Gregor Mendel chose the garden pea (Pisum sativum) as a favourable experimental subject for his work: it is easy to grow, has a high number of offspring with different characteristics that can be easily observed. Mendel selected suitable pea varieties for his experiments for two years and finally decided on 22 varieties that differed from each other in 7 characteristics:


Flower colour (violet or white)


Seed shape (smooth or wrinkled)


Seed colour (green or yellow)


Pod shape (full or narrow)


Pod colour (yellow or green)


Placement of the blossom on the stem (middle or at the top)


Length of the stem (short or long)

First, he wanted to know what happens when you cross plants with different characteristics. Using scissors, he first removed the male sex organs (stamens) in the flowers of one plant to prevent self-fertilisation. Then, for pollination, he transferred the pollen of another plant with a brush. This way he always knew both parents of the pea seeds, which then developed in the pods.


Mendel noticed that all the descendants of the first generation had only one expression of the respective characteristic, e.g. only purple flowers. If he now crossed plants of this generation with each other again, the disappeared characteristic (white flowers) reappeared in the second daughter generation, namely in a quarter of the offspring. This 3:1 ratio occurred in all seven traits studied. Since Mendel worked with a large number of plants – in total he used 28,000 plants – he was able to statistically validate the results.

Thus he introduced two terms for explanation, which are still used today: Dominance and recessiveness. The superior, dominant violet colour masked the recessive white in the first generation. The suppressed, recessive white colour reappeared in the second generation. From this, Mendel concluded that peas have two units of inheritance for each trait, such as seed colour, flower colour or seed shape.

Today, these units are called alleles. One always comes from the father and the other from the mother. The combination of both alleles is called the genotype. The genotype determines the external appearance, the phenotype.


On the basis of his work and research, Mendel established three rules of inheritance. They were called Mendel’s Rules or Mendel’s Laws after him:

Mendel’s First Rule or Uniformity Rule

If you cross two pure-bred parents who differ in one characteristic, all offspring will be genotypically and phenotypically the same (uniform).

Mendel’s Second Rule or Splitting Rule

In a cross between heterozygous individuals, the offspring will split in both genotype and phenotype – and always according to a certain numerical ratio.

Mendel’s Third Rule or Independence Rule

With crosses of homozygous parents who differ in two characteristics (dihybrid inheritance), the respective hereditary traits are passed on freely and independently of each other to the offspring.

Mendels rules (Myosotis) | © Photograph by S. Metzing-Blau, Germany, Wikimedia Commons