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.
  • Initially, he looked at only one trait at a time (flower color, height, etc.)
  • 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)
    • 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)

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

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.

For some practice with Punnet squares you can try the Punnet square application at the University of Cincinnati or work through some crosses with peas at the same site (the crosses are pretty basic so I would only recommend this if the Virtual Fly is too confusing for you).

Independent Assortment

After his initial experiments with individual traits, Mendel tried crosses with two traits at the same time, for instance, true breeding round and yellow peas crossed with true breeding wrinkled and green peas

As the F1 progeny all had round yellow peas round and yellow are both dominant, the cross was

RRYY x rryy to give all RrYy progeny

Crossing the F1 gave the following odd result

RrYy x RrYy

Results:

Observed

round, yellow

315

wrinkled, yellow

101

round, green

108

wrinkled, green

32

Total

556

If you examine the numbers, as Mendel did, you will notice that 416/556 of the plants had yellow peas, versus 140/556 with green peas. 423/556 had round peas and 133/556 had wrinkled peas. thus both show the normal 3:1 ratio that Mendel had observed when he studied the traits independently. Why 315/556 for round and yellow then? A ratio of .57? Where did that come from? Mendel reasoned that if 3/4 of the F2 had round peas and if 3/4 had yellow peas then if the traits were determined independently 3/4 x 3/4 = 9/16 (.5625) of the progeny should have both round and yellow peas. In a like manner 3/4 x 1/4 = 3/16 should have round and green peas, 1/4 x/ 3/4 = 3/16 should have wrinkled and yellow peas and 1/4 x 1/4 = 1/16 should have wrinkled and green peas. this can also be seen by counting up the squares in the punnet square, which will have 16 squares, as there are four possible combinations of alleles for each gamete

RY

Ry

rY

ry

RY

RRYY

RRYy

RrYY

RrYy

Ry

RRyY

RRyy

RryY

Rryy

rY

rRYY

rRYy

rrYY

rrYy

ry

rRyY

rRyy

rryY

rryy

Mendel concluded that

  • Which member of a pair of genes becomes included in a gamete is determined by chance (Law of Independent Assortment)

This result implied that the different genes were separate from each other (there is no connection between them, the round allele was not tied to the wrinkled allele in the F1 parents, for instance). While, as we will see later, this is not always true, the concept of genes as physical stuctures that were divided up independently of one another was important and is true for most genes.

Unfortunately, Mendel's results were ignored for over thirty years. Perhaps the biggest reason (other than the unfashionable use of mathmematics and probability in a biology paper!) was there there was no structure known in the cell that behaved in the odd manner that Mendel predicted for his genes. Whatever the genes were they would have to exist in pairs only and they would have to somehow separate during gamete formation to produce a new pair when two gametes fused to produce an embryo. In the ensuing thirty years a structure was discovered in the cell that behaved exactly as Mendel predicted for his genes. The structures were called chromosomes. In the next section we will look at how a special form of cell reproduction called meiosis separates the chromosomes just as Mendel had predicted for his alleles.

The following links contain additional information about Mendel and his work.

Bell CSU Chico Library
This document is copyright of Jeff Bell
Last Update: Wednesday, August 12, 1998