Gene interaction is the modification of normal phenotypic expression of a gene due to either of its alleles or other non-allelic genes, which is known as gene interaction.
Or, when the expression of one gene affected by the presence or absence of another gene/allele in an individual, it is known as gene interaction.
Gene interaction is of two types
1. Intragenic Gene interaction
In intragenic, two alleles of a gene that are present on the same gene locus on the two homologous chromosomes, react to produce a modified phenotype.
Intragenic Gene interaction can be seen in the following
(i). Incomplete Dominance:
It is the phenomenon where the dominant allele does not completely express itself. Or, when a dominant allele, does not completely mask the effects of a recessive allele, it is known as Incomplete dominance.
- This was first studied in flower colour of mirabilis Jalapa or four O`clock plant.
- The phenotypic, as well as genotypic monohybrid ratio in F2 generation in incomplete dominance, is 1:2:1 i.e., pure dominant: hybrid: pure recessive.
- E.g., when red and white-flowered plants are crossed, pink-flowered plants are obtained in the F1 generation.
It is the phenomenon of two alleles lacking a dominant-recessive relationship where both of them express themselves together and equally in the organisms. This results in offspring with a phenotype that is neither dominant nor recessive.
A typical example showing codominance is the ABO blood group system in humans. The codominant alleles are able to express themselves independently when preset alone. The phenotypic ratio is 1: 2: 1.
(iii). Multiple Alleles:
These alleles are multiple forms (more than two alternatives) of a gene that occur on the same gene locus, distributed in different organisms in the gene pool with an organism carrying only two alleles, and gamete contains only one allele.
ABO blood group system in human beings is an example of both co-dominant and multiple alleles.
- In human beings, the blood groups are determined by two types of antigens present in the surface coating of red blood cells – A and B.
- In the human population, 3 different alleles for ABO blood group systems are found lA, lB, and lO or i. IA and lB are mutant alleles and are dominant over Io or i, which is a wild allele.
- IA and IB are responsible for A and B antigens (glycoproteins) while Io or i does not produce any of these A or B antigens.
- A person is having only two of these three alleles and blood type can be determined by their antigen types. Six genotype combinations are possible with these three alleles.
(iv). Lethal Genes:
They are genes that control some vital function of the organism and cause the death of the organism either in homozygous recessive or homozygous dominant condition.
They are usually a result of mutations in genes that are essential to growth or development. Lethal alleles may be recessive, dominant, or conditional depending on the gene or genes involved.
2. Intergenic Gene interaction
Modification of the effect of a gene under the influence of a non-allelic gene i.e., a gene at the different locus is termed as intergenic interaction.
It may be expressed in the following forms:
(i). Complementary genes:
Complementary genes are the genes that are present on different genetic loci but interact with each other to express a single character in combination. They both together produce a particular phenotypic trait in an individual.
(ii). Supplementary genes:
Supplementary genes are two non-allelic genes, in which the first gene can produce its effect whether, the second gene is present or not, but the second (supplementary) gene produces its effect only in the presence of the first gene.
Epistasis is the interaction between genes present at two separate loci in which one gene suppresses or masks the expression of other genes.
- Dominant epistasis: When a dominant allele at one locus can mask the expression of both alleles (dominant and recessive) of another gene at another locus, it is known as dominant epistasis.
- Recessive epistatic: Recessive epistasis is a form of epistasis in which a pair of recessive alleles of one gene can cause a masking effect on the expression of alleles of another gene at another locus.
(iv). Duplicate genes:
Two identical genes showing the same phenotypic action but localized in different regions of a chromosome or on different chromosomes, it is known as Duplicate genes.
(v) Pleiotropic genes:
When a gene affects many aspects of phenotype or controls several phenotypes. It is said to be a pleiotropic gene and this phenomenon is called pleiotropy.