Multiple Factor Hypothesis (With Example) | Genetics

In this article we will discuss about the multiple factor hypothesis.

Laws of heredity by Mendel offer a simple and correct explanation of qualitative difference among plants and animals such as the flower colour, red or white and the seed colour, either yellow or green. But certain characters are quantitative instead of being qualitative such as weight, height, intelligence in man.

Some other important characters in cultivated plants and domestic animals such as yield of seeds, fruits, eggs and amount of milk or meat produced, do not fall into clear cut classes and all gradations come between the two extremes between large and small, heavy and light etc.

Such quantitative characters show a continuous variation. Mendel’s method of analysis is hard to apply in such continuously varying characters because they seem to mix or blend instead of segregating in the offspring of hybrids.

The problem of the inheritance of quantitative character was taken up by the Swedish botanist. H. Nilsson-Ehle (1908) and American E.N. East (1910, 1916).

These investigators showed that this apparent ‘blending inheritance’ can be explained by supposing that a continuously varying characters are due to the combined action of several genes, each of which has a small effect on the same character. Such genes are called the cumulative or additive or polygenes.

A cumulative gene is one which if added to another identical or similar gene affects the intensity or the degree of expression of a quantitative character. In other words, a character is governed by several genes (=polygenes) and their effects or actions are cumulative or additive in nature.

This is the essence of the multiple factor hypothesis. As quantitative inheritance it is controlled by many genes. Therefore, it is also known as polygenic inheritance.

A few common examples of polygenic inheritance are described as below:

Seed colour in Wheat:

Nilsson-Ehle, crossed two varieties of wheat, red and white in colour and found that all the F₁ offsprings were intermediate between red and white i.e., light red colour, showing that red is incompletely dominant over white.

When the F₁ hybrids were self-fertilized the F₂ progenies or offsprings showed a ratio of 15 red to 1 white. The red progenies, however, varied in shade from pure red to pink. The ratio 15:1 clarify that this was di-hybrid cross in which two identical genes were involved for producing 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 other examples, it is found that 63 out of 64 of the F₂ contains red colour and only 1 of 64 is white, suggesting that three genes are involved in this case, each producing red colour, the red parent will be represented then by the genotypes R₁R₁R₂R₂R₃R₃ and the white parents by r₁r₁r₂r₂r₃r₃.

The F₁ hybrids will have only three colour genes R₁r₁R₂r₂R₃r₃ and will show light red shade. In F₂ 1/64 will be completely like the red grand parent having six colour genes, 6/64 will have 5, 15/64 will have four, 20/64 will have three, 15/64 will have two, 6/64 will have one colour genes while 1/64 will be completely white without any colour gene.

Skin colour in Man:

It was idea of Davenport (1913) that the multiple factor hypotheses explains the mode of inheritance of skin colour in man. His assumption was that the Negroes differ from the whites in having 2 pairs of colour forming genes that do not show complete dominance. He carried on his studies in Jamaica and Bermuda where intermarriages between coloured and white people were very common.

A marriage between a Negro (P₁P₁P₂P₂) and a white (p₁p₁p₂p₂) results in children having intermediate shade and have only 2 colour forming genes (P₁p₁P₂p₂). These are mulattoes. When 2 mulattoes marry they may have children showing different degrees of colouration ranging from pure black to white.

In the F₂ 1/16 will be as dark as the negro grandparent having 4 colour genes. The rest 14/16 will show intermediate shades depending upon the number of colour genes contained by them. But all these gradations could be seen only when a large number of children are born. In a small family the mulattoes parents will produce a completely black or white child.

It has been proposed that the difference in the skin colour between Negros and whites is due to the presence of more than two pairs of colour forming genes bringing about a considerable variation in the skin colour. Gates has suggested for three where as Stern’s for four, five or six pairs of genes. Other geneticists have estimated the colour genes number from two to twenty pairs but the exact number involved is till unknown.

