1. Sex Link Traits: Genes on the X and Y chromosomes
Copy what
is in RED

2. Lets recall how we determine sex…
Sex Chromosomes: X and Y
Female: XX Male: XY
The sperm determines the sex because it gives an X or Y to the gametes. The egg only gives an X to the gamete
The X chromosome is Bigger

3. What are sex-linked genes?  genes found on a sex chromosome
X-linked genes are genes found on the X chromosome, symbolized by Xr, XR.
Y-linked genes are found on the Y chromosome, (we won’t look at any of these)
Thomas Morgan experimented with the eye colour of fruit flies (Drosophilia) to determine X- linkage

4. Y-linked Genes
Y-linked genes are uncommon because the Y chromosome is so small and does not contain many genes, Y-linked diseases are rare.
In humans, only males have a Y chromosome so traits / diseases are passed only from father to son

5. X-linked Genes X-linked traits can be dominant or recessive XD, Xd
If the trait is DOMINANT only one copy is needed for the trait to be expressed in either females or males: XDX, XDY
If the trait is RECESSIVE two copies are needed for the trait to be expressed in females XdXd. But only one copy is needed in males: XdY

6. X-linked Genes
X-linked recessive traits will show up more often in males because they have only one X chromosome
Example 1: female carrier of an x-linked gene and a normal male
What will the genotypes be from the following cross?
XXr and XY

7. Set up the Punnett SquareX Xr Y

8. Here’s the results X Xr X X X Xr Y X Y Xr Y
For girls: 0% have the trait
For boys 50% have it.
Probability is higher for boys because whatever X they get determines the trait, for girls they have to get 2 recessive X’s. X Xr
X X
X Xr Y
X Y
Xr Y

9. Examples of X-linked traits in humans:
1. Red/Green Colour Blindness
2. Hemophilia blood disease
3. Muscular Dystrophy
4. Icthyosis simplex (scaly skin)

10. Colorblindness
A person with normal colour vision sees a number seven in the circle above.
Those who are colour blind usually do not see any number at all.

11. Colorblindness RED-GREEN COLORBLINDNESS:
People with red-green colour blindness see either a three or nothing at all.
Those with normal colour vision see an 8.

12. Hemophilia Hemophilia- Lacking in the ability to clot blood
There is a gene on the “X” chromosome that control blood clotting
People who have hemophilia are missing the protein to clot blood
They can bleed to death by minor cut.

13. Muscular Dystrophy - Results in weakening/loss of muscles
- Caused by defective version of gene that codes for muscle

14. X-linked Genes
Example 2: an unaffected male marries an affected female
What will the genotypes be from the following cross?
XrXr and XY

15. Set up the Punnett SquareXr X Y

16. Here’s the results Xr X X Xr Y Xr Y
For girls: 0% have the trait but 100% are carriers
For boys 100% have it. Xr X
X Xr Y
Xr Y

17. Phenotypes of Children
100% girls normal fingers
100% boys with webbed fingers

18. Practice http://www.ksu.edu/biology/pob/genetics/xl inked.htm
_genetics/problem_sets/sex_linked_inherit ance/sex_linked_inheritance.html 18