Question

The shape of red blood cells is governed by a codominant allele. Normal red blood cells are described as being round or donut shaped. All of the red blood cells in individuals with two copies of the normal allele (NN) are round. The mutated form of this gene causes red blood cells to be sickle–shaped. These thin, elongated red blood cells can get stuck in blood vessels and block blood flow and oxygen from parts of the body. All of the red blood cells in individuals with two copies of the mutated allele (SS) are sickle–shaped. People who inherit one normal allele and one mutated allele (NS) have some round red blood cells and some sickle-shaped blood cells.

Sickle Cell Genotypes and Phenotypes

N N, S S, N S.

What are the chances of a child having both round and sickle-shaped red blood cells if one parent has all sickle–shaped cells and the other parent has both round and sickle-shaped red blood cells?

A
0%
B
50%
C
75%
D
100%

Answer 1

Cross SS × NS. The SS parent can only give S; the NS parent gives N or S. Offspring: 50% NS (both cell types) and 50% SS (all sickle). So the chance is 50% — answer B.