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Download day 11 sex linked traits
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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 Xlinkage 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 X Y r X Here’s the results • 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 Y X r X XX X Xr XY r XY 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 r X X Y r X Here’s the results • For girls: 0% have the trait but 100% are carriers • For boys 100% have it. r X r X X X Xr X Xr Y r XY r XY 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.zerobio.com/drag_gr11/sexlink. htm • http://www.biology.arizona.edu/mendelian _genetics/problem_sets/sex_linked_inherit ance/sex_linked_inheritance.html 18