Polygenic inheritance

When several genes control a single character. It is observed that the phenotypical ratio is modified and such modification is due to the number of doses of genes.Hence,the greater number of gene the greater will be effect.This type of interaction is called quantitative interaction or cumulative interaction or polygenic interaction.

Example:- Human Skin colour-

Human skin colour depends upon the presence of a Pigment i.e Melanin.The total amount of melanin pigment in a person depends upon the number of genes. Iaven port(1913) observed the skin colour in Negroes,which is control by the action of two dominant gene namely A and B with cumulative effect.But the skin colour of white is due to the presence of recessive alleli a & b.Thus A negro has a Genotype AABB and the Genotype of white aabb. Marriage between a negro and a white results in following ways in both F1 and F2.

                                AABB            *       aabb  

                              (Negroes)             (White)

Gamete-        AB                               ab

F1:-                                           AaBb        - All are Mullatoes

Gametes:- AB Ab ab ab  *  AB Ab ab ab

	AB	Ab	aB	ab
AB	AABB Negroes	AABb Dark	AaBB Dark	AaBb Mullatoes
Ab	AABb Dark	Aabb Mullatoes	AaBb Mullatoes	Aabb Light
aB	AaBB Dark	AaBb Mullatoes	aaBB Mullatoes	aaBb Light
ab	AaBb Mullatoes	Aabb Light	aaBb Light	Aabb White

      

In  F1 is gives an intermediate colour which is called Mullato.Hence the mullato possses the genotype AaBb.in that way the negroes have 4 dominant gene,the whites have 4 recessive gene whereas the mullatoes have 2 dominant and 2 recessive gene.  

  

On selfing these F1 mullatoes 16 individuel appear with various colour of skin depending upon the number of dominant gene.Thus out of the 16 individuels only one has 4 dominant pigment gene and that become a negro,4 have 3 dominant pigment geneand they become dark,6 have 2 dominant pigment gene which become mallatoes.4 have only one dominant pigment gene which has got light colour and finally only one has no dominant pigment gene which got the white colour.

In this way the ratio become 1:4:6:4:1. This indicate production of mallato or any other colour is due to cumolative os several gene.

The other example of such polygenic inheritance comes from –

(1)Colour of the grain in Wheat

(2)Eye colour in Drosophilla

(3)Body size in chicken

(4)Corolla length in Nicotina

(5)Coat colour in Cattles

(6)Height in man

Colour of grain in Wheat

Nilsson & Ehile made a very interesting study on colour of grain in wheat.There are two colour i.e dark red and white are due to the pairs of dominant gene AABB and recessive gene aabb respectively.In F1 they produce intermediate or medium coloured grains.But in F2(by self polinating) they produce offspring like 1 dark red,6 intermediate or medium ,4 red,4 light redand 1 white.

Corolla length in Nicotina

East studied the inheritance in corolla length of Nicotina between two varieties of species-one with average length(40 mm) & other with(93 mm) in length.He crossed this two varieties and in F1 he has got intermediate between the two parent.Where as in F2 he got more variations.Finally he come to the conclusion that the length of Corolla was control by 6 to 8 gene pairs. 

Body size in chicken

According to Punnet 4 pairs of dominant gene are responsible for the development of body size in chicken.Thus,the body size is determined by the number or quantity of dominant gene present in the animal.