Characters are determined by two or more gene pairs, and they have additive or cumulative effect. Such genes are called cumulative genes or polygenes or multiple factors.
Example Human skin colour:
i. Population derived from marriage between negro and white show intermediate skin colour and are called mulattoes.
ii. When such individuals marry each other, all shades of colour are observed in the population in the ratio, 1:6:15:20:15:6:1.
From this, it can be concluded that skin colour in humans is controlled by three pairs of genes, Aa, Bb, and Cc.
iii. The presence of melanin pigment in the skin determines the skin colour. Each dominant gene is responsible for the synthesis of fixed amount of melanin.
iv. The effect of all the genes is additive and the amount of melanin synthesized is always proportional to the number of dominant genes.
v. Genotype of negro parent is AABBCC, and that of albino (pure white, melanin is not produced at all) is aabbcc.
vi. Genotype of their offspring (mulatto) is AaBbCc.
vii. Mulattoes (F1 offspring) produce eight different types of gametes, and total sixty four combinations are possible in the population of next generation (F2); but there are seven different phenotypes due to the cumulative effect of each dominant gene as follows,
|i||Pure black (negro)||6 dominant genes||1/64|
|ii.||Black (less dark than negro parent)||5 dominant genes||6/64|
|iii.||Lesser black or brown||4 dominant genes||15/64|
|iv.||Mulatto (intermediate-‘sanwla’)||3 dominant genes||20/64|
|v.||Fair||2 dominant genes||15/64|
|vi.||Very fair||1 dominant genes||6/64|
|vii.||Pure white (albino)||Non dominant genes||1/64|
Example :Kernel colour in Wheat.
i. A variety of wheat with red kernel was crossed with wheat having white kernel.
ii. The F1 generation plants had red kernel, but of a shade intermediate between the red and white of the parental generation.
iii. When F1 plants were self-pollinated, the F2 individuals produced were of five different phenotypes, in the ratio of 1:4:6:4:1.
iv. 1/16 of the individuals of the progeny were darkest red (as red as a parent plant) resembled one of the parents and another 1/16 individuals were white (as white as a parent plant).
v. 4/16 of the individuals were medium red (less than parent but more than F1 hybrids), 6/16 of the individuals were intermediate red (as F1 hybrids) and 4/16 of the individuals were light red (less than F1 hybrids).
vi. It was concluded that the kernel colour is under control of two pairs of alleles. The two pairs of alleles segregate independently of each other as in Mendel’s dihybrid crosses. The two genes contribute in the production of pigment and a graded phenotype is produced.