sex linkage

Recall that the 23rd chromosome in humans dictates the gender of the individual

The Y chromosome is much smaller, and carries much less genetic info

X chromosome

Y chromosome

Colourblindness

Hemophilia

900-1600 genes

70-200 genes

Genes found on the either chromosome are called sex-linked traits

Far more are found on the X, and not found on the Y

What does this mean for males?? They are more susceptible to conditions

found on the Y chromosome All X-linked conditions are expressed more

so in males

Can result from a defect in one of three genes found on X chromosome

Possible Inheritance of Colorblindness Allele

Hemophilia Genes for clotting found on X chromosome Defects lead to deficiencies in expression of

coagulation factors Patients blood does not clot as easily

◦ Cuts can be fatal

Duchenne Muscular Dystrophy Another X-linked condition Sufferers lack a muscle protein, skeletal

muscle degrades

One of the X chromosomes is inactivated in females (randomly)

Chromosome forms a dense region that attaches to nuclear envelope

Stains darker, visible under light microscope Called a Barr Body Discovered by a Canadian cytogeneticist Dr

Murray L. Barr

Other dark spots are nucleoli

Used in 1966 Olympics to test for males attempting to pass as females

Resulted in the stripping of medals from individuals because of ambiguous genital sex◦ Genetic defect that allows males to appear as

females Also successfully indentified a number of

cases of testicular feminization syndrome (androgen insensitivity syndrome)◦ Error in androgen receptors resulting in XY

individual developing phenotypically as females