Mendel's Hypothesis

One major difficulty with Darwin's theory of evolution by natural selection was caused by the belief in blending inheritance.

  • Traits of the parents are blended together to produce progeny
    • big crossed to little will produce intermediate sized children, etc.
  • Problem: if a new more fit variant appears in a population they will have to mate with a normal individual from the population and the progeny will be less fit than the parent. (How likely is it that Michael Jordan's children will be as good at basketball as he is?)
    • blending will produce a population where everyone is average, i.e., mediocore
    • A partial solution was to proprose that most evolution occurs in small populations (on islands, for instance) where the beneficial new traits wouldn't be diluted as much

While Darwin wrestled with how to make his theory work with blending inheritance, an obscure Austrian monk was discovering that the inheritance of traits was much stranger than everyone supposed.

Gregor Mendel

Gregor Mendel (1822-1884) was trained as a physicist and brought a quantitative approach to the study of inheritance. His studies were based on crosses between different strains of peas grown at his monastary. He soon discovered that many traits did not behave as would be expected from blending inheritance. In a cross of white flowered peas with purple flowered peas, for instance, the progeny would all have purple flowers, not a light purple that would be expected from blending. Even more bizarre was that crosses of the hybrid blue flowered peas could produce white progeny! With his physics background, Mendel decided to cross large numbers of plants to see if there was a pattern.

Mendel's Experiment

  • Mendel isolated true breeding strains of peas with distinctive traits. In a true breeding strain crosses of two individuals from the same strain (or of one plant with itself) produce progeny who all have the trait. Thus a cross of purple flowers with purple produce all purple flowered progeny, white x white = all white, etc. This flash animation from "DNA from the Beginning" shows how Mendel did his crosses - Children resemble thier parents
  • Initially, he looked at only one trait at a time (flower color, height, etc.) This animation from "DNA from the Beginning" shows the different traits and has pictures of Mendel and the monastary where he worked - Genes come in pairs
  • carefully controlled the breeding (used paintbrushes to trnasfer pollen from one plant to another, etc.)
  • kept careful records
  • studied a large number of progeny
  • followed the traits for several generations (he crossed the progeny of each generation with one another)
  • The first cross was between true breeding peas with white flowers and true breeding peas with purple flowers (the parentals)
    • all progeny had purple flowers (the F1 generation)
      • This was an important result as it meant blending was not true. Check out this section of "DNA from the Beginning" and try the quiz to see if you understand this result - Genes don't blend
    • crosses of the F1 plants produced some peas with purple flowers and some with white flowers
      • out of 929 peas, 705 had purple flowers and 224 had white flowers (the F2)
      • Mendel noted that this was a 3.15 to 1 ratio of purple to white
    • the white flowered F2 were true breeding but only 1/3 of the purple F2 were true breeding (the others produced a mixture of white and purple progeny)
  • Mendel repeated the experiment with six other traits, in one case examining over 8,000 progeny
    • in all cases, one trait dissappeared in the F1 and then reappeared in the F2 in a ratio of 1 to three to the dominant trait (the trait which disappears in the F1 Mendel called the recessive trait)
      • The next module of "DNA from the Beginning" covers dominance and has a good audio glossary with descriptions of some key Mendelian terms - Some genes are dominant

Mendel's Hypothesis

  • Each trait is determined by pairs of discrete physical units (now called genes)
  • Pairs of genes separate from each other during gamete formation (Law of Segregation)
  • There may be two or more alternative forms of a gene (alleles)
  • Sometimes one allele (called the dominant allele) can mask the expression of the other (recessive) allele
  • True-breeding organisms have two of the same alleles (homozygotes), hybrids have two different alleles (heterozygotes)
  • Which member of a pair of genes becomes included in a gamete is determined by chance (Law of Independent Assortment)

Implications

  • Solved Darwin's problem - an allele may blend with another allele to produce an intermediate phenotype but the allele is not lost or blended
  • The phenotype (the observed characteristics of an organism) is produced by the interaction of a genotype (the collection of all pairs of genes in the organism) with the environment
    • a one way street - genotype produces phenotype but phenotype does not produce the genotype (no Lamarkism)

Punnett Square

Mendel wasn't understood in his time because chromosomes and their behavior during mitosis and meiosis had not been discovered yet. An easy way to visualize what happens in a Mendelian cross is to use a Punnett square

  • Each individual has two versions of a gene (the two alleles) so we use a symbol to stand for each allele
    • Mendel used a single letter from the phenotype, a capital letter stood for the dominant allele, a small letter for the recessive allele
    • In the purple white cross the gene is named purple so the dominant allele that produces the purple flowers would be P while the recessive allele would be p
      • a homozygous plant with the purple phenotype would have a genotype of PP
      • a homozygous plant with the white phenotype would have a genotype of pp
      • a heterozygous plant with the purple phenotype would have a genotype of Pp (or pP)
      • a cross can be represented by x so PP x pp is a cross between purple and white true breeding peas
  • In the Punnett square the alleles are separated from one another (Law of Segragation) and put on one side the square (usually one individuals allels go on the top and the others go on the left side) the possible progeny are then produced by filling the squares with one alle from the top and one from the left to produce the progeny genotypes
  • PP x pp would look like this
       
      P
      P
      p
      Pp
      Pp
      p
      Pp
      Pp
  • And Pp x Pp
      P
      p
      P
      PP
      pP
      p
      Pp
      pp
  • Note that if you count the squares for the progeny in the second cross one out of four has the genotype PP, two out of four have Pp and one out of the four is pp. As the peas with either PP or Pp will have the same phenotype, purple flowers, this produces a ratio of 3/4 purple flowers to 1/4 white, the same as what Mendel observed.
  • Another cross, that Mendel used to determine the genotype of a hybrid plant, was the test cross. In a test cross you cross the plant of unknown genotpe with a homozygous recessive plant. There are two possiblities, the unknown will either be homozygous for the dominant allele
       
      P
      P
      p
      Pp
      Pp
      p
      Pp
      Pp

    In which case all of the progeny will have purple flowers, or the unknown is heterozygous and

      P
      p
      p
      Pp
      pp
      p
      Pp
      pp

    one half of the progeny will have purple flowers and one half will have white flowers.

This module from "DNA from the Beginning" covers Punnet (and Punnet squares), Bateson and the quiz goes over a dihybrid cross - Genetic inheritance follows rules.


This document is copyright of Jeff Bell
Last Update: Tuesday, August 24, 2004