Meiosis

Goal: to understand the processes underlying Mendel's "laws" of inheretance.
vocabulary: diploid, haploid, chromasome, homologous chromasomes

  1. All cells in a diploid organism have 2 copies of each chromasome
    1. n=number of distinct chromasomes
    2. How many copies of each gene in each cell of a diploid organism?
  2. Offspring must have the same number of copies as their parents
    1. Therefore gametes must have only 1 copy of each chromasome
  3. Mitosis = production of new cells with same number of chromasomes as parent cell
    1. chromasomes are copied,
    2. new and old copies are separated into two daughter cells
  4. Meiosis = production of new cells with 1/2 number of chromasomes, one copy of each
    1. proceeds through two cell devisions
      1. chromasomes are copied
      2. meiosis I
        1. homologous chromasomes pair up
        2. parent cell divides, separating (duplicated) homologous chromasomes
      3. meiosis II
        1. daughter cells divide, separating duplicates of chromasomes (sister chromatids)
    2. result = four haploid cells
    3. fertilization = combination of two haploid cells to form new diploid cell
  5. Meiosis and Mendel's "laws"
    1.  separation of homologous chromasomes in meiosis division I explains character segregation
    2. random sorting of paternal and maternal chromasomes during meiosis division 1 explains independent assortment
  6. Modifications to Mendel's "laws"
    1. non-independent assortment, or linkage
      1. crossing over during meiosis phase 1
    2. sex determination
    3. gene x environment interactions
    4. quantitative traits
Study questions 1. Crossing over, separation of maternal and paternal chromasomes, and separation of chromatids, all are believed to occur randomly. Which of the following are every-day examples of random processes?
  1. selection of some individuals for extra security checks at airports
  2. selection of a page in a book "with your eyes closed"
  3. selection of a bingo chip out of a closed box
  4. selection of an individual using "enie-meenie-miney-moe"
  5. selection of a treatment in an experiment with "a flip of a coin"
2. Using what information you have from lecture and the book, describe how crossing over could result in genetic errors (or mutations)

3. Design an experiment that would test the models:

  1. fly weight at maturity is determined by the environment
  2. fly weight at maturity is determined genetically
specify the treatment(s) and genotype(s) you would want to use, and the data that would cause you to reject each model.

4.  Using the butterflies described in the genetics problem set, you have homozygous butterflies with black forewings (BB) with white stripes (ss), and homozygous butterflies with yellow forewings (bb) and no stripes (SS).
(A) When you cross these parents (BBss x bbSS), you get offspring that are all black with no stripes.  What is their genotype?
(B)  When you cross the offspring to each other, you get the following results:  25% black with white stripes, 25% yellow without stripes, and 50% black with no stripes.   How is this different from expectation, and what explanations (models) do you have to explain these results?

5.  (A) A person of genotype and blood type AA marries a person of genotype and blood type BB.  What is the genotype and blood type (phenotype) of their offspring?
(B) A  person of blood type AB marries and produces children with a person of bloodtype B (genotype BO).  What is the probability that one of their children will have blood type B?  What is the probability that one of their children will have genotype BB?

Answers