Sex-Linked Genes Print Friendly and PDF

Google Slides

Anchoring Phenomenon: Why are calico cats always female? Why would a cloned calico cat not resemble the original?

Chromosome Numbers

Some genes are located on the sex chromosomes.

There are 22 pairs of autosomes and 1 pair of sex chromosomes in a typical human.


The Y chromosome is small and contains few genes. (Think: Y is WIMPY!)

How Coat Color in Inherited in Cats

Black and orange is sex linked, and codominant.


Show the cross between a black female and an orange male.

CC and Rainbow

X Inactivation

-occurs when one copy of the X chromosome is inactivated
-prevents females from having too many gene products
-which X becomes inactive is random

*This is why some patches of the female cat display orange color, and other patches are black.

Why is CC (cloned cat) different color than her mother, whom she was cloned from?


Sex Linked Disorders

Colorblindness - inability to distinguish certain colors, most common is red-green

Hemophilia - inabiliy of the blood to clot, wounds will bleed uncontrollably (can be deadly)

Duchenne Muscular Dystrophy - muscles become weak over time

Rett Syndrome - caused by a mutation on the X chromosome, not inherited, only affects girls


Example: Genetic Crosses with Sex Linked Traits

How hemophilia is inherited from a female who is a carrier.
Note: males cannot be carriers.



punnett square

Resources on Sex Linked Genes

SRY not SRY - case study on the sex determining region of the Y chromosomes

Fruit Flies and X-Linked Traits -  practice crosses that involve sex-linkage; fruit flies and calico cats

Calico Cats and X Linked Traits - more practice with sex-linked traits, see also advanced version that goes into tortoiseshell vs calico

Sexy Chickens - inheritance patterns in birds, using the ZW sex determination concept