Sex-linked Disorders

Many inherited disorders result from mutations in a single gene.

When the mutated genes are located on the sex chromosomes, the resulting conditions are called sex-linked disorders.

Sex-linked disorders are grouped into three types: Y-linked, X-linked dominant, and X-linked recessive.

Y-linked disorders, such as hypertrichosis of the ear, are caused by mutations in genes on the Y chromosome.

Since only males have a Y chromosome, these traits pass directly from father to son.

An affected father passes the trait to all of his sons, while his daughters are not affected.

X-linked dominant disorders, such as fragile X syndrome, can result when one altered copy of a gene on the X chromosome is enough to affect the phenotype.

An affected mother has a 50% chance of passing the mutation to each child, regardless of sex.

An affected father passes the mutation to all of his daughters and none of his sons.

Mutations in genes on the X chromosome can also cause X-linked recessive disorders.

Males are affected more often because they have only one X chromosome, while females need mutations in both copies of the gene to be affected.

Carrier mothers, who have one copy of a mutated gene but often do not show symptoms, have a 50% chance of passing the mutation to each child.

Each son has a 50% chance of being affected, while each daughter has a 50% chance of being a carrier.

An affected father does not pass the disorder to his sons, but all of his daughters inherit the mutation and become carriers.

For example, Duchenne muscular dystrophy, or DMD, is an X-linked recessive disorder that primarily affects males and is caused by mutations in the dystrophin gene.

These mutations disrupt the production of the dystrophin protein in muscle fibers, leading to progressive weakness and muscle degeneration.