Sex linked genes

Human Chromosomes 46 total 23 pairs 22 pairs autosomal 1 pair sex chromosomes

Sex determination Sex chromosomes – determines the sex of an individual Y X Males have X and Y Two kinds of gametes Female have two X’s Only one type of gamete

Sex inherited traits Males only have one copy of the X chromosome (hemizygous), plus a Y Female have two X chromosomes, and may be homozygous or heterozygous for a trait X-linked genes are never passed from father to son. The Y chromosome is the only sex chromosome that passes from father to son.

Females with one copy of the normal gene and one copy of the mutated gene are called carriers. They don’t show the trait. Males are never carriers – if they have a mutated gene on the X chromosome, it will be expressed

X-linked disorders Recessive Dominant Red-green color blindness Hemophilia Duchenne muscular dystrophy Ichthyosis Dominant Hypertrichosis

Thomas Hunt Morgan was awarded the Nobel Prize in Physiology or Medicine in 1933. The work for which the prize was awarded was completed over a 17-year period at Columbia University, commencing in 1910 with his discovery of the white-eyed mutation in the fruit fly, Drosophila.

Morgan’s work with Drosophila demonstrated that genes for certain traits are located on the X chromosomes. These genes do not appear to have corresponding alleles on the Y chromosome. Since many sex-linked genes are recessive, they are expressed more frequently in males than in females.

XX x X’Y where X’=allele for white eye

Problem- Try this the key is on the next page In a cross between a white-eyed female fruit fly and red-eyed male, what percent of the female offspring will have white eyes? (White eyes are X-linked, recessive)

Sex linked cross In a cross between a white-eyed female fruit fly and red-eyed male, what percent of the female offspring will have white eyes? (White eyes are X-linked, recessive)

WHAT DO YOU SEE?

Queen Victoria Queen Victoria of England was a carrier of hemophilia and passed The disease to many of her descendants (including the Russian emperor’s family and the Spanish royal family).

The history of Queen Victoria's descendants illustrates the hereditary characteristics of hemophilia. We can take a look at her family tree(pedigree).

Explanation of the inheritance of hemophilia

Hemophilia in humans is due to an X-chromosome mutation Hemophilia in humans is due to an X-chromosome mutation. What will be the results of mating between a normal (non-carrier) female and a hemophilic male? All sons are normal and daughters are carrieers.

Co dominance Blood Typing There are three versions (called “alleles”) of this blood type gene: A, B, and O. The “A” allele is dominant and so is the “B” allele. Together though, the “A” and “B” alleles are co- dominant. The “O” allele is recessive.

Determining the Genotype The blood type gene has three different alleles: IAIA is Type A IAIB is Type AB IBIB results in Type B ii is Type O

Determining the Genotype Scenario: Mom has the alleles IAIA for blood type and Dad has the alleles IBIB blood type. What will be the blood type for their child?

Punnett Sqaures IA IB