SEX-LINKED TRAITSSEX-LINKED TRAITS

Solving pedigrees for traits with Solving pedigrees for traits with genes located on the X and Y genes located on the X and Y chromosomechromosome

AUTOSOMES AUTOSOMES VS. VS. SEX CHROMOSOMESSEX CHROMOSOMES Humans have 23 pairs of Humans have 23 pairs of

chromosomeschromosomes 22 pairs of chromosomes are not 22 pairs of chromosomes are not

related to gender (male or female). related to gender (male or female). These are called AUTOSOMES These are called AUTOSOMES

1 pair of chromosomes determines 1 pair of chromosomes determines gender. These are called SEX gender. These are called SEX CHROMOSOMESCHROMOSOMES

KARYOTYPEKARYOTYPEchart of homologous chart of homologous chromosomes (XX)chromosomes (XX)

KARYOTYPEKARYOTYPEchart of homologous chart of homologous chromosomes (XY)chromosomes (XY)

AUTOSOMAL TRAITSAUTOSOMAL TRAITS

Most of the traits in humans are Most of the traits in humans are controlled by genes on the autosomes.controlled by genes on the autosomes.

Autosomal traits are inherited the Autosomal traits are inherited the same way by both males and females.same way by both males and females.

All of the traits we have studied so far All of the traits we have studied so far have been autosomal – they are the have been autosomal – they are the simplest type of genetics problems to simplest type of genetics problems to solvesolve

SEX CHROMOSOMESSEX CHROMOSOMES

The sex chromosomes determine genderThe sex chromosomes determine gender The sex chromosomes are also called the The sex chromosomes are also called the

X and Y chromosomeX and Y chromosome Females are XXFemales are XX Males are XYMales are XY The Y chromosome is much smaller, and The Y chromosome is much smaller, and

does not have the same genes as the Xdoes not have the same genes as the X Most “sex-linked” traits are “X linked”Most “sex-linked” traits are “X linked”

INHERITANCE OF SEX INHERITANCE OF SEX CHROMOSOMESCHROMOSOMES Since Females are XXSince Females are XX

– They inherited an X chromosome They inherited an X chromosome from from bothboth of their parents of their parents

– They will only produce eggs with X They will only produce eggs with X chromosomeschromosomes

– All of their children will inherit one or All of their children will inherit one or the other of her X chromosomes the other of her X chromosomes (with equal probability) – but all of (with equal probability) – but all of her children, both boys and girls, will her children, both boys and girls, will inherit an X chromosome from herinherit an X chromosome from her

INHERITANCE OF SEX INHERITANCE OF SEX CHROMOSOMESCHROMOSOMES Since males are XYSince males are XY

– They inherited the They inherited the YY from their from their father father must be – their mother did not have a Y must be – their mother did not have a Y

chromosome because she is femalechromosome because she is female

– They inherited the They inherited the XX from their from their mothermother process of elimination – if the Y came process of elimination – if the Y came

from Dad, the X came from Momfrom Dad, the X came from Mom

Females vs. Males Females vs. Males X-Linked TraitsX-Linked Traits Since females are XXSince females are XX

– They have 2 alleles for the trait. It works out They have 2 alleles for the trait. It works out exactly like autosomal traitsexactly like autosomal traits

Homozygous dominant, heterozygous, and Homozygous dominant, heterozygous, and homozygous recessive are all possiblehomozygous recessive are all possible

Since males are XYSince males are XY– They only have one allele for the trait. There They only have one allele for the trait. There

is no corresponding allele on the Y is no corresponding allele on the Y chromosomechromosome

– Whatever alleles males have on the X Whatever alleles males have on the X chromosome will be expressed.chromosome will be expressed.

