Chapter 11, Problem 32A

Summary Introduction

To explain:

The reason a girl who has Turner’s syndrome has red- green color blindness even though her parents have normal vision.

Introduction:

Nondisjunction in human sex chromosomes causes Turner’s syndrome in females. This condition results from fertilization with a gamete that had no X chromosome. So the person with Turner’s syndrome has only one sex chromosome. Her genotype can be written as XO.

Answer to Problem 32A

A girl who has Turner’s syndrome has only one X chromosome. If that chromosome carries the recessive allele for red- green color blindness, she will also have color blindness. Since her parents were of normal vision, her mother must be a carrier for the trait.

Explanation of Solution

Red- green color blindness is a recessive X- linked trait. A male person will suffer from red- green color blindness if he has the recessive allele on X chromosome. A female will suffer from red- green color blindness only if she is homozygous for the recessive gene. She can be heterozygous for red- green color blindness as there are two X chromosomes in females but will not suffer from the disorder. Instead a heterozygous female will be a carrier for red- green color blindness.

A girl who has Turner’s syndrome has only one X chromosome, which must have the recessive allele for red- green color blindness. Her mother must have been a carrier for color blindness. Hence she suffers from color blindness as well as Turner’s syndrome.