2. SEX-LINKED TRAITS

3. Genetic Counseling Sometimes it’s a good idea to know the odds.... Especially when dealing with sex-linked traits.

4. 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 Sex-linked genes are shown as SUPERSCRIPTS on the X chromosome.

5. 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 - X A Y Female - X A X A or X A X a No Expression: Male - X a Y Female - X a X a 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 - X a Y Female - X a X a No Expression: Male - X A Y Female - X A X A or X A X a (Carrier) Most Sex-linked traits are Recessive!

6. Sex-Linked Recessive Traits More males are affected than females. An affected son can have parents who have the normal phenotype. (X A Y, dad) x (X A X a, mom) For a daughter to have the trait, her father must also have it. Her mother must have it or be a carrier. (X a Y, dad) (X a X a or X A X a, mom) The trait often skips a generation from the grandfather to the grandson. If a woman has the trait (X a X a ), ALL of her sons will be affected. Pedigrees show only female carriers--no male carriers.

7. 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 Duchene 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.

8. Karyotype: a picture of chromosomes.

9. Body Cells vs. Sex Cells Autosomes: the first 22 homologous pairs of chromosomes. Autosomes are the same for both males and females. Sex cells: Determine sex of offspring

10. Is it Male or Female? Explain

11. Sex chromosomes: determines the sex of the individual. The sex chromosomes are the 23rd pair of chromosomes. XX =female

12. XY=male

13. Which parent determines the sex of an offspring? DAD Why? All moms have the genotype XX. When egg cells are made, they will all carry a single X chromosome.

14. All dads have the genotype XY. When sperm cells are made, 50% will have an X chromosome and 50% will have a Y chromosome. Therefore, males and females are born in roughly a 50:50 ratio.

15. SEX-LINKED TRAITS Those traits that are controlled by genes on the X or Y chromosomes. NOTE: The Y chromosome is much smaller than the X chromosome and only contains a few genes. Most sex-linked traits are on the X chromosome.

16. Genetic Sex-Linked Disorders 2- Hemophilia A and B Recessive disorders that affect 1 of 5,000 males. These interfere with normal blood clotting and occur on the X chromosome.

17. In humans, hemophilia is a sex-linked trait. Having hemophilia is recessive (X h ) to being normal (X H ). The heterozygous female is called a carrier. Cross a carrier female with a normal male. male __ X H X h __ X __ X H Y ___ X H X H Y X h Y X H X h XHXH Y XHXH XhXh

18. Results: Genotypic ratio: 1 X H X H :1 X H X h : 1 X H Y :1X h Y X H X H Normal Female X H Y Normal Male X h Y Male w/ hemophilia X H X h Normal Female XHXH Y XHXH XhXh Phenotypic ratio: 2 normal females: 1normal male: 1 male with hemophilia

19. Now You Try Cross a carrier female with a male with hemophilia. X H X h X X h Y

20. Results: Genotypic ratio: 1 X H X h :1 X h X h : 1 X H Y :1 X h Y X H X H Normal Female X H Y Normal Male X h Y Male w/ hemophilia X h Female w/ hemophilia XHXH Y XHXH XhXh Phenotypic ratio: 1 normal female: 1 female w/ Hemopholia: 1normal male: 1 male with hemophilia

21. 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.

22. Pedigree Royale: Hemophilia

23. Genetic Sex-Linked Disorders Color Blindness Cause: x-linked recessive 1/10 males have, 1/100 females have. Why the difference? Individuals are unable to distinguish shades of red-green.

24. 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 = X R Y Female = X R X r

25. Does yours look like this? Pedigree of color-blind trait. Genotypes of Parents: Male = X R Y Female = X R X r XRYXRYXRXrXRXr XRYXRY XrYXrY XRXRXRXR XRXrXRXr

26. Can a female be color-blind? Think about it. If so, how? Why?

27. YES—she can! If dad (X r Y) is color-blind and mom (X R X r ) is a carrier = 50% chance X r X r She’s color- blind.

28. Polygenic Inheritance Traits that are influenced by several Genes. Show many degrees of variation. Examples: Height Hair color Skin Color Eye Color