Sex-Linked Inheritance:

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.

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, X^hX^h, while XX^h is a carrier female.
In males, it expresses when the single X-chromosome has the recessive gene X^hY.

Clotting factors:

The clotting factors are a group of chemicals in the blood which is responsible for the formation of blood clot.

Clotting factors are usually inactive but once there is an injury to the wall of the blood vessel, they get activated.

The function of clotting factors is to trigger the formation of a blood clot and stabilize it for as long as necessary.

Clotting factors are therefore known as procoagulants.

MNEMONIC for Clotting Factors:

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Factor I – Fibrinogen; common pathway; converted to fibrin.

Factor II – Prothrombin; common pathway; converted to thrombin that converts fibrinogen to fibrin.

Factor III – Tissue factor (TF) or thromboplastin; extrinsic pathway.

Factor IV – Calcium ions (Ca²⁺); always present, needed for coagulation.

haemophilia