SEX-LINKED INHERITANCE

What are Sex-Linked Traits? Traits controlled by genes located on the sex chromosomes. Most of these traits are X-linked – the Y chromosome is tiny, so there aren’t many genes on it. The symbol for the allele is attached to an X or Y.

Males are more Affected by Recessive X-Linked Traits This is because males only have one X chromosome There’s no other allele to dominate over the recessive allele. D is expressed d is expressed

Examples Red-Green Colour Vision Deficiency Hemophelia X-Linked Recessive Hemophelia Inability for blood to clot Duchenne Muscular Distrophy X-linked Recessive Causes muscle degeneration Fragile X Syndrome X-linked Dominant Can cause mental and social disorders Rett Syndrome Neurological Disorder

Example Problem 1: Red-green colour vision deficiency is an X-linked recessive disorder. A male with normal vision marries a heterozygous female. What is the chance the couple’s first child will be a male with normal vision?

Example Problem 2: Fragile X Syndrome is a dominant X-linked disorder Example Problem 2: Fragile X Syndrome is a dominant X-linked disorder. If a heterozygous female has a child with an affected male, what is the chance of the child having Fragile X Syndrome?

Example Problem 3: Hemophilia is a recessive X-liked disorder Example Problem 3: Hemophilia is a recessive X-liked disorder. A normal woman who’s father had hemophilia marries a normal man. What is the chance of the couple having a hemophiliac child?