Part 2: Pedigrees

Question

A genetic pedigree diagram for an autosomal trait. Tom does not show the recessive trait because he has brown eyes but his wife, Mary, does show the autosomal recessive trait (blue eyes). One of their male children exhibits the recessive trait (ee). (Note Tom (E? - brown eyes) Mary (ee- blue eyes) they have one child that shows the recessive trait of blue eyes(ee)

1. What are the genotypes of the following individuals in the pedigree?

A) Tom

B) Mary

C) Mary's Mother

1. Tom and Mary are expecting their second child. What are the chances that their child will have blue eyes? ( Show all work, i.e. Punnett Square)

Answer 1

Tom must be heterozygous for eye color. Ee

Mary must be homozygous recessive for eye color. ee

With the given information we can't tell if Mary's mother is homozygous recessive (ee) or heterozygous (Ee) but at least we know that she is not homozygous dominant (EE). (Because her daughter has the recessive allele).

The punnet square for Tom and Mary's offspring would look like this:

E e

e Ee ee

e Ee ee

Only two phenotypes at 50% each: 1/2 brown eyed and 1/2 blue eyed.

The second child still has a 50% chance of either eye color despite the outcome of the first child, that is just how probability works!