Genetics and Inheritance

Thomas Hunt Morgan linked Mendelian inheritance to chromosomes.
Used Drosophila melanogaster (fruit fly) as the experimental organism due to:
- Rapid breeding (new generation every 2 weeks).
- Only four pairs of chromosomes (three autosomes and one sex pair).
Discovery of mutant phenotype (white eyes) led to:
- Conclusion of eye color gene's location on the X chromosome.
- Established notation for alleles (e.g., w for white eyes, w+ for red).

Traits linked to sex chromosomes behave differently than autosomal traits.
Males (XY) inherit X-linked traits from their mothers; females (XX) can inherit from both parents.
Recessive X-linked traits more common in males due to hemizygosity (only one X).
- Example: Color blindness is X-linked; a male with the recessive allele will express the trait.
Notable X-linked disorders:
- Duchenne muscular dystrophy: Affects muscle coordination due to absence of dystrophin.
- Hemophilia: Lack of blood clotting proteins, historically prevalent in European royal families.

Females have two X chromosomes; one is inactivated (Barr body) during early development.
Mosaicism observed in traits linked to X chromosomes:
- Heterozygous females can express both alleles in patches of cells.
- Example: Tortie cats show different coat colors based on X chromosome active allele.
XIST gene plays a role in X inactivation through RNA interaction with the chromosome.