XLinked Inheritance Sex Linked Humans have 23 pairs

X-Linked Inheritance (Sex Linked)

• Humans have 23 pairs of chromosomes. • One pair of chromosomes is related to the sex of an individual , these chromosomes are called sex chromosomes • The other 22 pairs of chromosomes are called autosomes (1 -22) • In humans, the sex of an individual depends on the presence or absence of the Y chromosome

– The Y carries little genetic information, mainly those that contribute to male characteristics. (About 87 genes total. ) – The X carries a lot more genetic information. (About 2000!)

Example: A male and a female have offspring. What is the likelihood of them having male or female offspring?

• some genes are carried on the X chromosome = sex-linked genes • this means females have one allele on each chromosome (two total) • whereas males only have one X chromosome, and therefore only one allele

• Sex-linked traits are recessive so this means that for females both X chromosomes must have the allele for the trait for it to be expressed • But, since males only have one X chromosome if they have a recessive allele, they will display the phenotype

• A carrier is a person that has the trait on only one chromosome and does not express the trait. Carriers of sex linked traits are always women.

Example: Hemophilia is a disease in humans that causes blood to not clot properly. It shows up more often in males than females. The allele for hemophilia is present on the X chromosome. If a heterozygous female mates with an unaffected male, which of their offspring have hemophilia?

Example: Colorblindness is another disorder caused by a defective recessive allele on the X chromosome. If a color-blind male has a child with a carrier female, what percentage of the offspring will be colorblind?