Lecture on Genes and Chromosome Theory of Inheritance

Introduction to Genes and Chromosomes

Mendel's early work on inheritance factors called genes.
Early 20th-century progress: chromosomes observed under microscopes.
Development of chromosome theory of inheritance.
- Studies of mitosis and meiosis explained Mendel’s laws of segregation and independent assortment.
- Chromosomes carry genes at specific loci.

Morgan's studies on fruit flies provided evidence for chromosome theory.
Fruit flies have 4 pairs of chromosomes, including sex chromosomes (X and Y).
Discovery of eye color linked to sex (X-linked gene).
Experiments showed red (wild type) is dominant over white (mutant), with white eyes appearing only in males.
Explanation of experimental breeding:
- F1 generation: all red eyes.
- F2 generation: 3:1 ratio of red to white; white eyes only in males.
- Inferred that eye color gene is located on the X chromosome.

Corroborated the chromosome theory of inheritance.
Different systems of sex determination:
- X/Y system: as in humans and fruit flies.
- X-0 system: found in insects like grasshoppers (males have one X, females have two).
- Z-W system: in birds, some fish, and insects (females ZW, males ZZ).
- Haplo-diploid system: in bees and ants (unfertilized eggs develop into males, fertilized into females).

Example of color blindness (X-linked disorder).
- Normal allele: X with superscript N (XN).
- Recessive allele: X with superscript n (Xn).
Crossings and probabilities:
- Homozygous dominant female and colorblind male: 50% heterozygous female, 50% hemizygous dominant male.
- Heterozygous mother and dominant father: 25% homozygous dominant, 25% heterozygous, 25% hemizygous dominant, 25% hemizygous recessive.
- Carrier mother and colorblind father: 25% carrier, 25% hemizygous dominant, 50% colorblind, including rare colorblind females.