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The gene interaction in which the gene present in one gene locus interacts with the another gene present in another gene locus of the same or different chromosomes for the expression of phenotypic character is called intergenic or non-allelic gene interaction.
Some of the gene interactions are:
The suppression or hiding the effect of a gene of one locus by the gene of another locus of the same chromosome is called epistasis, The gene that suppresses the effect of a gene of another locus of the same chromosome is called epistatic gene or inhibitory gene. The gene which is suppressed by the gene of another locus of the same chromosome is called the hypostatic or suppressible gene.
So, epistasis is an example of intergenic gene interaction. The epistatic gene and the hypostatic gene may be dominant as well as recessive. Epistasis is of two types:
a) Dominant epistasis: The epistasis in which dominant gene of one locus suppresses the activity of dominant epistasis of another locus. So, the epistatic gene is dominant one but hypostatic gene may be dominant or recessive. Epistatic gene shows its effect in homozygous and heterozygous condition. This phenomenon can be illustrated by considering fruit color of summer squash. (Cucumber pepo).
In summer squash there are three types of fruit color- white (WW or Ww), yellow(YY or Yy), and green (yy). The White color is produced by a dominant gene which is epistatic to yellow and green color fruits. Similarly, yellow color dominates green color which is an allelic interaction.
When pure white fruit(WWYY) is crossed with pure green fruit (wwyy) of summer squash the F1 hybrid white fruit (WwYy) are formed in the F1 generation. On self-breeding of F1 hybrid, 3 types of fruits are formed in F2 generation. In phenotype ratio- 12 white : 3 yellow: 1 green i:e 12:3:1. So, the phenotypic ratio 12:3:1 is formed instead of Mendelian dihybrid phenotype ratio 9:3:3:1 due to dominant epistasis.
b) Recessive epistasis: The epistasis in which a recessive homozygous gene present in one locus suppresses the activity of a dominant gene present in the another locus of the same chromosome is called recessive epistasis. So, the epistatic gene is recessive homozygous gene and the hypostatic gene is the dominant gene. This phenomenon can be illustrated by considering the color of flower color of Snapdragon.
This includes monogenic inheritance too.
⇒ Monogenic inheritance: It is a type of inheritance in which a single dominant gene influences a complete trait or character. The presence of two such dominant genes doesn't change the phenotype. The genes which are involved in monogenic inheritance are called mono-genes. This monogenic inheritance is also called qualitative inheritance. Example- The 7 pairs of contrasting traits studied by Mendel are the examples of monogenic inheritance. Example- TT or Tt for tallness of pea plant.
⇒ Polygenic inheritance: It is a process of inheritance of traits by a number of dominant non-allelic genes (no. of dominant alleles) in a cumulative manner. It is also called It is also called quantitative inheritance. The genes which are involved in polygenic inheritance, are called polygenes or multiple genes or cumulative genes. Example, skin color in human, kernel color in wheat grain e.t.c.
• Skin color in human-
Skin color in human is due to the formation of melanin pigment from the tyrosine amino acid in the presence of tyrosinase enzyme and sunlight. This pigment formation in skin is controlled by three pairs of non-allelic dominant genes (AABBCC) i:e polygenes. These genes are present at three loci and each dominant gene is responsible for the synthesis of fixed amount of melanin pigment in the skin. The effect of melanin pigment appears in gradual gradation due to the addition of dominant genes. Thus, the amount of melanin pigment formation on the skin color is proportional to the number of dominant genes or alleles.
When a negro man (AABB) gets married to a white/albino woman (aabb), they produce F1 progeny with intermediate color or mulleto (AaBb) in F1 generation. When two such individuals of intermediate color are crossed, they produce offspring of the skin color of varying type that ranges from extreme black or negro to albino or white in the ratio of 1 negro: 4 dark: 6 mulleto: 4 fair: 1 albino i:e 1:4:6:4:1.
Plotting the graph-
It is a type of gene interaction when both the dominant alleles are present together and hence complement each other and produce a specific phenotype. It is due to the combined action of the complementary genes. These non-allelic genes, when present together can't dominate each other, but they interact and complement each other to produce a new trait.
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