Sex-Linked Disorders: Hemophilia and Color Blindness

10 Questions

What term is used to describe females who carry one abnormal X chromosome and one normal X chromosome?

Carriers

What is the probability that sons of a mother carrying a defective gene and a father without the gene will inherit the affected gene?

50%

Why are males who carry X-linked genes described as hemizygous?

Because they typically only possess one copy of the respective gene

In X-linked disorders, if both parents carry the affected allele, what happens to the probability of having an affected child?

It increases

What is the risk for daughters of a mother carrying a defective gene and a father without the gene?

50% chance of inheriting the normal gene

What is the primary characteristic of hemophilia?

Difficulty in blood clotting

How do carrier females typically experience X-linked recessive disorders?

They experience mild symptoms due to skewed X-inactivation

Which sex is more commonly affected by X-linked recessive color blindness?

Males

What is the approximate prevalence of hemophilia A among live male births worldwide?

1 in 4,500

What is the primary factor responsible for the development of color blindness?

A defective gene for photopigment formation

Study Notes

Sex-Linked Disorders

Introduction

Sex-linked disorders are a group of genetic conditions that are carried on the X chromosome. In humans, females have two X chromosomes, while males have one X and one Y chromosome. As a result, the sex of an individual plays a significant role in determining whether they will develop these disorders.

Hemophilia

Hemophilia is a classic X-linked recessive disorder. It is characterized by a deficiency in one of the clotting factors, specifically factor VIII or factor IX. This leads to impaired blood clotting and increased risk of bleeds. Hemophilia A affects approximately 1 in 4,500 live male births worldwide, and treatment typically involves administering the missing clotting factor.

Color Blindness

X-linked recessive color blindness is another well-known sex-linked disorder. It is caused by a defective gene responsible for the production of a protein critical to the photopigment formation necessary for red and green color perception. Approximately 10% of men are affected by this disorder, while only 1% of women will experience it due to X-linked inheritance patterns.

X-Linked Recessive Disorders

X-linked recessive disorders are rare in females, but a carrier female could potentially exhibit mild symptoms due to skewed X-inactivation, which means that one copy of the X chromosome might be more active than the other copy.

Genetic Inheritance Patterns

Sex-linked disorders follow a particular pattern of inheritance, where the risk of passing down a mutated gene depends on the sexes involved. For instance, in pregnancies where a mother carries the defective gene and the father does not, all daughters would have a 50% chance of inheriting the normal gene, ensuring they remain unaffected. On the other hand, 50% of sons would receive the affected gene, leading to hemophilia or another X-linked disorder. If both parents carry the affected allele, the probability of having an affected child increases, regardless of gender.

Hemizygosity

Males who carry X-linked genes are described as being hemizygous because they typically only possess one copy of the respective gene. Consequently, they are more susceptible to expressing recessive traits linked to those genes, since there is no masking effect from another copy. Females carrying one abnormal X chromosome and one normal X chromosome are referred to as carriers, as they do not show symptoms but can pass on the defective gene to their offspring.