Genetics ( Sem 5th )

        

Epistasis: ( Greek word: Epi = Upon

                                          Statis= Standing

Epistatis= Standing upon

Definition: Epistasis is a gene interaction between two genes in which one gene masks or suppress the expression of another gene at distinct locus.

Epistatic gene: Which gene is Supressed the expression of another gene and express its phenotype is called  Epistatic gene.

Hypostatic gene: It is the gene Supressed by Epistatic gene and can not express its phenotype traits called hypostatic gene.

Difference between Dominance and Epistasis:

Dominance: It is the interaction between two alleles of the same gene.

Epistatis: Epistatis is the interaction between two different gene. 

Types of Epistasis:

1. Dominant Epistasis

2. Recessive Epistasis

3. Duplicate Epistasis 

Dominant Epistasis:

Dominant allele takes the major role in dominant Epistasis. When the dominant allele of one gene supressed the expression of another gene known as dominant Epistasis. Say for example, A and B are two different allele present on different locus of the same chromosome. A is responsible for production of purple colour flower Whereas B is responsible for blue colour flower production. Here, A allele is Epistasis allele to B allele ( Hypostatis) . The dominant allele A is able to express itself in the presence of B or b allelic combination. In this case A allele is always express its phenotypes in the presence of B or b allele beacause A allele is function as Epistatic allele to B allele.When homogygous recessive combination aa of Epistatic gene  interacts with B or b combination of hypostatic B gene, the hystatic Allele can be expressed its phenotype. A-B, and A-bb allelic combination produce same phenotypes where aaB and aabb exhibits 2 additional phenotypes. The classical phenotype ratio of Mandle's 2nd law becomes modified into 12:3:1.

Recessive Epistasis: 

In recessive Epistasis, There is a pair of non allelic gene ( two different allele present on different genes.) One can produce its phenotypic effect independently in dominant combination but other allele does not express own phenotype trait independently. Latter B allele can produce its phenotype effect only one condition, where A and B allele are present in dominant state( A-B ).

Suppose, A and B are two non allelic genes present on different locus. A can produce phenotypic trait independently in dominant state, but other gene B can not produce phenotypic trait independently. The recessive genotype aa can Suppressed the expression of alleles which are present in B locus, but B allele can express its phenotype effect in the presence of dominant allele at the A locus ( AA/BB), ( AA/ BB). In recessive Epistasis A-B and A-bb allelic combination produce two additional phenotypes. Thus 9:3:3:1 ratio becomes into 9:3:4.

Polygenic Inheritance:

 Poly= Many

 Genic= gene

In short terms: One Phenotypic trait is determined by Two or more genes in organism's body.  Multiple allele trait also regulated by two or more alleles. 

But polygenic inheritance is totally different from multiple allele trait. We will discus about that later in details.

Polygenic Inheritance: 

This is a  quantitative Inheritance by two or more independent genes each have additive phenotypic effect on a specific trait of a organism called as Polygenic Inheritance.

Some heritable phenotypes don't follow Mendel's rule in their inheritance pattern. Here, Polygenic inheritance don't follow this rule.

Example: 

Human: Height, Eye color, Hair color, skin color,

Plant: Shape, Size, and color of the flower.

Size of seed, Time to mature, oil content in flower.

Characteristics of Polygenic inheritance:

1. Two or more genes are involved to determine a specific trait in organism body.Where each gene has a slight effect on phenotype trait.

2. Phenotypic effect of a single gene too small to difficult to detect it 

3. Some genes exert equal effect.

4. The expression of single gene is not Supressed by presence of all other gene. There is no effect of Epistatis.

5. This phenomena pattern is often remains unpredictable and highly complex in environment. 

Difference between Multiple allele and Polygenic inheritance:

1. Multiple alleles or genes present on the same locus in same chromosome Whereas, Polygenic genes are located different locus on different chromosome.

2. Multiple alleles may have dominant and recessive  phenotypic effect but in case of Polygenic inheritance, each gene have a slight effect on a trait without presence of any Dominant and Recessive traits.