Sex Linked Genes

The Xs and Ys of Genetics

Sex Linked Genes

•  There are 23 pairs of chromosomes and one of those pairs are the sex chromosomes.

•  There are either X chromosomes or Y chromosomes.

Sex Linked Genes

•  These chromosomes not only carry the genes that determine male and female traits, but also those for some other characteristics as well.

•  Genes that are carried by either sex chromosome (X or Y) are said to be SEX LINKED

Males

•  Men normally have an X and a Y combination of sex chromosomes.

•  Since only men inherit the Y chromosome, they are the only ones that inherit Y-linked traits.

Females

• Women have 2 X chromosomes

X Linked Traits

•  Men and Women can get the X-linked traits since both inherit X chromosomes.

Sex inheritance patterns for male and female children

X Chromosome Traits

•  Men only have one X chromosome •  Therefore genes on that chromosome

are expressed in the male phenotype, even if it is a recessive gene.

X-linked traits

•  In women, a recessive allele on one x chromosome is often masked on their phenotype

•  May have a dominant normal allele on the other carriers of X-linked traits but more rarely have them expressed in their own phenotypes

X-linked traits

•  In humans, at least 120 genes are known to be X-linked.

•  Genes for hemophilia, red-green color blindness, congenital night blindness, and Duchene muscular dystrophy

Hemophilia

•  An X-linked genetically inherited recessive disease

•  Can not clot their blood

•  Severe bleeding from minor cuts

•  Mostly in males

Red-Green Color Blindness

• X-linked • Cannot perceive red and green

in the same • Most often afflicts males

Muscular Dystrophy (MD)

•  Characterized by a gradual irreversible wasting away of skeletal muscle.

•  An X-linked trait most often passed on to sons from their mothers.

•  Begins to weaken the legs of boys by age 3 and gets worse every year.

•  No cure and inflicted usually die by age 30.

Y-linked Traits

•  The Y chromosome is small and therefore does not contain many genes

•  Y linked diseases are very rare •  Only passed from father to son.

Hypertrichosis Pinnae.

•  Hairy ears •  Can happen later in

life. •  Y-linked