What do you mean by qualitative and quantitative inheritance?
Answers
Quantitative Inheritance (Polygenic Inheritance):
It is a type of inheritance controlled by one or more genes in which the dominant alleles have cumulative effect with each dominant allele expressing a part or unit of the trait, the full trait being shown only when all the dominant alleles are present.
The genes involved in quantitative inheritance are called polygenes. Quantitative inheritance is, therefore, also called polygenic inheritance. It is also named as multiple factor inheritance. A few instances of quantitative inheritance are kernel colour in wheat, cob length in Maize, skin colour in human beings, human intelligence, milk and meat yield in animals, height in human beings and several plants, yield of crop plants including size, shape and number of seeds or fruits per plant.
A polygene is defined as a gene where one dominant allele controls only a unit or partial quantitative expression of a trait. It is also termed as a gene in which a dominant allele individually produces a slight effect on the phenotype but in the presence of similar other dominant allele controls the quantitative expression of a trait due to cumulative effect. Hence, polygenes are also called cumulative genes.
The traits controlled by quantitative inheritance are sometimes known as metric traits because they can be measured in terms of unit of size, height, weight or number. Quantitative inheritance is further characterised by the occurrence of intermediate forms (‘continuous variations’) between the parental types. Here a cross between two pure breeding parents does not produce dominant trait of one parent but instead an intermediate trait is exhibited.
Similarly in F2 generation apart from the two parental types there are several intermediate types which link the two parental traits. Because of the latter, quantitative inheritance is also called blending inheritance. The dominant polygenic alleles which contribute to the expression of the trait are called contributing alleles while the recessive polygenic alleles are known as non-contributing alleles.
Quantitative Inheritance:
Quantitative or polygenic inheritance was first studied by J. Kolreuter (1760) in case of height in tobacco and F. Galton (1883) in case of height and intelligence in human beings. Nilsson-Ehle (1908) obtained the first experimental proof of polygenic inheritance in case of kernel colour in wheat. Polygenic inheritance occurs in case of plant height, crop yield, milk yield, intelligence height and skin colour in humans. It is easily influenced by environment can be known from the frequency distribution of phenotypes. In monogenic or qualitative inheritance the phenotypes are two (3: 1) in case of single gene pair and 4 (9: 3: 3: 1) in case of two pairs of genes.
In polygenic or quantitative inheritance the number of phenotypes is 3 (1: 2: 1) in case of one polygene pair, 5 (1: 4: 6: 4: 1) in case of two polygene pairs and 7 (1 : 6: 15: 20: 15: 6: 1) when three polygene pairs are involved. Thus we see that the number of intermediate types increases with the increase in the number of polygenes but the number of parental types remain the same (2 in the above cases). The possible origin of polygenes is:
(i) Duplication of chromosome part
(ii) Polyploidy or increase in chromosomes number
(iii) Mutations producing genes having similar effect.
Ex : Seed coat and sees colour of pea plant
Quantitative Inheritance is a charecter of inheritance which depends upon the cumulative action of many genes, each gene produces only a small effect. In this case, continuous variation is observed.
Ex : Height of tree, seed shape and colour (for plants)
Height, Skin pigmentation in human (for animals) etc.
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