NonMendelian Sex Linked Traits A Determining Sex 1
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Non-Mendelian: Sex- Linked Traits A. Determining Sex 1. Humans have a diploid number of 46 chromosomes or 23 pairs 2. 22 of the pairs are autosomes, they are the same for males and females 3. The 23 rd pair of chromosomes differ in males and females The are the sex chromosomes and are indicated by X and Y XY = male XX = female
• Males determine the baby’s sex because only men give the Y chromosome
IX. Sex- Linked Traits B. Inheriting sex linked traits 1. Genes located on the sex chromosomes are called sex- linked traits 2. Because the Y chromosome is small it carries few genes, including the male determinant sex gene.
B. Inheriting sex linked traits IX. Sex- Linked Traits 3. Males who receive a recessive allele on the X chromosomes will express the recessive phenotype because he cannot inherit on the Y chromosome. o There is no dominant allele to overpower the recessive gene 4. Males have a higher percent chance of expressing the recessive trait
IX. Sex- Linked Traits C. Examples in Humans 1. 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 § Need two recessive alleles; one from mom and one from dad
A woman who is heterozygous (a carrier) for hemophilia marries a normal man:
Queen Victoria of England was a carrier of the gene for hemophilia. She passed the harmful allele to 4 sons and 2 daughters. Her son Leopold had the disease and died at age 30, while her daughters were only carriers. As a result of marrying into other European royal families, the princesses Alice and Beatrice spread hemophilia to Russia, Germany, and Spain. By the early 20 th century, ten of Victoria's descendents had hemophilia. All of them were men, as expected.
IX. Sex- Linked Traits C. Examples in Humans 1. 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 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 a. Key: XX= female XY= male C= normal c’= colorblind a. Parents: Xc’Y c. Genotype Ratio: 1: 2 X CY XCXc’ d. Phenotype Ratio: 1: 2 female carriers 1: 2 normal males X CX C
Do all 5 steps! 1. Cross a heterozygous B blood type with an AB blood type. 2. A colorblind woman has children with a normal sighted man. What are the chances their children will have normal sight?
