Sex Linkage

1. Sex Linkage

2. X –Linked Traits Definitions: • Autosomes - Chromosomes #1 – 22 • Sex chromosomes - #23 • In males, the 23rd pair of chromosomes is NOT homologous • Females have a homologous 23rd pair. • Traits located on the 23rd pair are sex-linked or X-linked. • Males: XY Females: XX

3. X –Linked Traits • Genes for sex-linked (X-linked) traits are carried on the X chromosome but not on the Y chromosome • Therefore in a male, the gene on the X chromosome is expressed whether it is dominant or recessive • In a female, she must have two recessive alleles to have the recessive phenotype

4. Thomas H. Morgan:Gender and Inheritance • Breakthrough work done by Thomas Hunt Morgan – American Geneticist • 1908 – Work on fruit flies (Drosophila melanogaster)provided deeper understanding of heredity based on gender

5. Drosophila melanogaster Ideal for genetic experiments for several reasons: • Rapid life cycle (10-15 days) • Hundreds of eggs at one time • Crossing experiments can be repeated many times • Offspring mature quickly • Males/females different • Small • Markedly contrasting traits • Only 8 chromosomes (4 pairs)

7. Female Male How are they different??

8. Male or Female? Male (XY) Non Homologous Chromosomes

9. Chromosomes in Fruit Flies • Morgan found the following: • Females  4 homologous pairs of chromosomes  3 autosomal pairs, one sex pair (XX) • Males 3 homologous pairs: • 3 autosomal pairs • Sex chromosomes were not homologous • one X and one Y (XY)

10. Solving X-Linked Problems: Eye Color In Drosophila • Let R = red, r = white • Assume these alleles are found on the X chromosome only (they are sex-linked) • Cross 1. Cross a red eyed homozygous female with a white eyed male. r R R Male: X Y Female: X X Now, put these gametes into a Punnett Square...

11. First Cross XR XR XRXr XRXr Xr XRY XRY Y ALL RED-EYED OFFSPRING

12. Second Generation (F1 Cross) XR Xr • 3 RED OFFSPRING • - 1 WHITE MALE XRXR XRXr XR What is the probability of these flies having a red-eyed daughter? XRY XrY Y 0.50 or 50 % or ½

13. Solving X-Linked Problems: Eye Color In Drosophila • Let R = red, r = white • Assume these alleles are found on the X chromosome only (they are sex-linked) • Cross 2. Cross a red eyed heterozygous female with a white eyed male. r R r Male: X Y Female: X X Now, put these gametes into a Punnett Square...

14. Cross 2 XR Xr What is the probability of these flies having a red-eyed son? XR Xr Xr Xr Xr 0.25 or 25 % or ¼ XRY XrY Y 1 Red female, 1 Red male, 1 White female, 1 White male

15. Female Male XR XR Xr Y XR Xr XRY Xr Xr Morgan’s Conclusions • Genes controlling eye color must be on X Chromosome • Traits coded for by genes on sex chromosomes are called SEX-LINKED TRAITS • Y does not carry gene for eye color • Males cannot be heterozygous for sex linked traits Traits controlled by genes carried on Y chromosome only affect males

16. Examples of Sex-Linkage in Humans • Color Blindness • genes for visual pigments responsible for perception of red and green are found on the X chromosome • Hemophilia A • caused by the lack of a blood protein, called Factor VIII, that is critical for blood clotting • Gene for Factor VIII on X Chromosome Both known as X-linked recessive

17. Female Genotypes XCXC XCXc XcXc 3 possible genotypes 2 phenotypes Male Genotypes XCY XcY only 2 possible phenotypes 2 possible genotypes Color BlindnessCaused by gene carried on X only

18. Link to Ishihara Test for Color Blindness • http://www.toledo-bend.com/colorblind/ishihara.html

19. Sex Limited Eg. Milk production in cows is controlled by an autosomal gene Bulls carry the genes...but they do not make milk!!!!!! The gene is not expressed for bulls (because they have low levels of female hormones which switch on this gene) Sex Influenced Eg. Male only needs one allele for baldness – male hormones stimulate the expression of this allele females need two recessive alleles to be bald Gene is dominant in males and recessive in females more common in males Other Gender Related Inherited Traits