Sex-linked Traits

Sex determination Sex chromosomes – determines the sex of an individual Y X Males have X and Y Two kinds of gametes Female have two X’s Only one type of gamete

Which parents 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 Traits Traits 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 genes Show inheritance pattern that differs for autosomal traits.

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 Recessive Dominant Red-green color blindness Hemophilia Duchenne muscular dystrophy Ichthyosis Dominant Hypertrichosis

Red-green color blindness Can not distinguish red from green. Normal red-green Red-green colorblind Some 10 million American men—fully 7 percent of the male population—either cannot distinguish red from green, or see red and green differently from most people. This is the commonest form of color blindness, but it affects only .4 percent of women. There are three basic variants of color blindness. Red/green color blindness (deuteranopia) is the most common deficiency, affecting 8% of Caucasian males and 0.5% of Caucasian females. The prevalence varies with culture. Blue color blindness (protanopia) is an inability to distinguish both blue and yellow, which are seen as white or gray. Protanopia is quite rare and has equal prevalence in males and females.

What a red-green colorblind person would see Normal Red-green colorblind

Hemophilia

X Chromosome Inactivation While 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.

Genes on the Y Chromosome Few genes are on the Y chromosome Hairy pinna