Course: Science Subtest II
Lesson: Genetics and Evolution
Topic: Inheritance of Traits      Page 15 of 18  

Linked Genes and Sex Linked Inheritance?

Linked genes are pairs of genes that are inherited together. Linked genes are usually on the same chromosome.

Sex Linked Inheritance is the inheritance of alleles that determine gender. All organisms that reproduce sexually have sex chromosomes. In humans and other mammals, the sex chromosomes are X and Y. Females have two X chromosomes. Males have one X and one Y chromosome.

In humans, most sex-linked genes are found on the X chromosome (called X-linked characteristics).Y-linked disorders are less common than X-linked disorders. Only males inherit Y-linked disorders.

A male's Y chromosome lacks the genes needed to block traits on X chromosomes. This explains why far more males than females inherit X-linked recessives disorders such as hemophilia. Since hemophilia is a recessive trait, femalesdo not express a recessive hemophilia trait unless it occurs in both X chromosomes.

A woman carries a recessive gene on one of the X chromosomes without expressing the trait. Yet she can pass the gene on to a son, who will express the trait. The male does not have the gene and cannot pass it on. Yet half of his sons will inherit and express the recessive trait.

A male usually expresses the traits on his X chromosome, because his Y chromosome does not have the X-chromosome traits he would need to block a recessive X-chromosome trait.