Mendel’s Principles of Inheritance

Based on the results of Mendel's experiments, Mendel postulated the following laws of heredity.

1. Law of segregation or purity of gametes

At formation of gametes, the two chromosomes of each pair separate (segregate) into two different cell which form the gametes. This is a universal law and always during gamete formation in all sexually reproducing organisms, the two factors of a pair pass into different gametes. Each gamete receives one member of a pair of factors and the gametes are pure.

Mendel’s factors later came to be known as genes.

2. Law of dominance

During inheritance of many traits (like eye colour, flower colour, seed shape) is controlled by one pair of genes. When the two genes of a pair are of the same kind (brown colour of eyes, red colour of flower) the condition is termed as homozygous.

When a pair of chromosomes has the gene controlling the same feature (flower colour) in two different forms (red flower gene on one chromosome and white flower gene on another member of the pair (termed its homologue) the condition is termed heterozygous.

The factors or genes for red and white flower colour are alternative forms of the same gene, that is, the gene for flower colour. Such alternative forms of the same gene are termed as Alleles.

The second law of inheritance maintains that when the two genes of a pair, represent contrasting characters the expression of one is dominant over that of the other. Thus if both genes of an allele are for tallness (represented as TT) that is homozygous or one gene is for tallness and another for dwarfness (Tt), that is heterozygous, the pea plants will be tall.

The opposite of dominant gene is termed recessive gene. The recessive feature (dwarfness of the plant) is expressed only when both the genes of allele are in the homozygous condition (tt).

The law of dominance was found to be true in both monohybrid and dihybrid crosses in cases of all the seven characteristics studied by Mendel in the garden pea.

3. Law of independent assortment

In the inheritance of two features (each feature controlled by a pair of genes), genes for the two different features are passed down into the offspring independently. The segregation of one pair of factors is independent of the segregation of the factors belonging to any other pair of factors or allelic pair.

Reasons for Mendel’s Success

1. Mendel succeeded in postulating laws of inheritance because of his choice of experimental plant garden pea which has a short life cycle, has self pollinated bisexual flowers so that cross-pollination is not allowed and the true breeding behaviour of parents could be maintained. Because of the property of self pollination in garden pea plants, a large number of pure line of plants with several pairs of contrasting characters could be obtained in the same field.

2. His selection of traits: All the seven pairs of contrasting characters of pea plants considered by Mendel in his experiments showed complete dominance that helped Mendel to postulate the law of dominance and the law of segregation.

3. The factors for all the seven traits selected by Mendel for his experiments were either present on separate homologous chromosomes or if they were present on the same chromosome, they were apart so that the factors segregated independently & were not inherited together so that Mendel failed to discover linkage and crossing over.

4. Mendel’s methodology: His technique of experimentation also helped him in discovering the Laws of Heredity:

  1. Homozygous pure line plants with contrasting characters were crossed.
  2. Self pollination was prevented by removing stamens to bring about cross pollination between the desired parents.
  3. Female plants were dusted with pollen grains from another plant with the contrasting feature and were tied in a bag to prevent any further pollination.
  4. Seeds were collected from plants of different generations and sown in time.
  5. The results of different generations were maintained, and analysed statistically, by counting the individuals exhibiting different traits.
  6. He considered the inheritance of one character at a time, then he considered inheritance involving individuals differing in two contrasting characters.
  7. He performed reciprocal crosses and test crosses to confirm the results.