Genetics

• A sex linked trait is a trait genetically determined by an allele located on a sex chromosome. A female can have two affected X's and actually have the disorder, one affected X and be a carrier, or no affected X's and be normal.  A male can only have two genotypes: an affected X and normal Y (having the disease), or a normal X and Y (not having the disease).
• An example of a sex linked disorder is Hemophelia, and it is a recessive sex linked disorder.  The biggest symptom of Hemophelia is uncontrollable bleeding.  Many people are able to live a pretty normal life, but some die prematurely due to severe bleeding. 
• X inactivation can only occur in females because they have two X chromosomes.  A barr body is an inactive X chromosome in a female somatic cell. 
 

Hemophelia Punnet Square 

 Genotypic Ratio- 1:1:1:1 XX*: 25%, X*X*: 25%, XY: 25%, X*Y: 25%)

Phenotypic Ratio- 1:1:1:1 (25% affected female, 25% unaffected female, 25% affected male, 25% unaffected male)