Unit – III A

Deviations from Mendel’s Experiments

  • Mendel performed his experiments with garden pea plant, which has traits or alleles having complete dominance and hence the laws of inheritanwasce were proved. Other scientists performed their experiments on different plants and animals and found deviations to Mendelian ratios.
  • Depending upon these experiments and observations, a different pattern of inheritance called gene interactions was discovered. This study is known as Post – Mendelian genetics or Neo-Mendelian genetics.

Incomplete Dominance:

  • In this case, both the genes of an allelomorphic pair express themselves partially. One gene can not suppress the expression of other completely.
  • e.g. four o’ clock plant, snapdragon.
  • Thus in this case the pair is not one dominant and other recessive. In such cases, there is an intermediate expression in F1 hybrid. In F2 generation, both the parent traits reappear again.


  • The phenomenon of incomplete dominance can be explained on the basis of Mendelian segregation. In complete dominance, the recessive factor can not express, but in incomplete dominance both alleles have equal chance to express, hence we g et hybrid intermediate in F1 generation.


In this case, both the genes of an allelomorphic pair express themselves equally in F1 generation. Such alleles express themselves independently even if present together in hybrids are called co-dominant alleles.

  • e.g. coat colour of cattle.


  • When black cattle is crossed with white cattle, the F1 generation has roan coat colour where black and white patches appear separately. When F1 generation is self-crossed, F2 Generation shows 4 phenotypes with ratio White :Both : Black = 1:2:1 and Genotypic ratio WW:WB:BB = 1:2:1

Gene Interactions:

  • It was observed that the phenotypic expression of a gene can be modified or influenced by the other gene. This phenomenon is called gene interaction. There are two types of gene interactions
  • Intragenic (Interallelic)Interaction:

    It occurs between alleles of the same gene. e.g. incomplete dominance, co-dominance, and multiple alleles.

  • Intergenic (nonallelic) Interaction:

    It occurs between the alleles of different genes on the same or different chromosomes. e.g. Pleiotropy, poygene, epistasis, supplementary and complementary genes.

Complementary Genes:

  • In this type of interaction, two separate pairs of genes interact to produce the phenotype in such a way that neither dominant is expressive unless other one is present. Thus the effect of one dominant is expressed if and only if other dominant complements it. This type of genes are called complementary genes. This inheritance was discovered by W. Bateson and R. C. Punnett. in sweet pea (Lathyrus odoratus).
  • When two certain white flowered varieties of sweet pea are crossed with each other. They produced F1 plant with red flowers. The F2 generation obtained by self-pollination of F1 generation,  The ratio of red-flowered plants to white-flowered plant is found to be 9:7, which was different than dihybrid ratio of 9:3:3:1.
  • The red colour in flower of sweet pea plant is produced by a pigment called anthocyanin. Its formation depends on two independent factors (C and P). Both these factors must be present to produce the pigment.

Deviations from Mendel’s Experiments

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