Sex Linked Inheritance

There is one pair of homologous chromosomes in animals that does not have exactly the same genes. These two chromosomes, the X and Y chromosomes, are called the sex chromosomes. All other chromosomes are called autosomes.

Sex-linked (or X-linked) genes are genes that reside on the X, or sex, chromosome. Y-linked genes are also possible, but since so few genes reside on the Y chromosome, Y-linkage is rarely encountered. In biology, the meaning of “linkage” in “sex-linkage” is different from its meaning when used alone. Used alone, the word linkage refers to two or more genes that reside on the same chromosome; sex-linkage refers to a single gene residing specifically on a sex chromosome.

There are additional considerations when working with sex-linked genes because when females (XX) inherit a sex-linked gene, they receive two copies of the gene, one on each X chromosome.

This situation is similar to that for autosomal inheritance. In contrast, however, a male (XY) will inherit only one copy of the gene because only the X chromosome delivers the gene. There is no similar gene delivered by the Y chromosome. As a result, whichever allele is on the X chromosome of a male, regardless of whether it is dominant or recessive, is the allele whose trait expressed. Hemophilia is caused by a sex-linked, recessive gene (h) in humans. Hemophiliacs cannot properly form blood clots and in the worst cases can die from minor injuries by bleeding to death. Females and males who inherit the normal allele (H) are XHXH and XHY, respectively, and are both normal. In order for a female to be a hemophiliac, she must have two copies of the defective allele (XhXh). A male, however, need inherit only one copy of the defective allele (XhY) to be a hemophiliac. As a result, hemophilia, as well as other sex-linked genetic defects, are much more common in males. Females who are XHXh have normal clotting abilities but are said to be carriers, since they can pass the defective allele to their offspring.