bio 1 ch 15

• Genes


  Hereditary units
• Chromosomes


  Location of Mendel's heritable factors
• Sutton, Boveri and others noted parallels between chromosome behavior and Mendel's proposed factors

  Around 1902
• First solid evidence associating a specific gene with a specific chromosome from Thomas Hunt Morgan


  Early 1900s
• Thomas Hunt Morgan


  • First native-born American to win the Nobel Prize, in 1933
  • Originally did not support Mendelian theory, but his work helped prove it was indeed true
  • Used the fruit fly, Drosophila melanogaster, as his model organism
• Drosophila melanogaster


  • Produces many offspring
  • A generation can be bred every two weeks
  • Has only four pairs of chromosomes
• Wild type


  Normal phenotype
• Mutant phenotype


  Trait alternative to the wild type
• Correlating behavior of a gene's alleles with behavior of a chromosome pair

  1. Mating male flies with white eyes (mutant) with female flies with red eyes (wild type)

  2. F1 generation all had red eyes

  3. F2 generation showed a 3:1 red to white eye ratio, but only males had white eyes
• The white-eyed mutant allele must be located on the X chromosome
• Sex chromosomes


  X chromosome and Y chromosome
• SRY gene


  Gene on the Y chromosome responsible for development of the testes in an embryo
• Sex-linked gene


  Gene located on either sex chromosome
• Y- linked gene


  Gene on the Y chromosome
• X-linked gene


  Gene on the X chromosome
• X- linked recessive trait


  • For a female to express, she needs two copies of the allele (homozygous)
  • For a male to express, he needs only one copy of the allele (hemizygous)
  • X-linked recessive disorders are much more common in males than in females
• X- linked recessive disorders in humans


  • Color blindness (mostly X-linked)
  • Duchenne muscular dystrophy
  • Hemophilia
• Duchenne muscular dystrophy


  • Dystrophin gene is largest gene in humans, 2.6 million bases
  • Protein links cytoskeleton to ECM
  • 427kDa
• Hemophilia


  • X-linked recessive disorder
  • Affects 1 in 5,000 men
  • All daughters of affected men are carriers of the gene, and their sons will have a 50% chance of contracting it
• Barr body


  Inactive X chromosome condensed in mammalian female cells
• X inactivation


  One of the two X chromosomes in each female cell is randomly inactivated during embryonic development
• XIST gene


  Gene involved in the inactivation of the X chromosome
• Linked genes


  Genes located on the same chromosome that tend to be inherited together
• How linkage affects inheritance

  1. Morgan crossed flies that differed in traits of body color and wing size

  2. P generation cross to generate dihybrid flies

  3. Testcross
• Genes on the same chromosome do not assort independently
• Genetic recombination


  Production of offspring with combinations of traits differing from either parent
• Parental types


  Offspring with a phenotype matching one of the parental (P) phenotypes
• Recombinant types/recombinants


  Offspring with nonparental phenotypes (new combinations of traits)
• 50% frequency of recombination is observed for any two genes on different chromosomes
• Crossing over


  Mechanism that occasionally breaks the physical connection between genes on the same chromosome
• Recombinant chromosomes bring alleles together in new combinations in gametes, increasing genetic variation for natural selection
• Nondisjunction


  Pairs of homologous chromosomes do not separate normally during meiosis, resulting in gametes with abnormal chromosome numbers
• Large-scale chromosomal alterations in humans and other mammals often lead to spontaneous abortions (miscarriages) or cause a variety of developmental disorders
• Plants tolerate such genetic changes better than animals do
• Crossing Over


  1. Morgan observed that although some genes are linked, nonparental allele combinations are still produced

  2. He proposed that some process must occasionally break the physical connection between genes on the same chromosome

  3. That mechanism was the crossing over of homologous chromosomes
• Recombinant chromosomes


  Bring alleles together in new combinations in gametes
• Random fertilization


  Increases even further the number of variant combinations that can be produced
• This abundance of genetic variation is the raw material upon which natural selection works
• Nondisjunction


  1. Pairs of homologous chromosomes do not separate normally during meiosis

  2. As a result, one gamete receives two of the same type of chromosome, and another gamete receives no copy
• Aneuploidy


  • Results from the fertilization of gametes in which nondisjunction occurred
  • Offspring with this condition have an abnormal number of a particular chromosome