Eye Color in Fruit FliesEye Color in Fruit Flies

White eyes in fruit flies is X linked recessiveWhite eyes in fruit flies is X linked recessive

(the normal “wild type” eye color is red)(the normal “wild type” eye color is red)

Predict the offspring of a cross Predict the offspring of a cross between a white eyed male and a between a white eyed male and a purebred red eyed femalepurebred red eyed female

Predict the offspring of a cross Predict the offspring of a cross between a white eyed male and a between a white eyed male and a red eyed femalered eyed female

All offspring will All offspring will have red eyeshave red eyes

Females will all Females will all be carriersbe carriers

Predict the offspring of a cross Predict the offspring of a cross between a normal male and a between a normal male and a heterozygous femaleheterozygous female

Predict the offspring of a cross Predict the offspring of a cross between a normal male and a between a normal male and a heterozygous femaleheterozygous female

All female offspring will All female offspring will have red eyes, but half have red eyes, but half of them will be carriersof them will be carriers

Half of the males will be Half of the males will be normal, and half will normal, and half will have white eyeshave white eyes

Colorblindness: X Colorblindness: X linked recessivelinked recessive

People affected by red-green People affected by red-green colorblindness are not able to colorblindness are not able to distinguish red from greendistinguish red from green

The pictures above are used to test for The pictures above are used to test for colorblindnesscolorblindness

Colorblindness: Colorblindness: Genotypes and Genotypes and PhenotypesPhenotypes Females:Females:

– XXNNXXN N normal vision, not a carriernormal vision, not a carrier

– XXNNXXnn normal vision, carriernormal vision, carrier

– XXnnXXnn colorblindcolorblind

MalesMales– XXNNYY normal visionnormal vision

– XXnnYY colorblindcolorblind

Colorblindness:Colorblindness:Pedigree solvingPedigree solving All males are XYAll males are XY All females are XXAll females are XX Males inherit X from Males inherit X from

mother and Y from mother and Y from father.father.

Males give X to Males give X to daughters and Y to daughters and Y to sonssons

Trace the X and Y Trace the X and Y through the through the generationsgenerations

Colorblindness:Colorblindness:Pedigree solvingPedigree solving All males are XYAll males are XY All females are All females are

XXXX

Colorblindness:Colorblindness:Pedigree solvingPedigree solving #3 is colorblind#3 is colorblind Colorblindness is X Colorblindness is X

linked recessivelinked recessive #3 must have a #3 must have a

recessive allele for recessive allele for colorblindness on colorblindness on his X chromosomehis X chromosome

All other males are All other males are normal, and must normal, and must have dominant have dominant alleles for color alleles for color vision on their X vision on their X chromosomeschromosomes

Colorblindness:Colorblindness:Pedigree solvingPedigree solving All the females in All the females in

this pedigree this pedigree have normal color have normal color vision, so all of vision, so all of them must have them must have at least one at least one dominant allele dominant allele (maybe both) for (maybe both) for normal color normal color vision on an X vision on an X chromosomechromosome

Colorblindness:Colorblindness:Pedigree solvingPedigree solving #3 inherited his Y #3 inherited his Y

chromosome from chromosome from his father (his his father (his mother doesn’t mother doesn’t have one because have one because she’s female)she’s female)

He inherited his X He inherited his X chromosome from chromosome from his motherhis mother

#2 (mother of #3) is #2 (mother of #3) is a carriera carrier

Colorblindness:Colorblindness:Pedigree solvingPedigree solving #5 is the daughter #5 is the daughter

of #3 (colorblind of #3 (colorblind male) and #4 male) and #4 (normal female)(normal female)

#5 has 2 X #5 has 2 X chromosomes, one chromosomes, one from her Mom and from her Mom and one from her Dadone from her Dad

#5 is a carrier #5 is a carrier because she because she inherited her Dad’s inherited her Dad’s X chromosomeX chromosome

Colorblindness:Colorblindness:Pedigree solvingPedigree solving The only person The only person

left is #4 – a left is #4 – a normal femalenormal female

Without knowing Without knowing more about her more about her parents, we can’t parents, we can’t determine if she determine if she is a carrier or not. is a carrier or not. Her genotype is Her genotype is unknownunknown