Multiple Factor Hypothesis or Quantitative inheritance (Hypothesis)
According to the multiple-factor hypothesis, one of a number of nonallelic genes that affect various quantitative hereditary traits.
Yule proposed the multiple factor hypotheses for the first time in 1906. Nilsson-Ehle, on the other hand, gave experimental proof for the existence of many factors in 1908, and so they are attributed with the notion of multiple factor inheritance.
Nilsson-Ehle found 3:1, 15:1, and 63:1 ratios between colourful and white seeds from different crosses in investigations on the inheritance of seed colour in wheat and oat.
These ratios show that seed colour was controlled by one (3:1 ratio in F2), two (15:1 ratio in F2), or three (63:1 ratio in F2) genes in these crosses.
Mendel’s Laws of Heredity provide a straightforward and accurate explanation for qualitative differences across plants and animals, such as flower colour (red or white) and seed colour (yellow or green). However, certain characteristics, such as weight, height, and intelligence in men, are quantitative rather than qualitative.
Other key characteristics of cultivated plants and domestic animals, such as seed yield, fruit yield, egg output, and milk or meat yield, do not fall into neat categories, and all gradations fall somewhere between large and little, heavy and light, and so on.
Quantitative features like this have a lot of variety. In the progeny of hybrids, Mendel’s method of analysis is difficult to apply since the traits appear to mix or blend rather than segregate.
H. Nilsson-Ehle (1908), a Swedish botanist, and E.N. East, an American, took up the topic of quantitative character inheritance (1910, 1916).
These researchers demonstrated that the apparent ‘blending inheritance’ can be explained by assuming that continually varying characters are the result of the combined activity of numerous genes, each of which has a minor effect on the same character. Cumulative, additive, or polygenes are terms used to describe such genes.
A cumulative gene is one that affects the strength or degree of expression of a quantitative property when combined with another identical or similar gene. To put it another way, a character is influenced by numerous genes (=polygenes), and the effects or activities of these genes are cumulative or additive in nature.
The multiple factor hypothesis boils down to this. It is controlled by a large number of genes because it is quantitative inheritance. As a result, it’s often referred to as polygenic inheritance.
Common examples of polygenic inheritance are :(Hypothesis)
Seed colour in Wheat:
Nilsson-Ehle crossed two wheat kinds, red and white in colour, and discovered that all F1 offspring were intermediate between red and white, i.e., light red, indicating that red is only partially dominant over white.
The F2 progenies or progeny of the F1 hybrids had a ratio of 15 red to 1 white when self-fertilized. The red progenies, on the other hand, ranged in colour from bright red to pink. The 15:1 ratio indicates that this was a di-hybrid cross in which two identical genes were responsible for the red colour.
(Member of several gene pairs which act in a cumulative way on a trait or character are known as multiple factor-Altenburg.)
In some cases, it has been discovered that 63 of 64 F2 has red colour and only 1 of 64 contains white colour, implying that three genes are implicated in this situation, each creating red colour; the red parent will be represented by the genotypes R1R1R2R2R3R3 and the white parent by r1r1r2r2r3r3.
The F1 hybrids will only have three colour genes, R1r1R2r2R3r3, and will be light red in colour. In F2, 1/64 will be totally red, with six colour genes, 6/64 will have five, 15/64 will have four, 20/64 will have three, 15/64 will have two, 6/64 will have one, and 1/64 will be completely white, with no colour genes.
Skin colour in Man:
Davenport (1913) proposed the various factor theories to explain how skin colour is inherited in humans. His hypothesis was that black people vary from white people in that they have two sets of color-forming genes that aren’t completely dominant. In Jamaica and Bermuda, where intermarriage between black and white people was frequent, he continued his studies.
Children born from a marriage between a Negro (P1P1P2P2) and a white (p1p1p2p2) have an intermediate shade with just two color-forming genes (P1p1P2p2). Mulattoes are people who are both black and white. When two mulattoes marry, their progeny may have varying degrees of coloration, ranging from completely black to completely white.
With four colour genes, 1/16 of the F2 will be as black as the negro grandfather. Depending on the quantity of colour genes they possess, the remaining 14/16 will display intermediate hues. However, only a vast number of children can reveal all of these gradations. In a small family, the parents of mulattoes will have a kid that is all black or white.
It has been hypothesised that the skin colour difference between Negros and whites is owing to the existence of more than two pairs of colour generating genes, resulting in a significant diversity in skin colour. Stern proposes four, five, or six pairs of genes, whereas Gates suggests three. Color genes have been calculated by other geneticists.Other geneticists have estimated the number of colour genes to be anywhere from two to twenty, but the precise number is still unclear.
Corolla length in tobacco (Nicotiana):
East (1916) published his findings on the inheritance of petal length in Nicotiana longiflora, a tobacco variety that is self-pollinated. Multiple genes influence this personality trait. He crossed a variety with a corolla average length of 52 mm with a variety with a corolla average length of 70 mm. Both of these kinds had been inbred for a long time and were homozygous.
The significant variations in corolla lengths were heritable, indicating that genes rather than the environment regulate them. F1 was determined to be moderate, with a mean corolla length of 61 mm, according to East.
In F2, there was a substantially bigger range in corolla length than in F1. The fluctuation was also continuous. East grew 444 F2 plants and didn’t obtain a single one that looked like either of the parents. This revealed that the length of the corolla in Nicotiana longiflora is determined by more than four pairs of genes.
Quantitative inheritance is based on the following facts: (Hypothesis)
(i)Variation that is constant.
(ii) A strong influence of their surroundings on their expression.
(iii) Multiple or polygenes are in charge.
(iv) Each gene has a single or unit impact. Genes can have additive or cumulative effects.
(v) There is no or only partial dominance. F1 hybrids have traits that mix together, or in other words, the F1 hybrid is intermediate.
(vi) In F2, segregation and independent assortment of genes follow Mendelian inheritance, although the phenotype is continuous between the parents’ extreme limits. The amount and kind of genes have an effect on the phenotypic percentage of F2.
(vii) Polygenic traits are sometimes regulated by a single gene as well. i.e., a single gene mutation might have the same effect as several cumulative gene alterations. Polygenes, for example, govern the height of sweet peas. Tall plant size variations are partially environmental and partly polygenic, however a single mutation can also result in dwarf plants.
(viii) We owe a debt of gratitude to Mather, Haldane, and Fisher, among others, for their contributions to statistical analysis of polygenic inheritance. Because biological samples are unlimited, statistical parameters are difficult to specify. Sampling is necessary, but it can only get us close to the truth, never to the truth or reality.
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