CHAPTER 2 - MENDELIAN ANALYSIS

  1. MENDEL’S LIFE
    1. Born 1822 – Heinzendorf, Moravia (then part of Austria)
    2. Novice at 21, priest at 25 in Brunn Augustinian monastery
    3. Attended U. of Vienna where he studied chemistry, zoology, botany, entomology, paleontology, and physics
    4. Kept bees, mice, amateur meteorologist, kept records all his life (records of sunspots, groundwater level via monastery well, detailed report of tornado), director of institute for deaf mutes, chairman of a mortgage bank, became abbot of monastery
    5. Began experiments with garden pea, Pisum sativum in 1854-1856
    6. "Experiments in Plant Hybridization" first published in Brunn Society for Natural History in 1866.
  2. MENDEL’S EXPERIMENTS
    1. Mendel started his experiments during a period in history when the prevailing idea was that the characteristics of an organism were due to the blending of the traits from each parent (blending inheritance). His work suggested otherwise though. Mendel proposed the idea that an element determined a particular characteristic of an organism. This theory that an element/ factor/ gene controlled heredity was called particulate inheritance. Now it is known that that the gene is the fundamental unit of heredity.
    2. Mendel’s work was not appreciated at the time, and it was rediscovered by Hugo deVries, Carl Correns, and Erik von Tschermak in the early 1900’s.
    3. General considerations why Mendel’s expts. with garden peas were successful
      1. Garden pea which was easily manipulated so that controlled matings could be done. He could easily control pollination.
      2. Pea plants had a short generation time.
      3. Mendel chose easily identifiable characteristics to examine in the pea plants. Later on we will also see that he was lucky in his choices in that they are simple cases of inheritance, uncomplicated by other factors.
      4. Mendel carefully quantitated his results.
      5. Pea plants generated a large number of progeny, which was good for statistical analysis.
      6. Pure breeding lines had been established. Pure lines meant that the trait of interest remains unchanged generation after generation in self fertilized plants
  3. CHARACTERS OR TRAITS THAT MENDEL EXAMINED.

    4. (From: AN INTRODUCTION TO GENETIC ANALYSIS 6/E BY Griffiths, Miller, Suzuki, Leontin, Gelbart ã 1996 by W. H. Freeman and Company. Used with permission.)

    **Different forms of each character are called phenotypes.**

  4. EXP. SET #1 (THE MONOHYBRID CROSS)
    1. Trait was seed color

      2. Y=yellow and G=green

      parental generation Y x G ------------->

      F1 (first filial) generation were all Y ----selfed-----> F2 were 3Y and 1G

      Among the F2 progeny that were selfed
      1/4 Y --------->all Y
      2/4 Y --------->3 Y and 1 G
      1/4 W --------->all G

      Therefore the Y plants in the F2 were really all not the same because they gave different type progenies and the underlying ratio if we consider what type progeny they give was 1 true breeding Y: 2 mixed breeding Y: 1 true breeding G

    2. Mendel found this same pattern for all the traits he examined
    3. What did all of Mendel’s monohybrid crosses lead him to propose?
      1. The particulate theory of inheritance: Characters are determined by discrete units (genes) that are transmitted from parent to progeny in gametes. Each pea plant has two factors which control each trait.
        1. No blending because F1 progeny did not have intermediate color
        2. Reappearance of G in F2 which suggested that even though the parents were Y, they still had the something that gave them the directions to produce G.
      2. One factor of the pair is dominant over the other.
      3. Mendel's First Law or the Principle of Equal Segregation: Each factor independently segregates into the egg/sperm. This is: 2 members of a gene pair segregate (separate) from each other into gametes. 1/2 the gametes carry one member of the pair, the other 1/2 the gametes carry the other.
      4. The union of the gametes is random.
    4. Testing the Principle of Equal Segregation

      5. Mendel set up additional crosses to test his model. He made predictions based on the Principle of Equal Segregation and then examined the progeny of the crosses to see if the predictions held up. The most important cross is the test cross (cross to a pure breeding recessive)

      Trait was seed color again (Y=yellow and G=green)

      parental generation Y x G -------------> F1 generation were all Y

      crossed F1 Y with G

      Based on Mendel’s laws

      1. Trait for seed color was controlled by a pair of factors (genes) were Y = yellow and y = green
      2. Yellow is dominant to green
      3. Equal segregation of factors

    ***Phenotypic ratio of test cross is 1:1 for Mendelian inheritance

    (From: AN INTRODUCTION TO GENETIC ANALYSIS 6/E BY Griffiths, Miller, Suzuki, Leontin, Gelbart ã 1996 by W. H. Freeman and Company. Used with permission.)

