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.
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.
• Example: Male infertility