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SEX LINKED INHERITANCE
▢ Definition: Inheritance of traits present on sex chromosomes
▢ Discovery:
Table 1: Sex-linked inheritance facts
Point | Data |
|---|---|
First X-linked trait observed by | T. H. Morgan |
Organism | Drosophila |
Observation | White-eyed fly among red-eyed flies |
Inheritance pattern | Criss-cross inheritance |
Transmission | Father → daughter → grandson |
▢ Key Rule:
- •X-linked gene transmitted from father to grandson through daughter
- •Pattern → criss-cross inheritance
▢ Human Sex-linked Inheritance:
❖ X-linked Inheritance:
◉ Colour Blindness / Red-Green Blindness / Daltonism:
▢ Cause: Defective recessive gene on X-chromosome
▢ Defect: Absence of colour-sensitive cells in retina
▢ Feature: Unable to distinguish red + green colour; appear grey
▢ Transmission: Colour-blind man → daughter → half of next generation
◉ Haemophilia / Bleeder's Disease / Royal Disease:
▢ Cause: Recessive gene on X-chromosome
▢ Defect: Lack of clotting factor in blood; Factor VIII
▢ Clotting Time:
Table 1: Blood clotting time
Condition | Clotting time |
|---|---|
Normal | |
Haemophilia | 30 minutes–24 hours |
▢ Special Points:
- •Common in royal families of England + Russia
- •Inheritance pattern described by Nasse → Nasse’s law
- •Haemophilic son → non-viable / does not attain sexual maturity
- •Males → sufferers
- •Females → carriers
◉ Muscular Dystrophy: Listed only