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 phenotype individual to one with the recessive phenotype.
By observing the resulting phenotypes, you can figure out the genotype

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

7. I guess not….but what does this have to do with sex?
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 Structure10
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)
Females have two X chromosomes, (XX)
Males have one X and one Y Chromosome (XY) 10

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. 1st Law: 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, CDCd XHXh XHY

15. Hemizygous XHY XHXh or XHXH
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

20. 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.

21. 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?

22. 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! Xb Y XB
XBXb
Carrier Female
XBY
Normal Male

23. 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 XB Y
XBXB
Normal Female
XBY
Normal Male Xb
XBXb
Carrier Female
XbY
Affected Male

24. What about a bald female?
It could happen, but you’d need
Bald or Carrier Female x Bald Male
XbXb or XBXb 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