Corolla length in tobacco (Nicotiana):

East (1916) reported his studies on the inheritance of corolla length in Nicotiana longiflora, a self-pollinated variety of tobacco. This character is governed by multiple genes. He crossed a variety, the corolla of which had an average length of 52 mm, to a variety, the corolla of which had an average length of 70 mm. Both these varieties had long been inbred and therefore were homozygous.

The marked differences in corolla lengths were heritable pointing out that they are controlled by genes rather than environmental. East found that F₁ was intermediate with mean corolla length of 61 mm.

In F₂ a much larger variation for corolla length than F₁ was observed. The variation was continuous as well. East raised 444 F₂ plants and failed to get even a single plant like either of the parents. This pointed out that more than 4 pair of genes are involved in determining the length of corolla in Nicotiana longiflora.

ABC, ABc, AbC, Abe, aBC, aBc, abC, abc X ABC, ABc, AbC, Abc, aBC, aBc, abC, abc

Quantitative inheritance is based on the following facts:

(i) Continuous variation.

(ii) A marked effect of the environment on their expression.

(iii) Governed by multiple or polygenes.

(iv) Each gene produces unit or individual effect. The effects of genes are additive or cumulative.

(v) Dominance is absent or partial. F₁ hybrids show blending in characters or in other words the F₁ hybrid is intermediate.

(vi) Segregation and independent assortment of genes in F₂ is according to Mendelian inheritance but the phenotype is in continuous range between the extreme limits of the parents. The phenotypic proportion of F₂ is modified according to the number and nature of genes.

(vii) Some times polygenic characters are governed by single gene too. i.e., single gene mutation may have the same effect as changes in many cumulative genes. For example, in sweet peas tallness is controlled by polygenes. Variations in the size of tall plants is partly environmental and partly polygenic but single mutation as well can result in to dwarf plants.

(viii) For statistical analysis of polygenic inheritance we owe a great deal to Mather, Haldane & Fisher etc. Biological samples are infinite and therefore, statistical parameters are not well defined. Sampling is essential and this can lead us only near the truth but never to the truth or reality.

Difference between Qualitative and Quantitative characters:

Qualitative characters:

(i) It shows discontinuous variation. It may be put in to clear cut classes.

(ii) Governed by major genes. Their ef­fect is definite.

(iii) Not usually affected by environment.

Quantitative characters:

(i) It shows continuous variation. It may not be put up in to clear cut classes.

(ii) Governed by minor genes. Their effect is additive.

(iii) Well affected by environment.

Transgressive segregation (= inheritance or variation):

The range of variation in F₂ progeny remains normally well within the limits of both the parents involved in a cross. But some times the extremes of F₂ exceed those of the parents. This type of variation is called Transgressive variation. K.Mather firstly used the term polygene in 1941.

Clearly speaking, transgressive segregants surpass the parental limits for a quantitative character and they are the result or effect of segregation. This is the reason for their name.

A classical example of transgressive variation was found in the experiments carried out by Punnett in chicken. He made a cross between a large Hamburg chicken with a small Sebright Bantam and found that the F₁ were of intermediate size.

But the F₂ progeny, however, contained some birds which were larger and some which were smaller than the parental varieties. Such results are obtained if the parents do not represent the extreme genotypes.

For example, 4 pairs of genes are responsible for determining the size of the chicken and in Hamburg variety only 3 are recessive and 1 dominant. A cross between the genotypes AABBCCdd X aabbccDD will produce only one type of F₁ which will be heterozygous for all the 4 genes (AaBbCcDd) thereby determining intermediate size. In the F₂ generation the offspring with the genotype AABBCCDD will be larger or heavier than the original Hamburg parent.

Likewise, those with the genotype (aabbccdd) will be smaller than the Sebright Bantam parent. However, these extreme types would be very few or limited in number. Thus, new and desirable types in plants and animals may be produced through proper crossings.