X-linked dominance

X-linked dominance is a mode of inheritance in which a dominant allele on the X chromosome is sufficient to cause a particular trait or disorder, regardless of whether the individual is male or female. This means that the presence of just one copy of the dominant allele on one of the X chromosomes is enough to express the trait or disorder, even if the other X chromosome carries a different allele.

Key Points about X-linked Dominance:

1. Expression in Males and Females:

   • In males: Males have one X chromosome and one Y chromosome (XY), so they will express X-linked dominant traits if they inherit the dominant allele on their X chromosome.
   • In females: Females have two X chromosomes (XX). If a female inherits one copy of the dominant allele, she will express the trait. However, if she inherits a recessive allele on the other X chromosome, the dominant allele will mask the expression of the recessive allele, resulting in dominance.

2. Vertical Transmission:

   X-linked dominant traits are often transmitted vertically through multiple generations, as affected individuals (both males and females) have a 50% chance of passing on the dominant allele to each of their offspring.

3. Manifestation in Heterozygous Females:

   Heterozygous females (carriers) who inherit one copy of the dominant allele may exhibit variable expressivity of the trait, depending on factors such as X inactivation and genetic modifiers.

4. No Male-to-Male Transmission:

   There is no male-to-male transmission of X-linked traits because males pass their Y chromosome to their sons, not their X chromosome.

Examples of X-linked Dominant Disorders:

1. Rett Syndrome:

   A neurodevelopmental disorder primarily affecting females, caused by mutations in the MECP2 gene located on the X chromosome.

2. Incontinentia Pigmenti:

   A rare genetic disorder characterized by skin abnormalities, dental abnormalities, hair loss, and eye abnormalities, caused by mutations in the IKBKG gene (also known as NEMO) located on the X chromosome.

3. Fragile X Syndrome (FXS):

   While typically associated with X-linked recessive inheritance, in rare cases, FXS can exhibit X-linked dominant inheritance patterns when there are premutation alleles with expanded CGG repeats in the FMR1 gene.

Genetic Counseling:

• Understanding the inheritance pattern of X-linked dominant traits is important for genetic counseling, especially when evaluating the risk of recurrence in families with affected individuals.
• Females who are carriers of X-linked dominant disorders have a 50% chance of passing the dominant allele to each of their offspring, regardless of the child's sex.

X-linked dominance is relatively rare compared to X-linked recessive inheritance patterns but is nonetheless important to consider in genetic counseling and understanding the inheritance of certain traits and disorders.