Sex Link Traits: Genes on the X and Y chromosomes Copy what is in RED

Lets recall how we determine sex… Sex Chromosomes: X and Y Female: XX Male: XY The sperm determines the sex because it gives an X or Y to the gametes. The egg only gives an X to the gamete The X chromosome is Bigger

What are sex-linked genes?  genes found on a sex chromosome X-linked genes are genes found on the X chromosome, symbolized by Xr, XR. Y-linked genes are found on the Y chromosome, (we won’t look at any of these) Thomas Morgan experimented with the eye colour of fruit flies (Drosophilia) to determine X- linkage

Y-linked Genes Y-linked genes are uncommon because the Y chromosome is so small and does not contain many genes, Y-linked diseases are rare. In humans, only males have a Y chromosome so traits / diseases are passed only from father to son

X-linked Genes X-linked traits can be dominant or recessive XD, Xd If the trait is DOMINANT only one copy is needed for the trait to be expressed in either females or males: XDX, XDY If the trait is RECESSIVE two copies are needed for the trait to be expressed in females XdXd. But only one copy is needed in males: XdY

X-linked Genes X-linked recessive traits will show up more often in males because they have only one X chromosome Example 1: female carrier of an x-linked gene and a normal male What will the genotypes be from the following cross? XXr and XY

Set up the Punnett Square X Xr Y

Here’s the results X Xr X X X Xr Y X Y Xr Y For girls: 0% have the trait For boys 50% have it. Probability is higher for boys because whatever X they get determines the trait, for girls they have to get 2 recessive X’s. X Xr X X X Xr Y X Y Xr Y

Examples of X-linked traits in humans: 1. Red/Green Colour Blindness 2. Hemophilia blood disease 3. Muscular Dystrophy 4. Icthyosis simplex (scaly skin)

Colorblindness A person with normal colour vision sees a number seven in the circle above. Those who are colour blind usually do not see any number at all.

Colorblindness RED-GREEN COLORBLINDNESS: People with red-green colour blindness see either a three or nothing at all. Those with normal colour vision see an 8.

Hemophilia Hemophilia- Lacking in the ability to clot blood There is a gene on the “X” chromosome that control blood clotting People who have hemophilia are missing the protein to clot blood They can bleed to death by minor cut.

Muscular Dystrophy - Results in weakening/loss of muscles - Caused by defective version of gene that codes for muscle

X-linked Genes Example 2: an unaffected male marries an affected female What will the genotypes be from the following cross? XrXr and XY

Set up the Punnett Square Xr X Y

Here’s the results Xr X X Xr Y Xr Y For girls: 0% have the trait but 100% are carriers For boys 100% have it. Xr X X Xr Y Xr Y

Phenotypes of Children 100% girls normal fingers 100% boys with webbed fingers

Practice http://www.ksu.edu/biology/pob/genetics/xl inked.htm http://www.biology.arizona.edu/mendelian _genetics/problem_sets/sex_linked_inherit ance/sex_linked_inheritance.html 18