Determining Sex Females are XX, producing only X gametes. Males are XY, producing both X & Y gametes.

2 Sex-linked Traits Traits (genes) located on the sex chromosomes Traits (genes) located on the sex chromosomes Sex chromosomes are X and Y Sex chromosomes are X and Y XX genotype for females XX genotype for females XY genotype for males XY genotype for males Many sex-linked traits carried on X chromosome Many sex-linked traits carried on X chromosome

The X-Chromosome contains over 2300 genes Most of these genes are unrelated to sex Genes on X-Chromosome are expressed in both sexes X-Chromosome has a centromere in the center of the chromosome X-Chromosome

The Y-Chromosome contains around 25 genes It is short & stubby The centromere is near the end Most important function is to determine sex Y-Chromosome

5 Sex-linked Traits Sex Chromosomes XX chromosome - femaleXy chromosome - male fruit fly eye color Example: Eye color in fruit flies

6 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 traits. RR = red eyed Rr = red eyed rr = white eyed XY = male XX = female XRXR XrXr XrXr Y

7 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

Red/Green Colorblindness Hemophilia Baldness Muscular Dystrophy Some Sex-Linked Traits

Only females can be carriers It is all or nothing for males Notice the shape of Y-Chromosome Sex-Linked Recessive Traits

Trait will show up in phenotype if in genotype Sex-Linked Dominant Trait

Pedigrees Another method used to study human heredity is a pedigree. It is a a visual tool that is easier to follow.

12 Female Carriers

A circle represents a female. A square represents a male. A horizontal line connecting a male and female represents a marriage. A vertical line and a bracket connect the parents to their children. A half-shaded circle or square indicates that a person is a carrier of the trait. A completely shaded circle or square indicates that a person expresses the trait. A circle or square that is not shaded indicates that a person does not express the trait but don’t know if they are carrier. Section 14-1 Figure 14-3 A Pedigree

Tay-Sacs Pedigree 1. How many generations are there in this pedigree? 2. Who died in Generation 2? 3. How many kids were produced from generation 1?

Tay-Sacs Pedigree 4. Who got married in generation 3? 5. How many kids lived in generation 4?

What is Hemophilia?  It is a blood clotting disorder.  Victims lack ONE of EIGHT proteins required for blood to clot.  Victims may die from cuts…or bruises!  It is a…  …sex-linked trait!