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Sex-Linked Traits. Genetic Counseling. Sometimes it’s a good idea to know the odds . . . . Especially when dealing with sex-linked traits. It’s all about the SEX!. Sex-linked genes are located on the 23 rd chromosome (sex chromosome). Sex chromosomes are X and Y. XY = Boy XX = Girl
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Genetic Counseling Sometimes it’s a good idea to know the odds . . . . Especially when dealing with sex-linked traits.
It’s all about the SEX! • Sex-linked genes are located on the 23rd chromosome (sex chromosome). • Sex chromosomes are X and Y. • XY = Boy • XX = Girl • Sex-linked genes are shown as SUPERSCRIPTS on the X chromosome.
Sex-Linked Traits • Sex-linked traits can be Dominant or Recessive. • A = dominant a = recessive • What would be the genotypes of a male and female that have a Sex-linked Dominant trait and do not express (have) the trait? • Expresses Trait: Male - XA Y Female - XA XA or XA Xa • No Expression: Male - Xa Y Female - Xa Xa • What would be the genotypes of a male and female that have a Sex-linked Recessive trait and do not express the trait? • Expresses Trait: Male - Xa Y Female - Xa Xa • No Expression: Male - XA Y Female - XA XA or XA Xa (Carrier) • Most Sex-linked traits are Recessive!
Sex-Linked Recessive Traits • More males are affected than females. • An affected son can have parents who have the normal phenotype. (XAY, dad) x (XAXa, mom) • For a daughter to have the trait, her father must also have it. Her mother must have it or be a carrier. (XaY, dad) (XaXa or XAXa, mom) • The trait often skips a generation from the grandfather to the grandson. • If a woman has the trait (XaXa), ALL of her sons will be affected. • Pedigrees show only female carriers--no male carriers.
Examples of Sex-Linked Recessive Disorders • Red/Green Colorblindness – Difficulty perceiving differences between colors (red or green, blue or yellow). • Hemophilia – Absence of one or more proteins necessary for normal blood clotting. • Deafness • Cataracts – opacity in the lens that can lead to blindness • Night blindness – (Nyctalopia) rods do not work so that can not see in the dark • Glaucoma – pressure in the eye that can lead to optic nerve damage and blindness • Duchenne Muscular Dystrophy – progressive weakness and degeneration of skeletal muscles that control movement due to absence of dystrophin (protein that maintains muscle integrity). Mainly in boys, onset 3-5 yrs, by 12 years can’t walk, and later needs respirator.
To See or Not To See Color-Blind -- cannot see the numbers or designs in the circles. -- What do you see?
Color-Blind Pedigrees • Draw a pedigree showing a cross between a man with normal vision and a woman who carriers the color-blind trait. • Color-blind is a recessive trait. • Genotypes of Parents: • Male = XR Y Female = XR Xr
XRY XRXr XrY XRXr XRY XRXR Does yours look like this? • Pedigree of color-blind trait. • Genotypes of Parents: • Male = XR Y Female = XR Xr
Can a female be color-blind? • Think about it. • If so, how? Why?
YES—she can! • If dad(XrY) is color-blind and mom (XRXr)is a carrier = 50% chance XrXr • She’s color-blind.
Hemophilia It is a rare blood disorder. The blood does not clot. A hemophiliac will bleed “freely”, while a normal person will eventually stop bleeding because a scab forms. Hemophiliacs require medical intervention to stop the flow of blood. Usually they are given clotting factors, which help them form scabs. Hemophilia is inherited. About 1 out of 5,000 males are born with hemophilia each year.
