|
Mendel's Hypothesis
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 ob
|
Quick Reply
