1. How is our sex determined?

2. Pre-assessment Questions What are the sex chromosomes? What does it mean when a trait is sex- linked? What are some examples of sex-linked traits or genes?

3. Punnett Squares for Sex Determination In females both chromosomes are known as X chromosomes while in males one chromosome is X while the other is Y.

4. The Y chromosome The y chromosome is known as a degenerate chromosome because it does not have many genes on it ◦ Only has genes related male anatomy Unlike the X chromosome which has many

5. Sex-linked Inheritance Sex linked inheritance occurs when certain genes are located on the X or Y chromosome (almost always the X). This means that some traits will come directly from one parent or another, or even from one grandparent or another.

6. Punnet Square for Sex-linked Inheritance When illustrating sex linked inheritance we always use the X and Y letters. If we are talking about gene “A” on the X chromosome we will illustrated this way: X A or X a. Why doesn’t the Y have any allele on it?

7. Sex-Linked Inheritance Imagine if the dominant allele “A” means normal vision and “a” means colour blind. Pretend the mother is heterozygous and the father has normal vision What percentage of males will be colour blind? How about females?

8. Sex linked diseases E.g. Haemophilia

9. Sex linked disease Muscular Dystrophy

10. Sex linked disease Fragile-X syndrome

11. Sex linked trait Red-green colour blindness

12. Sex linked diseases While there are diseases that exist on the Y chromosome we will only look at diseases on the X chromosome X-linked diseases can come in two forms: 1. Dominant 2. Recessive

13. Dominant X linked disease Some X-linked diseases are dominant. This means they will be expressed regardless of whether the other X chromosome contains the disease. e.g. Disease = X D X D or X D X d or X D Y (males)

14. Recessive X-linked disease Other X-linked diseases are recessive. This means they will only be expressed if the individual has both alleles with the recessive diseased allele. e.g. Disease = X d X d or X d Y (males)

15. Carriers of the disease Carriers of the disease are those individuals that have the diseased allele within them but do not have expression of the disease because the allele is recessive. Example of X-linked carriers: - X D X d (where X d is the recessive diseased allele)

16. Pedigree Charts Genetic researchers use pedigree charts to track traits from parents to children ◦ Pedigree charts are similar to family trees

17. Pedigree Charts

18. X-Linked Recessive Disease Example Assume d = diseased and D = normal What is the genotypes for the F1 generation? F1 F2 P

19. Practice: X-linked Dominant Here is an example of a pedigree chart for an X- linked dominant disease. Let us pretend that the X- linked dominant disease is Fragile X syndrome X F Y (where F is fragile X syndrome dominant allele) Solve the genotypes for the II generation

20. Homework #13, 15-18 (p.258) #10 – 13 (p.259)