1. Concept 3: Applying the chromosomal basis of inheritance to analyze the effects of sex chromosomes, linked genes, and recombined recombines genes
Campbell: Chapter 15

2. Learning Intentions You must know:
Goal: to analyze mechanisms of chromosomal inheritance
You must know:
How the chromosome theory of inheritance connects the physical movement of chromosomes in meiosis to Mendel’s laws of inheritance
The unique pattern of inheritance in sex-linked and linked genes

3. Remember...Chromosome Theory of Inheritance
What is the relationship between a pair of alleles and a homologous pair of chromosomes?

4. Remember...Chromosome Theory of Inheritance
Alleles are located on chromosomes.
The homologous pair of chromosomes code for the same genes, each having one allele.
Homologous chromosomes

5. Remember...Chromosome Theory of Inheritance
Since genes have specific locations on chromosomes (loci), when chromosomes segregate and assort independently so do the genes!

6. Sex-Linked Genes
A sex linked gene is a gene located on the X sex chromosome.
Would this individual be a male or a female? How do you know?
Answer: Male. Males are XY, females are XX

7. Morgan’s Sex-Linked Discovery in 1910!

8. He got a Nobel Prize... He’s kind of a big deal.
Thomas Hunt Morgan The Fly Room!

9. Conclusion?

10. The gene for fly eye colour is found on the “X” sex chromosome!

11. Sex-Linked Genes – Checking In...
Can Fathers pass sex-linked genes to their sons?
For males, do the terms homozygous and heterozygous apply?
Do most genes on the “X” sex chromosomes have to do with sex characteristics?

12. Sex-Linked Genes
NO. Fathers pass sex-linked genes to their daughters but not their sons.
NO. For males, the terms homozygous and heterozygous do not apply for sex-linked genes since males only have one copy of the gene.
NO. Most genes on the “X” sex chromosomes have nothing to do with sex characteristics.

13. Colour Blindness: Sex-Linked Gene
Can you read the number?
Number 8

14. Colour Blindness: Sex-Linked Gene
What happens if you cross a normal female with a colour blind male?

15. Colour Blindness: Sex-Linked Gene
All females are carriers
All males are normal

16. Colour Blindness: Sex-Linked Gene
What happens if you cross a carrier female with a normal male?

17. Colour Blindness: Sex-Linked Gene
½ females are carriers
½ females are normal
½ males are colour-blind
½ males are normal

18. Colour Blindness: Sex-Linked Gene
What happens if you cross a carrier female with a colour-blind male?

19. Colour Blindness: Sex-Linked Gene
½ females are colour blind
½ females are carriers
½ males are colour blind
½ males are normal

20. Colour Blindness: Sex-Linked Gene
½ females are colour blind, ½ females are carriers. ½ males are colourblind
½ females are carriers and ½ males are colour-blind
All females are carriers

21. Other examples of Sex-Linked Genes:
Duchene Muscular Dystrophy
Progressive muscle weakening, loss of coordination
Hemophilia
Reduced ability to clot blood

22. Try This!
Neither Tom nor Rhonda has muscular distrophy, but their firstborn son has it.
What is the probability that a second child will have this disease?
What if the child was a boy?
A girl?
Child:

23. Try This!
Neither Tom nor Rhonda has muscular distrophy, but their firstborn son has it.
What is the probability that a second child will have this disease?
What if the child was a boy?
A girl?
Child:

24. Try This!
Neither Tom nor Rhonda has muscular distrophy, but their firstborn son has it.
What is the probability that a second child will have this disease? 1/4
What if the child was a boy? 1/2
A girl? 0, but ½ carrier
Child:

25. X-Inactivation
Since females get two copies of the “X” sex chromosomes, they need to inactivate one of them!
Done by adding methyl groups to DNA Bunches up the chromosome
Happens randomly in each cell when the female is an embryo
Inactive chromosome is called a Barr body

26. Example of X-Inactivation: Calico Cats!

28. The kitten on the left “CC” is an identical clone of the cat on the right “Rainbow”. How can you explain the different colouring?
Since X-inactivation is done randomly during embryonic development, the cat has a different pattern of inactivation from her kitten clone
CC (copy cat) was born in She is the first cloned cat.

29. Discuss...
During early embryonic development of female carriers for colour blindness, the normal allele is inactivated by chance in about half the cells. Why, then, aren’t half of female’s colour blind?

30. Discuss...
During early embryonic development of female carriers for colour blindness, the normal allele is inactivated by chance in about half the cells. Why, then, aren’t half of female’s colour blind?
Eye cells must come from multiple cells in early embryo
Having ~1/2 of mature eye cells expressing proteins for colour detection must be enough for normal colour vision

31. Try This! Handout with questions... Chapter 15!
“Sex Linked Practice”
Chapter 15!
Next Class: Linked Genes