third eye? What are the odds that a child is born that is both female and has a third eye?

Answer 1

The pedigree for the family can be found as below:

This pedigree clearly shows that unaffected parents had an unaffected son, who had only one affected daugter with a normal woman. Since not all the males are affected, the disease is not Y-linked in nature. On the other hand, the disease seems to be X-linked recessively inherited in nature since a female would require two affected chromosomes, as in the daughter. Further, the father might have inherited a defected allele from the father, and another from the unaffected or carrier mother. Similarly, the daughter of this man must have inherited one copy each from his affected father and normal/carrier mother.

This suggests that whereas Bob is homozygous, his wife is heterozygous for this trait. The Punnett square can be formulated as below:

	Xa	Y
Xa	XaXa	XaY
X	XXa	XY

Thus, genotype: XaXa: XaY : XXa : XY :: 1:1:1:1

Phenotype: Affected females : affected males : normal female : normal male :: 1:1:1:1

Thus, the odds that any child of Bob and his partner will have a third eye are 1/4+1/4 or 1/2.

The odds that a child is born that is both female and has a third eye are only 1/4 as shown in the Punnett square and the phenotypic ratio.