Non-Mendelian: Sex- Linked Traits Determining Sex (Gender) Humans have a diploid number of 46 chromosomes or 23 pairs 22 of the pairs are autosomes, they are the same for males and females The 23rd pair of chromosomes differ in males and females These are the sex chromosomes and are indicated by X and Y XY = male XX = female

The Male determines the baby’s sex because only men give the Y chromosome

Sex- Linked Traits Inheriting sex-linked traits Genes located on the sex chromosomes are called sex-linked traits Because the Y chromosome is small it carries few genes, along with the male determinant sex gene.

Sex- Linked Traits Inheriting sex linked traits Males who receive a recessive allele on the X chromosomes will express the recessive phenotype because he cannot inherit an allele on the Y chromosome. There is no dominant allele to overpower the recessive gene so it is expressed. Males have a higher percent chance of expressing the recessive trait

Sex- Linked Traits Examples in Humans Hemophilia Causes a problem with blood clotting Caused by a recessive allele on the X chromosomes Males: 1/10,000 Need one recessive allele from carrier mom Females: 1/10,000,000 Need two recessive alleles; one from carrier mom and one from affected dad

A woman who is heterozygous (a carrier) for hemophilia marries a normal man:

Sex- Linked Traits Examples in Humans Red-Green Color Blindness People who have red-green color blindness cannot differentiate between the two colors. Caused by a recessive allele on the X chromosomes

Are you colorblind? Colorblindness The allele that controls colorblindness is found on the X chromosome and is recessive . What genotype must a female have to be diagnosed as colorblind?

Sample problem Xc’Y XCXC A colorblind man marries a woman who is not colorblind. Assuming there is no colorblindness in the woman’s ancestry, what will most likely be the genotype and phenotypes of their sons and daughters? Show your work Key: Parents: c. Genotype: d. Phenotype: XX= female XY= male C= normal c’= colorblind Xc’Y XCXC 2 XCY 2 XCXc’ 2 female carriers 2 normal males

Do all 5 steps! A colorblind woman has children with a normal sighted man. What are the chances their children will have normal sight?