Sex-linked TraitsSex-linked Traits A gene that is found A gene that is found

only on the X only on the X chromosome and not chromosome and not the Y chromosomethe Y chromosome

Examples:Examples: Red-green Red-green

colorblindnesscolorblindness Male Pattern BaldnessMale Pattern Baldness HemophiliaHemophilia Duchenne Muscular Duchenne Muscular

DystrophyDystrophy

Female Male

CharacteristicsCharacteristics More common in men More common in men

because they only have 1 because they only have 1 X chromosomeX chromosome

Females have 2 X Females have 2 X chromosomes and would chromosomes and would need the gene on both need the gene on both chromosomes to have the chromosomes to have the traittrait

Females that are Females that are heterozygous for the trait heterozygous for the trait are called carriersare called carriers

Carriers appear normal Carriers appear normal but can pass the trait to but can pass the trait to their offspringtheir offspring

ColorblindnessColorblindness Recessive gene (b) Recessive gene (b)

on X chromosomeon X chromosome Certain colors cannot Certain colors cannot

be distinguished be distinguished Red/Green color Red/Green color

blindness is most blindness is most common (about common (about 99%)99%)

No treatment but life No treatment but life is normalis normal

HemophiliaHemophilia Recessive gene (h) on the Recessive gene (h) on the

X chromosomeX chromosome Bleeding disorder in which Bleeding disorder in which

the blood does not clot the blood does not clot normallynormally

Persons with hemophilia Persons with hemophilia may bleed for a longer may bleed for a longer time than others after an time than others after an injury or accident injury or accident

Live life cautiously, Live life cautiously, receive treatments to help receive treatments to help blood clot normallyblood clot normally

Duchenne Muscular DystrophyDuchenne Muscular Dystrophy Recessive gene (d) on the Recessive gene (d) on the

X chromosomeX chromosome Rapidly-worsening muscle Rapidly-worsening muscle

weakness that starts in weakness that starts in the legs and pelvis, and the legs and pelvis, and later affects the whole later affects the whole bodybody

No cure (death usually No cure (death usually occurs by age 25)occurs by age 25)

Treatment is aimed at Treatment is aimed at control of symptoms to control of symptoms to maximize the quality of maximize the quality of life life

Sample Problem: Male Pattern Sample Problem: Male Pattern BaldnessBaldness

1.1. Unlike all the other Unlike all the other crosses there is crosses there is information that we know information that we know before we ever read the before we ever read the problem. We know that problem. We know that a XX female is going to a XX female is going to mate with a XY male. mate with a XY male. So, you can set up your So, you can set up your square ahead of time.square ahead of time.

XX XY

XX XY

X Y

X

X

2.2. Next, you have to read the problem and Next, you have to read the problem and figure out what the sex-linked condition is figure out what the sex-linked condition is and write down the allele you want to use and write down the allele you want to use for the problem and set up a key!for the problem and set up a key!

Baldness is a sex-linked trait. The allele for Baldness is a sex-linked trait. The allele for baldness is baldness is recessiverecessive and is carried on the X chromosome. The Griffin and is carried on the X chromosome. The Griffin family lives in a town called Quahog. The family lives in a town called Quahog. The father,father, Peter, has a Peter, has a gene for gene for normal hair.normal hair. The The wife,wife, Lois, is a Lois, is a carrier for baldnesscarrier for baldness. . Peter and Lois have 3 kids: Chris, Meg, and Stewie. The kids Peter and Lois have 3 kids: Chris, Meg, and Stewie. The kids want to know what is the probability they will develop want to know what is the probability they will develop baldness?baldness?

Setting up your keySetting up your key What letters will Peter have since he is a What letters will Peter have since he is a

male?male? What letters will Lois have since she is a What letters will Lois have since she is a

female?female? Decide what letter you will use for the Decide what letter you will use for the

trait trait B = normal, b= baldB = normal, b= bald

3.3. Reread the problem and find the parents Reread the problem and find the parents genotypes using the key you just madegenotypes using the key you just made

So, Peter is XY and Lois is XXSo, Peter is XY and Lois is XX Now you have to add the allele for Now you have to add the allele for

baldness to all of the X chromosomes as baldness to all of the X chromosomes as described in the problemdescribed in the problem

Peter XPeter XBBYY Lois XLois XBBXXbb

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

XBXB XBY

XBXb XbY

XB Y

XB

Xb

Pheno: ¼ female, normal

¼ female, carrier

¼ male, normal

¼ male, bald

Geno: ¼ XBXB

¼ XBXb

¼ XBY ¼ XbY

5.5. Answer the question:Answer the question: What is the probability the children will What is the probability the children will

develop baldness?develop baldness? Females = 0% chanceFemales = 0% chance Males = 50% chance Males = 50% chance

Sample CrossSample Cross

Cross a colorblind male Cross a colorblind male with a female who is a with a female who is a carrier for carrier for colorblindness.colorblindness.

XXbbY x XY x XBBXXbb

Geno: ¼ XGeno: ¼ XBBXXbb; ¼ X; ¼ XbbXXbb; ; ¼ X¼ XBBY; ¼ XY; ¼ XbbYY

Pheno: ½ females will Pheno: ½ females will be colorblind; ½ be colorblind; ½ females will be carriers; females will be carriers; ½ males will be normal; ½ males will be normal; ½ males will be ½ males will be colorblindcolorblind