If n is the number of contributing dominant alleles then the ratio like 1 : 4 : 6 :
4 : 1 or 1 : 6 : 15 : 20 : 15 : 6 : 1 can be obtained by the expansion of binomial
equation^1
2
1
2 +
n. Here, in the case of two pairs of multiple genes n = 4
and in the case of three pairs of multiple genes n = 6.
height in man
The inheritance of height in man is a more complex phenomenon involving
perhaps ten or more pairs of genes. The character of tallness is recessive
to shortness, thus, an individual having the genotype of more dominant
genes will have the phenotype of shortness. because, this quantitative trait is
controlled by multiple pairs of genes and is variously influenced by a variety
of environmental conditions, the heights of adults range from 140 cm to 203
cm.
Table: Differences between monogenes and polygenes
Monogenes/Monogenic Inheritance Polygenes/Polygenic Inheritance(i) They produce discontinuous variations in the
expression of traits.Polygenes produce continuous variations in the expression of
traits.
(ii) a single dominant allele expresses the complete
trait.a single dominant allele expresses only unit of the trait.(iii) Monogenic inheritance controls qualitative traits. Polygenic inheritance controls quantitative or metric traits.(iv) a character is represented in an individual by a
pair of alleles.a character is represented by three to several pairs of alleles.(v) f 1 individuals are similar to dominant parent. No
intermediates are produced.f 1 individuals are intermediate between the two parents.(vi) f 2 individuals resemble both the parents in the ratio
of 3 : 1.Depending upon the number of polygenes, 2/4 (one pair), 2/
(two pairs) or 2/64 (three pairs) f 2 individuals resemble the
parental types.
(vii) There is no cumulative action in the presence of
two dominant genes.The dominant genes have cumulative effect on the expression
of the trait.
(viii) Individuals with dominant phenotype are more
numerous than with recessive phenotype.Individuals with dominant trait are usually as few as with
recessive trait. Intermediate forms are more numerous.
(ix) environment has lesser effect on phenotypic
expression.environment has greater effect on phenotypic expression.