  5. RELATING MENDEL’S EXP. TO TODAYS GENETIC TERMS
  1.   EXP. SET #2 (THE DIHYBRID CROSS)
    1. Trait 1 was seed color: (either yellow or green)
      Trait 2 was seed shape: (either round or wrinkled)

      2. (From: AN INTRODUCTION TO GENETIC ANALYSIS 6/E BY Griffiths, Miller, Suzuki, Leontin, Gelbart ã 1996 by W. H. Freeman and Company. Used with permission.)

      F1 ratio told Mendel that the dominance of one trait is unaffected by another trait.

      At first, F2 ratio seemed really confusing; however, Mendel carefully analyzed each trait separately and found that color was 3:1 and shape was 3:1. This is the same phenotypic ratio Mendel saw for the monohybrid crosses! So Mendel proposed his 2nd law or the Law of Independent Assortment: During gamete formation the segregation of one gene pair is independent of other gene pairs.

    2. There are several ways to graphically represent this. It is important to understand all of them.
      1. Punnett Square

        2. (From: AN INTRODUCTION TO GENETIC ANALYSIS 6/E BY Griffiths, Miller, Suzuki, Leontin, Gelbart ã 1996 by W. H. Freeman and Company. Used with permission.)

      2. Branching method
      3. Statistical method

        4. Used the laws of Probability (Probability of an event is its frequency of occurrence over a very large number of trials.)

        or Probability = # of times event expected to happen  /  # of opportunities for it to happen (or # of trials)

        For instance: With a quarter that you toss there only two possibilities if you toss it it lands head side up or tails. Doing this many many times results in very nearly 1/2 times heads and 1/2 times tails. With a die, the probability of rolling a 4 is 1/6.

        1. Product Rule: The probability of two or more independent events (i.e A and B) occurring simultaneously is the product of their separate probabilities. Here the KEY WORD IS AND.

          2. P(A and B) = P(A)*P(B)

          For example, with a pair of dice the probability of getting two 4's (one 4 AND another 4) is P(4)*P(4) = (1/6)(1/6) = 1/36

          For the genetic cross RrYy X RrYy
          The probability of rryy = P(rr)*P(yy) = (1/4)(1/4) = 1/16

        2. Sum Rule: The probability of one or another of a set of mutually exclusive events occuring is the sum of the probabilities of the separate events. Here the KEY WORD IS OR

      P(A or B) = P(A) + P(B)

      With a pair of dice the probability of getting two 4's or two 6's on each dice is P(4)*P(4) + P(6)*P(6) = (1/6)(1/6) + (1/6)(1/6) = 1/18

      For the genetic cross RrYy X RrYy
      The probability of rryy or RRYY =
      P(rr)*P(yy) + P(RR)*P(YY) = (1/4)(1/4) + (1/4)(1/4) = 1/8

    3. Testing the hypothesis...... with a test cross

      4. F1 RrYy X rryy ---------->
      1 yellow/round RrYy
      1 yellow/wrinkled Rryy
      1 green/round rrYy
      1 green/wrinkled rryy

  2. .  MENDELIAN GENETICS IN HUMANS:

    3. Analysis is done via a pedigree analysis (analysis of a family history to find patterns of genetic transmission of traits.

    1. Rare Recessive Traits

      2. (because it is rare, unrelated individuals who do not have affected children are presumed homozygous dominant)

      1. Parents not usually affected because it is rare and recessive.
      2. Unaffected parents can produce affected children if both are heterozygous.
      3. Two affected parents always have affected offspring because they are homozygous recessive.
      4. Approx. 1/4 of siblings of affected individuals are affected provided that parents were heterozygous (based on 3:1 Mendelian ratios).
      5. Recessive traits often result from consanguineous matings.
      6. Examples PKU (phenylketonuria), CF (cystic fibrosis), and albinism

    2. Rare, Dominant Traits

      3. (because it is rare, assume affected people of unknown genotype (either AA or Aa) are Aa.  An affected person must have at least one A allele because they are affected.  The assumption that the second allele is an "a" is made because the A allele is RARE and so it is more likely that they would have an "a" for the second allele vs. an "A"

      1. Trait occurs every generation .
      2. At least one parent of an affected child is affected.
      3. With one parent affected, approx. ½ of the children are affected (because of Mendelian ratios)
      4. Unaffected individuals cannot produce affected offspring (because they are homozygous recessive).
      5. Two affected parents can produce unaffected offspring if they are both heterozygotes and pass on the recessive alleles.
      6. Examples include achondroplasia (dwarfism), Huntington’s Disease, Polydactyly (extra digits), Brachydactyly (short fingers).

      .

    3. Normal phenotypic variation
      1. Continuous – phenotypes arranged around a continuous spectrum.
      2. Discontinuous – phenoytpes arranged into distinct classes
        1. Polymorphism = a set of two or more common alternative phenotypes (i.s PTC tasting)

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