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Sex-Linked Genes

Biology: IB Diploma Higher Level, topic 15.1.8

Sex-Linked Genes

Linkage is where genes that are close to one another on a chromosome are likely to be inherited together. Sex linkage is different from autosomal linkage because it takes place in the sex chromosomes.

Sex chromosomes

Sex chromosomes

  • In many organisms, the sex chromosomes (X and Y) determine the sex of an individual.
  • Sex chromosomes differ from autosomes because they are non-homologous.
  • Non-homologous chromosomes do not consist of the same genes in the same order along the chromosome.
Y chromosome

Y chromosome

  • Human females have a homologous pair of X chromosomes (XX) and human males are heterozygous (XY).
  • X and Y chromosomes contain a small region of similarity.
  • The Y chromosome is considerably shorter than the X chromosome and contains fewer genes.
  • If a gene is only found on the X chromosome, it is X-linked.
X-linked genes

X-linked genes

  • Males only have one copy of an allele for X-linked genes.
  • There is no allele for the same gene on the Y chromosome. This is called hemizygosity.
  • Hemizygosity means that there is no role of dominance and recessiveness.
  • If a recessive gene is inherited on the X chromosome, it will always be present in the phenotype as it is the only allele present.
Hemophilia

Hemophilia

  • Hemophilia is a blood disorder that is X-linked.
  • Because human males need to inherit only one recessive mutant X allele to be affected, X-linked disorders are disproportionately observed in males.
  • Females must inherit recessive X-linked alleles from both of their parents in order to express the trait.
  • When they inherit one recessive X-linked mutant allele and one dominant X-linked wild-type allele, they are carriers of the trait and are typically unaffected.
X-linked disorders

X-linked disorders

  • The son of a woman who is a carrier of a recessive X-linked disorder will have a 50 per cent chance of being affected.
  • A daughter will not be affected, but she will have a 50 per cent chance of being a carrier like her mother.

Test yourself on Sex-Linked Genes

  • How are sex chromosomes different from autosomes?

  • Which of the following statements are true for X-linked genes?

  • What is the term given to the fact there is no role of dominance and recessiveness in X-linked genes?

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