Lesson 4

• Thomas Hunt Morgan: non-random segregation of linked genes
• THM says that genes are arranged in a linear order
• The closer the genes are = higher probability to be inherited
• Crossing over is the exchange of parts between homologous chromosomes
• Sex:

  Heterogametic sex of species = ↓ crossover frequencies
• Maternal age:

  ↑ maternal age increases = ↓ crossing over
• Temperature:

  Cooler or warmer than 22C = ↑ crossing over
• Nutrition (young drosophila):

  ↑ calcium = ↓ crossing over
• Chemical effects:

  ↑ injection of antibiotics (mitomycin & antinomycin D) = ↑ crossing over
• Radiation:

  ↑ exposure in X-ray = ↑ crossing over
• Barr Body is discovered Murray Barr
• Barr body is observed in interphase nucleus if XX females
• No. of Barr Body is one less than the number of X chromosomes
• Mary Lyon says that Barr Body is an inactive X chromosome
• Any person regardless of X number chromosomes has only 1 active X chromosome
• McClung showed association of sex chromosomes
• Linkage is the natural result of the number of genes exceeding the number of chromosomes
• The principle of independent assortment assumes that gene pairs are on different homologous chromosome pairs and thus can randomly recombine
• Incomplete linkage:

  parental types >50%

  recombinant types <50%
• How does recombination occur?

  • Homologous chromosomes undergo synapsis
  • They align and form 4 chromatids
  • Crossing over occurs between nonsister chromatids
  • Completed.
• Linkage group = physical association of genes on a chromosome
• Chromosome number of dog:

  2n=76
• Chromosome number of corn:

  2n=10
• Chromosome number of Fruit fly:

  2n=8
• Percent (%) recombination is used as a measurement of distance since the frequency of crossing over between two genes
• If genes are closer, chance of recombination is lower
• The higher the % recombination, the farther apart the genes are and vice versa
• Strength of linkage is measured by coefficient of coincidence (cc) which estimates how the actual DCO coincides with theexpected DCO.
• 0 (no coincidence)

  1 (complete coincidence)
• Sex Linkage is seen in genes found on sex chromosomes.
• Sex chromosomes carry other genes aside from genes for sex determination.
• X-linked Inheritance:

  due to X-linked recessive or dominant genes.
• X-linked recessive traits:

  colorblindness

  hemophilia

  absence of central incisors

  congenital deafness

  congenital cataract
• Y-linked inheritance:

  genes show holandric transmission or father to son transmission
• 3 ways for sex determination:

  • Genetic
  • Environmental
  • Chromosomal
• Murray Barr observed females of humans, cats, and mice attached to the nuclearmembrane at interphase.