Gregor Johann Mendel (1822 - 1884) was a member of an Augustinian order (Monastic) in Brunn Austria (Now part of Czechoslovakia).

Mendel began studying plant breading by trying to find the effects of crossing different strains of common garden pea.

He carried out his research with more precision than had yet been used. He also used the new science of statistics to analyse the results of his experiments. This use of mathematics to describe biological phenomena was a new concept.

Mendel did his pioneering work from 1856 to 1865 and his results were published in one paper (reports) in 1866. With the lack of communication at those times, it is not surprising that his work went unnoticed until 1900 when another group if scientists repeated the experiments and then found that Mendel had already developed the ideas some 40 years previous.

Out of Mendel's work came two "Laws" of inheritance:

1) Mendel proposed that heredity was controlled by paired factors that segregated when gametes formed and rejoined at fertilisation and,

2) The principle of independent assortment indicates that the segregation of one pair of factors, or Alleles, has no influence over the way any other pair segregates.

Mendel used 34 "true-breeding" strains of the common garden pea for his experiments. These strains differed from each other in very pronounced (visible) ways so that there could be no doubt as the results of a given experiment. Pea plants were perfect for such experiments since their flowers had both male (anthers) and female (pistils) flower parts and the flower petals never open therefore no foreign pollen could enter and back crosses (self fertilisation) was easy.

Mendel used seven different pairs of traits:

1) Seed form - round or wrinkled.

2) Colour of seeds - yellow or green (contents).

3) Colour of seed coat white or grey.

4) Colour of unripe seedpods - green or yellow.

5) Shape of ripe seedpods - inflated or constricted between seeds.

6) Length of stem - short 9 - 18 inches of long 6 - 7 feet.

7) Position of flowers - axial (on stem) or terminal (at tip of stem).

The Monohybrid Cross

If we take one of Mendel's traits e.g. seed colour, yellow vs. green and breed yellow x yellow we will always get yellow, or green x green will always give green. This is then described as "true breeding" plants.

When Mendel crossed a "true breeding" yellow seed plant with a "true breeding" green seed plant he obtained all yellow seed!

E.g.

yellow x green         Parental Generation

all yellow                 F₁ Generation (first filial)

Whatever the factor that caused the yellow colour it was more powerful than the factor for green colour. Mendel said that the yellow factor "dominated" the green factor.

We now recognise that these factors are Alleles genes with slightly different instructions on homologous chromosomes and that one allele is dominant to the other.

 Therefore the Y yellow allele is said to be the dominant allele while the y which is covered up is said to be the recessive allele.

NB Dominance when a dominant allele is present it will show its trait recessive allele only shows its trait when no dominant allele is around.

Homozygous: This genotype is one in which the alleles on homologous chromosomes are the same.

E.g. YY homozygous dominant

yy homozygous recessive

Heterozygous: This genotype is one in which the alleles on the homologous chromosomes are different.

Eg. Yy = heterozygous

Therefore our "true breeding" plants must have been homozygous. The resulting F₁ generation were heterozygous.

"The "factors" of inheritance are paired and segregated when gametes are formed." "Mendel's principal of segregation". We know of homologous chromosomes, alleles and meiosis, all of which explains Mendel's results.

The F₁ generation is allowed to "self fertilize":

Yellow X Yellow                 F₁ Generation

3/4 Yellow : 1/4 Green     F₂ Generation

 

Phenotypic ratio: 3/4 yellow : 1/4 green or 3 yellow : 1 green