Sex-Linked Genes and Pedigrees

Linked Genes Some genes are linked because they are on the SAME chromosome. A chromosome is a set of linked genes. Remember: Offspring get one copy of each chromosome from each parent. Chromosomes assort independently, NOT individual genes. Crossing-over during meiosis can “un- link” genes.

3 Sex-linked Traits Traits can be either Autosomal: traits (genes) are located on the non-sex chromosomesAutosomal: traits (genes) are located on the non-sex chromosomes Sex-Linked: traits (genes) are located on the sex chromosomesSex-Linked: traits (genes) are located on the sex chromosomes Sex chromosomes are X and Y XX genotype for females XY genotype for males Many sex-linked traits are carried on X chromosome copyright cmassengale

Examples of Autosomal Traits Autosomal Dominant: Huntington’s Disease Achondroplasia (Dwarfism) Osteoporosis Autosomal Recessive: Cystic Fibrosis Sickle Cell Anemia

Examples of Sex-Linked Traits X-linked Recessive: Colorblindness Hemophilia Duchenne Muscular DystrophyY-linked: Ear Hair

X vs. Y

What this means… Males ALL X-linked alleles are expressed Males have only one X chromosome, so ALL X-linked alleles are expressed in males, even if the trait is recessive. This is why males exhibit some traits more frequently than females. X-linked mother to son X-linked traits are passed from mother to son. Y-linked father to son Y-linked traits are passed from father to son.

8 Sex-linked Traits Sex Chromosomes XX chromosome - femaleXY chromosome - male fruit fly eye color Example: Eye color in fruit flies copyright cmassengale

9 Sex-linked Trait Problem Example: Eye color in fruit flies (red-eyed male) x (white-eyed female) X R Y x X r X r Remember: the Y chromosome in males does not carry an allele for the trait. RR = red eyed Rr = red eyed rr = white eyed XY = male XX = female XRXR XrXr XrXr Y copyright cmassengale

10 Sex-linked Trait Solution: X R X r X r Y X R X r X r Y 50% red eyed female 50% white eyed male XRXR XrXr XrXr Y copyright cmassengale

Pedigrees

Carriers Heterozygote (has the recessive allele but the trait is NOT expressed) In autosomal traits, carriers can be Male or Female. In Sex-Linked traits, ONLY Females can be carriers. All Sex-Linked traits are expressed in Males. copyright cmassengale12

13copyright cmassengale