Meiosis and Chromosomes Notes

Chromosome Theory of Inheritence

• 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.

Sex-linked Genes

• 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.

Symbols

• 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.

Linked Genes

• 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.

Crossover

• 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.