Ch. 13

• Mendelian inheritance is grounded in the behaviour of chromosomes
• Early 1900s: Genes are located on chromosomes
• The behavior of chromosomes during meiosis was said to account for Mendel's laws
• Mendelian laws


  • Segregation
  • Independent assortment
• Chromosome theory of inheritance

  • Mendelian factors have specific locations on chromosomes
  • Chromosomes undergo segregation and independent assortment
• Modern genetics begins in early 1900s with the rediscovery of Mendel's work
• Multiple geneticists independently worked on Mendel's work with various model organisms and duplicated his findings
• Walter Sutton (American) & Theodor Boveri (German) also supported Mendel with their chromosomal theory
• Mendel 1st presented his work at a conference in 1865
• Mendel's work was largely ignored
• Sutton and Boveri's theory

  Chromosomes carry genes and are the units of heredity; this is consistent with Mendel's Laws
• During meiosis


  • Homologous chromosome pairs migrate as discrete structures that are independent of other chromosome pairs
  • Chromosome sorting from each homologous pair into gametes is random
• Sperm and egg differ in genetic content, but they have the same number of chromosomes
• Mom and dad contribute equally - each gamete made contains half of their chromosome complement
• Gametes (and their chromosomes) combine during fertilization to produce offspring with the same chromosome number as parents: (n+n=2n)
• Fruit flies (Drosophila melanogaster)

  • They breed at a high rate
  • A new generation can be bred every 2 weeks
  • They have only 4 pairs of chromosomes
  • Super easy to deal with in a lab setting
• Wild type (wt)


  "normal" common phenotypes in the fly populations
• Mutant phenotypes


  Traits alternative to the wild type
• The F2 generation showed a typical Mendelian 3:1 ratio of red eyes to white eyes. However, no females displayed the white-eye trait; they all had red eyes. Half the males had white eyes and half had red eyes.
• The mutant white-eye trait (w) is recessive to the wt red-eye trait (w+)


  In any XX/XY system, males are always more prone to x-linked diseases. They have no chance of "hiding" behind being a carrier (het).
• Since the recessive trait was expressed only in males in the F2 generation, the eye-color gene must be located on the X chromosome
• Sex-Linked Genes


  • A gene located on either sex chromosome is called a sex-linked gene
  • Usually associated with X chromosome but can be Y chromosome
  • Exhibit different patterns of inheritance
• Types of sex-linked inheritance

  • X-linked Dominant (Females)
  • X-linked Dominant (Males)
  • X-linked Recessive (Females)
  • X-linked Recessive (Males)
• Linked genes tend to be inherited together and are physically located close to each other on the same chromosome
• This violates the Law of Independent Assortment
• Recombination


  The result of crossing over, where new combinations of the linked traits can form
• Recombination Frequency (RF)


  The percentage of offspring that show new combinations of the linked traits
• When 50% of offspring are recombinants there is a 50% recombination frequency (RF)
• Linked genes have a RF of less than 50%
• Recombination frequency (RF)


  Related to the distance between genes on the chromosome
• Morgan discovered that genes could be linked, but the appearance of recombinant phenotypes made the linkage appear incomplete
• Homologous recombination (crossing over) was the mechanism that occasionally broke the physical connection between genes on the same chromosome
• Calculating Recombination Frequency (RF) - Example

  1. In Drosophila, true-breeding normal-winged, & eyed flies are crossed with true-breeding curled-wing, eyeless flies. All of the offspring have normal wings & eyes. In a testcross of these F1 individuals, the following offspring were obtained: 628 normal wings, eyes, 116 normal wings, eyeless, 655 curled-wings, eyeless, 101 curled wings, eyes

  2. Determine the frequency of recombination between the locus for wing shape & the locus for having eyes
• Recombination Frequency (RF)


  Can be expressed in centimorgans (cM) - a genetic distance, not a physical distance
• Genetic Maps


  An ordered list of the gene loci along a particular chromosome
• Linkage Maps


  The actual map of a chromosome based on the RF or cM
• The further apart genes are on a chromosome, the more likely a recombination event will occur between them
• The closer genes are, the lower the recombination frequency
• X Inactivation


  In mammalian females: One of the two X chromosomes in each cell is randomly inactivated during embryonic development (via packaging)
• XIST


  1. inactive specific transcript