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Sex Linkage

Sex Linkage. Here Comes the Sun King. The tale of King Henry VIII and Anne Boleyn. T.H. Morgan’s Flies. Context: 1908 – before anyone knew the link between chromosomes and heredity Working with the fruit fly, Drosophlia melanogaster A model organism for genetics

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Sex Linkage

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  1. Sex Linkage

  2. Here Comes the Sun King • The tale of King Henry VIII and Anne Boleyn

  3. T.H. Morgan’s Flies • Context: 1908 – before anyone knew the link between chromosomes and heredity • Working with the fruit fly, Drosophlia melanogaster • A model organism for genetics • Won the Nobel Prize in 1933 for discovering that chromosomes carried heritable material

  4. One day in the lab… • Was looking at flies, who normally have wild type red eyes, saw a fly with white eyes • Wanted to do a breeding analysis to understand about white eyes • Did a test cross

  5. Test Cross • A test cross is used to determine the genotype of an individual exhibiting a dominant trait • Determine if this individual is homozygous or heterozygous (HH or Hh) • How? Cross a dominant phenotypeindividual to one with the recessive phenotype. • By observing the resulting phenotypes, you can figure out the genotype

  6. So he crossed them Maybe white eyes are lethal in females?

  7. So he crossed them I guess not….but what does this have to do with sex?

  8. Morgan’s Results • The appearance of white eyes in females shows that this trait is not lethal in females. • All possible combinations of white eyes and sex are possible. • The white-eye trait can be carried over to females when F1 females are crossed with white-eyed males.

  9. Did it have to do with chromosomes? • The male and females seemed to have slightly different chromosomes • Morgan found that the gene for white eyes seemed to follow the inheritance of sex • From these and other crosses, he was able to figure out that genes were carried on chromosomes!

  10. Recall: Chromosome Structure 10 • Females have two X chromosomes, (XX) • Males have one X and one Y Chromosome (XY) • Each human cell has 46 chromosomes • 23 pairs in total • 22 are autosomes (not sex chromosomes) • 1 pair of sex chromosomes • Each pair is homologous(similar but not the same)

  11. X and Y on a Karyotype • A Karyotype is an arrangement of the chromosomes of an individual that has been sorted according to size and type

  12. 1stLaw: Law of Segregation • Mendel’s law of segregation states that every individual possesses a pair of alleles and passes a randomly selected copy (one or the other) to its offspring.

  13. The same happens for sex chromosomes

  14. Sex Linked Inheritance • Autosomal inheritance: inheritance of alleles located on autosomal (non-sex) chromosomes • (This is all the inheritance we have dealt with up until now) • Sex-linked: describes an allele that is found on one of the sex chromosomes (X or Y) • Aa, CDCdXHXh XHY

  15. Hemizygous • Just like before, females can be homozygous or heterozygous for a trait • XHXh or XHXH • Males are called hemizygous because they are neither heterozygous nor homozygous. They only have one possibility! • XHY

  16. Sex Linked Problems • For example, hemophilia A is a blood disease where it takes a long time for the blood to clot. The gene for hemophilia is located on the X chromosome and is recessive. • If a woman carrying the hemophilia allele marries a man who does not have hemophilia, what are the odds their children will have hemophilia?

  17. How does it work • Let XH represent the normal allele • Let Xh represent the allele for hemophilia • (Y is the Y chromosome)

  18. Results • 50% of the males are affected • 0% of the females are affected, although one is a carrier

  19. Carrier • Carrier: someone who does not have the phenotype of a condition but has the allele for the condition. • This usually applies to recessive genes • For sex-linked genes, only FEMALES can be carriers.

  20. Sex-Linked Genes • Male Pattern Baldness • Located on the X chromosome • Recessive • If you are male and your mothers father had it, you will get it. It is rare in females. • Why?

  21. Male Pattern Baldness - P • Let XB represent the normal hair allele, and Xb represent the baldness allele • P - Normal Female x Bald Male XBXB x XbY Both sons are normal, both daughters are carriers!

  22. Male Pattern Baldness = F1 • F1- Carrier Female x Normal Male XBXb x XBY 100% of females are normal, ½ of sons are normal, ½ of sons are affected Altogether, ¼ of children are affected

  23. What about a bald female? It could happen, but you’d need Bald or Carrier Female x Bald Male XbXborXBXb x XbY There are also Y-linked diseases • Obviously, only males can get it. • If your dad has it, you will get it • Less common because the Y chromosome is smaller and has less genes

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