X-linked genetics traits

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

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

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

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

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!

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)

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’

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)

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…

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

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