X-Linked Inheritance

X-linked inheritance is marked by family pedigrees in which most, if not all, individuals who are affected with the trait or condition in question are males. Mothers of the affected males are heterozygotes or carriers, and the sisters of affected males may be either heterozygotes or not carry the gene at all. The basis for X-linked inheritance is that females have two X chromosomes and males have only one X chromosome.

As illustrated in this figure....

Pedigree Illustrating X-Linked Recessive Inheritance

• affected males are hemizygous (that is, their one X chromosome has the mutant allele [gene]); affected females are usually homozygous (that is, both X chromosomes have the mutant allele).
• affected males transmit the gene to all daughters, but not to any of their sons.
• the daughters of an affected male will usually be a carrier (heterozygote) and thus not show the trait.
• sons of heterozygous females have a 50% (1 in 2) chance of receiving the gene and thus expressing the trait or condition.

Occasionally, heterozygous females will express the trait or condition in question; this is because, despite the presence of two X chromosomes in a female's cells, only one of the X chromosomes is "active." Which X chromosome in any given cell is active or inactive is thought to be randomly determined very early in embryonic development. So occasionally, and by chance, a disproportionate number of X chromosomes carrying the mutant gene remain in the "active" state, and thus the mutant phenotype gene is visible in its effects.

Of lesser consequence....

X-Linked Dominant Inheritance

X-linked dominant inheritance is very rare. It is marked by pedigrees in which:

• all the daughters of an affected male are themselves affected;
• none of the sons of an affected male is affected;
• a heterozygous (carrier) female is affected and transmits the trait to half of her children, with males and females being equally affected; 
• twice as many females as males will be affected, in general.

Y-Linked Inheritance

A small number of genes have been mapped to the Y chromosome, and others have recently been discovered. Genes on the Y chromosome pass from father to sons only; no daughters of affected males can inherit the gene, since none of their cells has a Y chromosome.