During the analysis of Mendel’s dihybrid and monohybrid crosses, it was noticed that for the determination of a single phenotypic trait of an organism, two alleles or allelomorphs of a single gene interact in various ways.

For instance, out of two allelomorphs of a single gene, one allelomorph might show simple (complete) dominance over the action of other (recessive) or both allelomorphs might have a partial or incomplete dominant relationship or both parallelograms might have an equal expression or codominant relationship.

These kinds of genetic interactions occur between two allelomorphs of a single type of gene and are usually termed as Intra-allelic or allelic genetic interactions.

These kinds of genetic interactions give the classical ratios of 3: 1 (monohybrid ratio) and 9 : 3 : 3: 1 (dihybrid ratio).

In addition to intra allelic genetic interactions, non-allelic or inter-allelic (genic) interactions also occur. In inter-allelic genetic interactions, the independent genes (non-homologous genes) located on the same or on different chromosomes interact with one another for the expression of a single phenotypic trait or an organism.

Thus, the genes are inherited as units but interact in some complex fashion to produce the trait. This is known as gene interaction.

Sometimes the interaction between alleles is more complex and phenotypic ratios completely different from 3: 1 and 9 : 3 : 3: 1 ratio are obtained. A few modifications depending on gene interaction are discussed as follows-

Lethal Genes (2: 1 Ratio)

l      There are several examples of conditions where a single gene may affect several characteristics, including mortality.

l      Certain genes, if present in organisms causes the death of those organisms during the early stages of development and are termed as lethal genes or factors.

l      The first case of lethal genes in the plant was discovered by Baur (1907) in Antirrhinum majus, in which a yellow-leaved dominant genotype called aurea when crossed (aurea × aurea), produced normal green plant as well as aurea heterozygous in the 2: 1 ratio.

Aurea homozygous lacked the ability to make chlorophyll after causing the death of an organism in which they are present in heterozygous condition.

l      Dominant lethal genes are lost from the population generally after causing the death of an organism in which they are present in heterozygous condition.

l      Recessive lethal genes are causative agents of death or an organism when they are in homozygous condition.

l      Cuenot (1905), soon after the discovery of Mendelism, studied the inheritance of colour and discovered the recessive lethal gene in mice.

l      Lethal genes may be dominant or recessive, for example, yellow coat colour in mouse is a heterozygous (Yy) character. l Yellow mice with homozygous dominants (YY) do not survive. Yellow coated mice never breed true. When inbred, these yellow mice always give a progeny of 2 yellow: 1 black instead of monohybrid 3: 1 ratio. The result of a mating between two heterozygous mice may be represented as follows-

l The homozygous offspring (YY), in the above cross, die because the gene Y in the homozygous condition has a lethal effect on the embryo. This monohybrid F2 ratio 1: 2: 1 is converted into a 2: 1 ratio due to the death of homozygous yellow. It is evident from the following cross –

Additive or Polymeric Genes (9: 6: 1  Ratio)

l      Different varieties of a common gourd (pumpkin), Cucurbita pepo, have long spherical or discoid fruits.

l      If one of the genes is dominant and the other is recessive, spherical fruits are produced.

l      When two varieties with spherical fruits are crossed, only discoid-fruited progeny are produced.

l      When both the genes are dominant, discoid-shaped fruits (gourd) are produced.

l      when both the genes are recessive, long fruits are produced.

l           In the F2 generation, discoid, spherical and long fruits are obtained in the ratio of 9: 6: 1, It is clear from the following crosses –