X-linked dominance

X-linked dominance is a mode of genetic inheritance by which a dominant gene is carried on the X chromosome. In genetics, when a trait is said to be "dominant," it means that an individual only needs one copy of a dominant allele to express the trait. Since males have one X and one Y chromosome (XY) and females have two X chromosomes (XX), the way X-linked dominant diseases manifest can differ significantly between the two sexes.

Mechanism

In X-linked dominance, a mutation in a gene on the X chromosome causes a phenotype to be expressed in individuals who have only one copy of the mutation. This is in contrast to X-linked recessive inheritance, where two copies of the mutation (one on each X chromosome) are usually required for a female to express the phenotype, while a single copy will affect males.

For females (XX), having one mutated gene (heterozygous) on one of their X chromosomes is enough to express the dominant trait. However, because females have two X chromosomes, the effect of the mutation may be moderated by the presence of a normal allele on the other X chromosome, depending on the condition.

For males (XY), possessing the mutated gene on their single X chromosome means they will express the trait, as they lack a second X chromosome that could potentially carry a normal allele to counteract the mutation.

Examples

Some examples of X-linked dominant disorders include:

• Incontinentia Pigmenti
• Rett Syndrome
• Fragile X Syndrome (a condition that has a more complex inheritance mechanism, often associated with both dominant and recessive patterns)

Genetic Counseling

Genetic counseling for families affected by X-linked dominant disorders is crucial. It can provide information on the risk of passing the disorder to offspring, discuss the implications of the disorder, and offer support and resources. The inheritance pattern means that:

• Affected men will pass the condition to all of their daughters but none of their sons, as males pass their Y chromosome to their sons and X chromosome to their daughters.
• Affected women have a 50% chance of passing the condition to each child, regardless of the child's sex.

Diagnosis and Management

Diagnosis of X-linked dominant conditions often involves genetic testing, which can identify mutations in the X chromosome. Management and treatment depend on the specific condition but may include medical care to manage symptoms, physical therapy, and supportive services.

See Also

• Genetic disorder
• Chromosome
• Heredity
• Mendelian inheritance