Sex-linked Traits

Sex Determination

Sex 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 parent determines the sex of the offspring?

XY

male

X Y

XX

Female

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 genesA Y has very few genes

Show inheritance pattern that differs for autosomal traits.

T.H. Morgan worked with fruit flies

• Fruit flies normally have red eyes.• One day, in one of his cultures of fruit flies,

Morgan found a fly with white eyes. • This white-eyed fly was male, so Morgan

mated it to red-eyed female fly to see if he could determine how this particular trait is inherited.

• When he mated it with a normal female, the offspring all had red eyes. This was expected in light of Mendel's experiments.

• The trait reappeared in the F2 generation. However, there was a twist because the trait only reappeared in males.

• There were no female flies with white eyes!

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 disorders

RecessiveRed-green color

blindnessHemophiliaDuchenne 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 seesNormal Red-green colorblind

Hemophilia

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.