Sex-linked Traits A gene that is found only on the X chromosome and not the Y chromosome A gene that is found only on the X chromosome and not the Y chromosome Examples: Examples: Red-green colorblindness Red-green colorblindness Male Pattern Baldness Male Pattern Baldness Hemophilia Hemophilia Duchenne Muscular Dystrophy Duchenne Muscular Dystrophy Female Male

Characteristics More common in men because they only have 1 X chromosome More common in men because they only have 1 X chromosome Females have 2 X chromosomes and would need the gene on both chromosomes to have the trait Females have 2 X chromosomes and would need the gene on both chromosomes to have the trait Females that are heterozygous for the trait are called carriers Females that are heterozygous for the trait are called carriers Carriers appear normal but can pass the trait to their offspring Carriers appear normal but can pass the trait to their offspring

Colorblindness Recessive gene (b) on X chromosome Recessive gene (b) on X chromosome Certain colors cannot be distinguished Certain colors cannot be distinguished Red/Green color blindness is most common (about 99%) Red/Green color blindness is most common (about 99%) No treatment but life is normal No treatment but life is normal

Hemophilia Recessive gene (h) on the X chromosome Recessive gene (h) on the X chromosome Bleeding disorder in which the blood does not clot normally Bleeding disorder in which the blood does not clot normally Persons with hemophilia may bleed for a longer time than others after an injury or accident Persons with hemophilia may bleed for a longer time than others after an injury or accident Live life cautiously, receive treatments to help blood clot normally Live life cautiously, receive treatments to help blood clot normally

Duchenne Muscular Dystrophy Recessive gene (d) on the X chromosome Recessive gene (d) on the X chromosome Rapidly-worsening muscle weakness that starts in the legs and pelvis, and later affects the whole body Rapidly-worsening muscle weakness that starts in the legs and pelvis, and later affects the whole body No cure (death usually occurs by age 25) No cure (death usually occurs by age 25) Treatment is aimed at control of symptoms to maximize the quality of life Treatment is aimed at control of symptoms to maximize the quality of life

Sample Problem: Male Pattern Baldness 1. Unlike all the other crosses there is information that we know before we ever read the problem. We know that a XX female is going to mate with a XY male. So, you can set up your square ahead of time. XXXY XXXY X

2. Next, you have to read the problem and figure out what the sex-linked condition is and write down the allele you want to use for the problem and set up a key! Baldness is a sex-linked trait. The allele for baldness is recessive and is carried on the X chromosome. The Griffin family lives in a town called Quahog. The father, Peter, has a gene for normal hair. The wife, Lois, is a carrier for baldness. Peter and Lois have 3 kids: Chris, Meg, and Stewie. The kids want to know what is the probability they will develop baldness?

Setting up your key What letters will Peter have since he is a male? What letters will Peter have since he is a male? What letters will Lois have since she is a female? What letters will Lois have since she is a female? Decide what letter you will use for the trait Decide what letter you will use for the trait B = normal, b= bald B = normal, b= bald

3. Reread the problem and find the parents genotypes using the key you just made So, Peter is XY and Lois is XX So, Peter is XY and Lois is XX Now you have to add the allele for baldness to all of the X chromosomes as described in the problem Now you have to add the allele for baldness to all of the X chromosomes as described in the problem Peter X B Y Peter X B Y Lois X B X b Lois X B X b

4. Now, set up the Punnett square and do the cross…Don’t forget the genotype and phenotype ratios!

XBXBXBXB XBYXBY XBXbXBXb XbYXbY X B Y X B X b Pheno: ¼ female, normal ¼ female, carrier ¼ male, normal ¼ male, bald Geno: ¼ X B X B ¼ X B X b ¼ X B Y ¼ XbY

5. Answer the question: What is the probability the children will develop baldness? What is the probability the children will develop baldness? Females = 0% chance Females = 0% chance Males = 50% chance Males = 50% chance

Sample Cross Cross a colorblind male with a female who is a carrier for colorblindness. Cross a colorblind male with a female who is a carrier for colorblindness. X b Y x X B X b X b Y x X B X b Geno: ¼ X B X b ; ¼ X b X b ; ¼ X B Y; ¼ X b Y Geno: ¼ X B X b ; ¼ X b X b ; ¼ X B Y; ¼ X b Y Pheno: ½ females will be colorblind; ½ females will be carriers; ½ males will be normal; ½ males will be colorblind Pheno: ½ females will be colorblind; ½ females will be carriers; ½ males will be normal; ½ males will be colorblind