T H Morgan while working on eye color in Drosophila, found that the gene for eye color did not follow the Mendelian pattern of Inheritance.
- This was due to the gene being present on the X-chromosome.
- Genes present on the X-chromosome are known as Sex-linked genes, the characters are sex-linked characters.
- Sex-linked Inheritance is the study of inheritance of sex-linked characters.
- Genes present on Y-chromosome are Holandric genes and they are restricted to the male sex only.
- Females have XX chromosomes while males have XY chromosomes.
- Males are hemizygous because they have only one allele of X-chromosome. Males will express the character of any gene on the X-chromosome whether it is dominant/recessive.
Examples of Sex-Linked Inheritance:
- Also referred as Bleeder’s disease or the Royal disease.
- Occurs due to a sex-linked recessive gene.
- Clotting of blood does not occur even to minor injuries.
- Haemophilia is of 2 types: Haemophilia A & Haemophilia B/Christmas disease.
- Haemophilia A occurs due to the absence of Factor VIII, Haemophilia B occurs due to the absence of Factor IX.
- It expresses in the female when both the X chromosomes have the recessive gene, XhXh, while XXh is a carrier female.
- In males, it expresses when the single X-chromosome has the recessive gene XhY.