Mendelian inheritance deals with characters which are contrasting and which have only two states of existence i.e. dominant or recessive are controlled by a single pair of genes.
In pea plants, the characters chosen by Mendel were clear cut representing two extremes without any intermediary or intergradations stages.
These variations are called discontinuous variations and represent a qualitative difference between the two genes.
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Even in Mendel’s experiments some minor quantitative differences did appear which he ignored on a statistical basis.
However Mendel was able to formulate his laws, because by and large the characters he studied (luckily for him) represented the simplest pattern of inheritance.
Not all characters in organisms can be analyzed by a qualitative assessment. Certain characters like height, weight, size, yield etc. are on a quantitative dimension and cannot be explained on the basis of Mendelian pattern which is wholly qualitative.
These characters do not fall into two clear cut extremes and hence lead to confusion if one uses Mendelian tools to assess them. Characters such as these seem too intergraded and indeed there is not one, but many intermediaries linking both the extremes.
Consequently these characters seem to blend instead of segregating into separate classes or entities, characters which vary within a range and represent all intergrading steps.
One such typical example of inheritance of intergrading characters is that of height in Nicotiana unsuccessfully studied by Joseph Kolreuter, the pre Mendelian plant breeder.
He reported that when tall and dwarf plants were crossed, the F, was intermediate and the F2 progeny showed all grades in between tall and dwarf. He was not able to explain his results.
The answer to the apparent failure of Kolreuter in rationalizing the confusion depends on the fact that continuously varying quantitative characters are controlled by not one pair but many pairs of genes.
The number of pairs of genes involved decides the number of intergradations. Each of these genes cumulatively affect the phenotypic trait, whether it is skin color in man, yield of milk in cows, aleurone color in wheat or size of the fruit in squashes etc.
The inheritance of such characters controlled by many pairs of genes which interact producing various grades is called multiple factor inheritance or polygenic inheritance or quantitative inheritance.
This inheritance which represents yet another exception to the monogenic inheritance of Mendel is different from the qualitative interaction of genes.
While the genes (more than one pair) in qualitative interaction produce clear cut characters (e.g. rose or pea, purple or white), cumulative interaction produces grades like white, whitish, purplish, purple etc., if it were to be a quantitative character.
Further, in qualitative inheritance the genes are non allelic independent genes. In quantitative inheritance, the genes involved are duplicates or replicas of the same gene, (e.g. R, R, R2 R2 – P, P, P2 P2 etc).
There is also one more difference between genes that affect quality and quantity. The quantitative genes (characters) largely depend on the environment for their expression. For instance, a cow may be genotypically suited to produce five liters of milk per day, but it may increase or decrease this, depending on its health, nutrition age etc.
Qualitative characters however are not largely influenced by the environment. The riddle of blending of characters and the problem of rationalizing the quantitative inheritance was first taken up by Swedish geneticist Nillsson Ehle (1908) and American geneticist east (1910, 1916).
These two scientists unraveled the secret of apparent blending inheritance by proposing that continuously varying traits are controlled by the cumulative action of many genes each of which contributes in a limited measure towards that trait. English scientist Mather has evolved statistical methods for the analysis of the multiple genes.
Polygenic inheritance may be defined as the inheritance in which several duplicates of genes cumulatively affect the quantity of a character.