Sex linkage
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Sex Linkage. Here Comes the Sun King. The tale of King Henry VIII and Anne Boleyn. T.H. Morgan’s Flies. Context: 1908 – before anyone knew the link between chromosomes and heredity Working with the fruit fly, Drosophlia melanogaster A model organism for genetics

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Sex Linkage

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Sex linkage

Sex Linkage

Here comes the sun king

Here Comes the Sun King

  • The tale of King Henry VIII and Anne Boleyn

T h morgan s flies

T.H. Morgan’s Flies

  • Context: 1908 – before anyone knew the link between chromosomes and heredity

  • Working with the fruit fly, Drosophlia melanogaster

  • A model organism for genetics

  • Won the Nobel Prize in 1933 for discovering that chromosomes carried heritable material

One day in the lab

One day in the lab…

  • Was looking at flies, who normally have wild type red eyes, saw a fly with white eyes

  • Wanted to do a breeding analysis to understand about white eyes

  • Did a test cross

Test cross

Test Cross

  • A 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 them

So he crossed them

Maybe white eyes are lethal in females?

So he crossed them1

So he crossed them

I guess not….but what does this have to do with sex?

Morgan s results

Morgan’s Results

  • The 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

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

Recall: Chromosome Structure


  • Females have two X chromosomes, (XX)

  • Males have one X and one Y Chromosome (XY)

  • Each human cell has 46 chromosomes

    • 23 pairs in total

    • 22 are autosomes (not sex chromosomes)

    • 1 pair of sex chromosomes

    • Each pair is


      but not the same)

X and y on a karyotype

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

1 st l aw law of segregation

1stLaw: 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

The same happens for sex chromosomes

Sex linked inheritance

Sex Linked Inheritance

  • Autosomal 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, CDCdXHXh XHY



  • Just 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 problems

Sex Linked Problems

  • For 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 work

How does it work

  • Let XH represent the normal allele

  • Let Xh represent the allele for hemophilia

  • (Y is the Y chromosome)



  • 50% of the males are affected

  • 0% of the females are affected, although one is a carrier



  • Carrier: someone who does not have the phenotype of a condition but has the allele for the condition.

  • This usually applies to recessive genes

  • For sex-linked genes, only FEMALES can be carriers.

Sex linked genes

Sex-Linked Genes

  • Male Pattern Baldness

    • Located on the X chromosome

    • Recessive

    • If you are male and your mothers father had it, you will get it. It is rare in females.

    • Why?

Male pattern baldness p

Male Pattern Baldness - P

  • Let XB represent the normal hair allele, and Xb represent the baldness allele

  • P - Normal Female x Bald Male

    XBXB xXbY

    Both sons are normal, both daughters are carriers!

Male pattern baldness f 1

Male Pattern Baldness = F1

  • F1- Carrier Female x Normal Male


    100% of females are normal, ½ of sons are normal, ½ of sons are affected

    Altogether, ¼ of children are affected

What about a bald female

What about a bald female?

It could happen, but you’d need

Bald or Carrier Femalex Bald Male


There are also Y-linked diseases

  • Obviously, only males can get it.

  • If your dad has it, you will get it

  • Less common because the Y chromosome is smaller and has less genes

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