Sex-linked Traits

Sex determinationSex chromosomes – determines

the sex of an individualY X

Males have X and YTwo kinds of gametes

Female have two X’sOnly one type of gamete

Which parents determines the sex of the offspring?

XYmale

X Y

XXFemale

X

X

It is the male that determines the sex of the offspring.

Sex Linked TraitsTraits controlled by genes located on

sex chromosomes (X or Y)Most are X-linked

The X chromosome is larger and has more genes

A Y has very few genesShow inheritance pattern that differs

for autosomal traits.

X-linked Inheritance

Males only have one copy of the X chromosome (hemizygous), plus a Y

Female have two X chromosomes, and may be homozygous or heterozygous for a trait

X-linked genes are never passed from father to son. The Y chromosome is the only sex chromosome that passes from father to son.

Females with one copy of the normal gene and one copy of the mutated gene are called carriers. They don’t show the trait.

Males are never carriers – if they have a mutated gene on the X chromosome, it will be expressed

X-linked disordersRecessive

Red-green color blindness

HemophiliaDuchenne muscular

dystrophy Ichthyosis

DominantHypertrichosis

Red-green color blindnessCan not distinguish red from

green.Normal red-green Red-green colorblind

What a red-green colorblind person would see

Normal Red-green colorblind

X Chromosome InactivationWhile females have two X

chromosomes, only one is active.The other is turned off.

This is random, in some cells the maternal X is turned off and in others the paternal X is turned off.

All descents of that cell will have same X turned off

It is condensed and known as a Barr Body.

Hemophilia

Non-carrier Mother + Father With Hemophilia

Carrier Mother + Non-hemophiliac Father

Carrier Mother + Father With Hemophilia