Sex-Linked Inheritance

Two copies of each autosomal gene affect phenotype Mendel studied autosomal gene traits (these genes are on autosomes), like hair texture. The “either-or” traits that were seen in your tutorial. Genes found on autosomes (1-22 for humans) = autosomal traits Genes found on sex chromosomes (pair 23 for humans) = sex-linked traits

There is a 50/50 chance of child being a boy/girl Sex Determination Sex determined by genes found on the X and Y chromosome Females: egg produced with an X chromosome X chromosome genes in mammals affect many traits. Males: ½ sperm cells contain X; and the other ½ contain Y Chromosome genes in mammals are responsible for male characteristics. X X X Y X X X X X Y X Y There is a 50/50 chance of child being a boy/girl

Males & Females have Different Inheritance Patterns ALL of a male’s sex-linked genes are expressed. Males have no second copies of sex-linked genes. Because of this, males can NEVER be “carriers” of sex-linked traits or disorders. Ex: Colorblindness, Hemophilia, Muscular dystrophy

Sex-Linked Disorders Defined: Inherited conditions found on X chromosome Usually recessive Females: XX chromosomes XH XH = healthy XH Xh = healthy carrier Xh Xh = disease Males: XY chromosomes XHY = healthy XhY = disease Rare in women (back-up X chromosome) Ex: Colorblindness, Hemophilia, Muscular dystrophy

Sample Colorblind Tests 4 Sex-Linked Traits:   1. Normal Color Vision:  A: 29,  B: 45,  C: --,  D: 26   2. Red-Green Color-Blind:  A: 70,  B: --,  C: 5,  D: --   3. Red Color-blind:  A: 70,  B: --,  C: 5,  D: 6   4. Green Color-Blind:  A: 70,  B: --,  C: 5,  D: 2

Sex-Linked Punnett Squares Let’s take a look at… Sex-Linked Punnett Squares

Jen is a healthy carrier (XH Xh) of hemophilia and Adam has no history in his family (XH Y). What is the probability of getting: Daughter with hemophilia? 0% Carrier child? 25% Child with hemophilia? Two healthy children? ¾ x ¾ = 9/16 (56%) Healthy girl Healthy boy Healthy girl (carrier) Disease boy

Kelly is a healthy carrier of hemophilia, but Jim suffers from the disease. What is the probability of getting a: Healthy son? 25% Healthy daughter? Child with hemophilia? 50% Carrier child? Healthy girl (carrier) Healthy boy Disease girl Disease boy

If the phenotype is more common in males, the gene is likely sex-linked.

Let’s try a sex-linked problem: A man and a woman marry. The woman is a carrier of colorblindness, an X-linked disorder. They have 4 children, 3 boys and one girl. Two of the boys are colorblind, and the girl is a carrier. The carrier daughter marries a healthy man. a)What is the probability they will have a carrier? b)What is the probability they will have a child that is colorblind? XD Xd XD Y Xd Y Xd Y XD Y XD Xd XD Y ? XD Xd XD XD XD Xd XD Y 25% XD Y Xd Y 25%

REVIEW How are sex-linked disorders different from autosome disorders? Why are sex-linked disorders more common in males? Write the genotype of a heterozygous dominant female. Write the genotype of a carrier female. Examine Kelly and Jim’s Punnett square. What are the chances they would have three children, all of whom are healthy?