Meiosis and Chromosomes Notes
Chromosome Theory of Inheritance
- 1870's cytologists deciphered process of mitosis.
- 1890's meiosis documented. People noticed parallels between Mendelian factors and chromosomes.
- Early 20th century, Thomas Morgan working with Drosophila melanogaster demonstrated genes were located on specific chromosomes at specific locations.
- Drosophila was a good choice.
- Has only four chromosomes.
- Easy to raise
- Prolific with a short breeding cycle.
- Morgan discovered sex-linked genes - gene located on sex chromosomes
- These X-chromosome genes are expressed only on males (also called X-linked genes).
- Example: only Morgan's F2 male drosophlia had white eye color vs. normal red.
- This linkage confounds Mendel's independent assortment.
- Alleles are indicated as dominant by first capital letter
- Example: eye color allele w.
- The normal (wild-type) allele indicated with a + superscript.
- Example: w+ is the normal red eye-color allele.
- Example: w is the mutant white eye-color allele.
- Since each chromosome can have 1000's of genes, often genes associate.
- Linked genes are genes on the same chromosome.
- This linkage also confounds Mendel's independent assortment.
- Sometimes the linkage seemed to break, but only between homologous pairs of chromosomes.
- This genetic recombination is called crossover.
- The farther apart genes are the greater the chance of crossover.
- By examining frequency of crossover, the relative distance of genes was mapped.