Mendelism or Mendel’s Principles of Inheritance


Gregor Johann Mendel, a monk carried his experiments by crossing varieties of garden peas and recording his results in a monastery garden of a town in Moravia, from 1857 to 1865. In 1866 he published his important conclusions in regard to heredity in the Proceedings of the Natural History Society of Brunn. But unfortunately, the scientists did not pay much attention to his findings and thus his result remained unnoticed until 1900. About the beginning of the 20th century, 16 years after Mendel’s death, his laws were rediscovered by three European scientists, Correns, Hugo de Vries, and von Tschermak. The rediscovery created a sensation all over the scientific world. Mendel’s theory brought a dramatic change in the thinking about man and heredity. His basic rules of heredity are applicable to all forms of life.

Mendel is called the Father of Genetics.

The basic principles of inheritance as formulated by Mendel are collectively called Mendel’s principles of inheritance or Mendelism.

Monohybrid Cross:

Monohybrid Cross refers to hybridization where two forms of a single trait are brought together.

Mendel carried the monohybrid cross on garden pea plants (Pisum sativum) by artificially transferring the pollen grains of one plant to the stigma of another plant. The individuals produced to such parents are called hybrids. The first generation of hybrids is called as first filial generation (F1 generation).

He crossed pure tall pea plant with a pure dwarf pea plant and obtained the F1 generation with all tall plants. He noticed that in the F1 generation, only one of the contrasting characters (tallness) appears and the other remained hidden (dwarf). The character that appears in F1 individuals is called a dominant character and the character that is not expressed in F1 individuals is known as recessive.

Then, he intercrossed the F1 hybrids and obtained the hybrids of the second filial generation (F2). In the F2 generation, the tall and dwarf were obtained in the ratio of 3 : 1 (phenotypic ratio) but genetically, this ratio was recognized in three groups of plants viz. TT, Tt, and tt with ratio 1 : 2 : 1 (genotypical ratio).

monohybrid cross - Mendelism or Mendel's Principles of Inheritance

He came to two important conclusions from these experimental results

Law of Dominance:

When a cross is made between two homozygous (pure) individuals, only one of the two characters of the pair appears in the F1 generation. There is an appearance of both the characters in the F2 generation.


The characters are controlled by discrete units called factors and these occur in pairs. In a dissimilar pair of factors, one member of the pair dominates (dominant) and the other (recessive).

Law of Segregation:

The two allelomorphic characters of an individual do not get mixed up but they segregate during gametic formation. Each gamete receives only one character of the two allelomorphs and a paired condition is restored by random fusion of gametes during fertilization. It is also called the Law of Purity of Gametes.

Dihybrid Cross and Law of Independent Assortment:

The inheritance of two or more genes at a time, their distribution in the gametes, and in the progeny of subsequent generation is independent of each other.

The law can be deduced from a dihybrid cross. A cross is made between a pure round yellow seeded Pea plant (RR YY) with a wrinkled green seeded Pea plant (rr yy). The yellow colour is dominant over the green and the rounded seed shape over the wrinkled seed shape. F1 plants or hybrid plants are all round and yellow seeded. F1 plants are allowed to self breed and produce the F2 generation. F2 generation has four types of plants- round yellow, wrinkled yellow, round green, and wrinkled green in the ratio of 9 : 3 : 3 : 1. Each of the character if considered separately shows a ratio of 3:1 as found in the monohybrid cross.

Seed Colour:

Yellow 9 + 3 = 12

Green 3 + 1 = 4

Ratio between yellow and green = 12 : 4 or 3 : 1

Seed Texture:

Round 9 + 3 = 12

Wrinkled 3 + 1 = 4

Ratio between round and wrinkled = 12 : 4 or 3 : 1

The F2 ratio of 9 : 3 : 3 : 1 further shows two types of recombinants, wrinkled yellow and rounded green. They can be produced only if the alleles of the two different characters are free to recombine, i.e., assort and combine independent of each other. The same can be confirmed with the help of Punnett square or checkerboard.

dihybrid cross - Mendelism or Mendel's Principles of Inheritance

What is Punnett Square?

It is a graphical representation to calculate the probability of all possible genotypes of offsprings in a genetic cross.

All possible gametes are written on the top row and left columns. All possible combinations are represented in boxes, which generates a square output form.

Important Terms:

  • Hybrids: The offsprings produced after a cross between two opposite sexes of two different species or genera are called hybrids.
  • Hybridization: The crossing of two individuals of the same species or genus is called hybridization.
  • Homozygous and heterozygous: Bateson and Saunders (1902) coined the term homozygous and heterozygous. The diploid individuals possessing sets of similar traits or characters (TT or tt) are called homozygous. On the other hand zygote or diploid individuals carrying both the factors of a pair of alleles is called heterozygous or heterozygote. Heterozygote (Tt) arises when two individuals with different genetic traits are crossed.
  • Phenotype: Johanson (1909) coined the term to represent visible external characters of an organism.
  • Genotype: The genetic constituents of an individual is called genotype.
  • Monohybrid cross: The cross between two parents having only one pair of contrast characters is called monohybrid cross.
  • Dihybrid cross: The cross between two parents having two pairs of contrast characters is called dihybrid cross.

Biological Significance of Mendel’s Laws:

Mendel’s laws offer several advantages in producing hybrids of plants and animals of desired traits that normally do not exist in nature. Even in a single variety, the desired characters can be combined and maintained. Many disease resistant and high yielding varieties of crop and ornamental plants are being produced. Bases of success of all these varieties lie on Mendel’s laws. These laws form a base for predicting the outcome of crosses in eukaryotic organisms.

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