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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

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

  • Won 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 eyes

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

  • Did a 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

Maybe white eyes are lethal in females?

So he crossed them

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

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?

  • 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


  • 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

  • A Karyotype is an arrangement of the chromosomes of an individual that has been sorted according to size and type

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

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

  • 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

  • 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

  • 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

  • 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 = 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?

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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