1. X-linked genetics traits

2. Karyotype
A karyotype shows the chromosomal map of all 23 pairs – 22 pairs of autosomes and one pair of sex chromosomes
XX means to be female
XY means to be male

3. Sex-linked traits
Traits found on Chromosomes 1 – 22 are called autosomes
Traits found on either the X or Y chromosome are called sex-linked traits
These chromosomes do more than just determine gender

4. X-Linked Traits
We will focus on the X chromosome due to its larger size
Linked traits refer to those expressed together due to close proximity of the genes on the chromosome

5. X-Linked Dominant Traits
Inherited through the X-chromosome
For dominant transmission, abnormal gene from one parent causes the disease; homologous gene from the other parent is normal
Very few diseases: Aicardi syndrome, Fragile X, Rett Syndrome

6. X-Linked Recessive Traits
If a recessive gene is on the ‘X’:
-In males it will always be expressed since there is no homologous pair on the ‘Y’.
-Therefore (sorry guys) recessive, X-linked diseases are much more common in males!

7. X-Linked Recessive Traits
many common examples:
colour-blindness
Duchenne muscular dystrophy
Male-patterned baldness
Charcot-Marie Tooth disease
Hemophilia A
Hemophilia B
(Christmas disease)

8. Hemophilia – an example
Since gender and the gene are important, both must be noted in the genotype.
Example:
Dominant allele – H
Recessive allele – h
XH – dominant gene on ‘X’
Xh – recessive gene on ‘X’
Y – no gene on the ‘Y’

9. Hemophilia Female genotypes XHXH – Normal
XHXh – Normal but a ‘Carrier’
XhXh – Hemophiliac (very rare)
Male genotypes
XHY – Normal
XhY – Hemophiliac
(cannot have a male carrier)

10. Example
What would be the probability of any child having hemophilia if the mother is a carrier and the father has normal blood clotting?
Mom: XHXh
Dad: XHY
There’s a Punnett Square for that…

11. Example XH Y Xh XHXH XH Y XH Xh Xh Y
DAD →
MOM ↓ XH Y
XHXH
XH Y Xh
XH Xh
Xh Y
Therefore, one chance in 4 that their child (a son) could have hemophilia – daughter could be a carrier though

12. Sex-linked diseases
You can now continue working on your genetics problems up to Set # 6
Have fun!