Sex Linkage

Here Comes the Sun KingThe tale of King Henry VIII and Anne Boleyn

T.H. Morgan’s FliesContext: 1908 – before anyone knew

the link between chromosomes and heredity

Working with the fruit fly, Drosophlia melanogaster

A model organism for geneticsWon the Nobel Prize in 1933 for

discovering that chromosomes carried heritable material

One day in the lab…Was looking at flies, who normally have wild

type red eyes, saw a fly with white eyesWanted to do a breeding analysis to understand

about white eyesDid a test cross

Test CrossA test cross is used to determine the genotype

of an individual exhibiting a dominant trait Determine if this individual is homozygous or

heterozygous (HH or Hh)How?

Cross a dominant phenotypeindividual to one with the recessive phenotype.

By observing the resulting phenotypes, you can figure out the genotype

So he crossed themMaybe

white eyes are lethal in

females?

So he crossed themI guess not….but what does this have to do with

sex?

Morgan’s ResultsThe appearance of white eyes

in females shows that this trait is not lethal in females.

All possible combinations of white eyes and sex are possible.

The white-eye trait can be carried over to females when F1 females are crossed with white-eyed males.

Did it have to do with chromosomes?

The male and females seemed to have slightly different chromosomes

Morgan found that the gene for white eyes seemed to follow the inheritance of sex

From these and other crosses, he was able to figure out that genes were carried on chromosomes!

Recall: Chromosome Structure

Each human cell has 46 chromosomes23 pairs in total 22 are autosomes

(not sex chromosomes)

1 pair of sex chromosomes

Each pair ishomologous (similar but not the same)

10

Females have two X chromosomes, (XX)Males have one X and one Y Chromosome (XY)

X and Y on a Karyotype A Karyotype is an arrangement of the chromosomes of

an individual that has been sorted according to size and type

1st Law: Law of Segregation

Mendel’s law of segregation states that every individual possesses a pair of alleles and passes a randomly selected copy (one or the other) to its offspring.

The same happens for sex chromosomes

Sex Linked InheritanceAutosomal inheritance: inheritance of alleles

located on autosomal (non-sex) chromosomes(This is all the inheritance we have dealt with up

until now)Sex-linked: describes an allele that is found on

one of the sex chromosomes (X or Y)

Aa, CDCd XHXh XHY

HemizygousJust like before, females can be homozygous or

heterozygous for a trait

XHXh or XHXH

Males are called hemizygous because they are neither heterozygous nor homozygous. They only have one possibility!

XHY

Sex Linked ProblemsFor example, hemophilia A is a blood disease

where it takes a long time for the blood to clot. The gene for hemophilia is located on the X chromosome and is recessive.

If a woman carrying the hemophilia allele marries a man who does not have hemophilia, what are the odds their children will have hemophilia?

How does it workLet XH represent the normal alleleLet Xh represent the allele for hemophilia(Y is the Y chromosome)

Results

50% of the males are affected0% of the females are affected, although one is

a carrier

CarrierCarrier: someone who does not have the

phenotype of a condition but has the allele for the condition.

This usually applies to recessive genesFor sex-linked genes, only FEMALES can be

carriers.

Sex-Linked GenesMale Pattern Baldness

Located on the X chromosomeRecessive If you are male and your mothers father had it,

you will get it. It is rare in females.Why?

Male Pattern Baldness - PLet XB represent the normal hair allele, and Xb

represent the baldness alleleP - Normal Female x Bald Male

XBXB x XbY

Both sons are normal, both daughters are carriers!

Xb YXB XBXb

Carrier FemaleXBYNormal Male

XB XBXb

Carrier FemaleXBYNormal Male

Male Pattern Baldness = F1

F1 - Carrier Female x Normal MaleXBXb x XBY

100% of females are normal, ½ of sons are normal, ½ of sons are affectedAltogether, ¼ of children are affected

XB YXB XBXB

Normal FemaleXBYNormal Male

Xb XBXb

Carrier FemaleXbYAffected Male

What about a bald female?

It could happen, but you’d needBald or Carrier Female x Bald Male

XbXb or XBXb x XbY

There are also Y-linked diseasesObviously, only males can get it. If your dad has it, you will get itLess common because the Y chromosome is smaller and

has less genes