Multiple factor hypothesis
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.
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.
Qualitative and quantitative traits
In these three classes, RR should be red, Rr should be intermediate in colour and rr should be white. When 2 mulattoes marry they may have children showing different degrees of colouration ranging from pure black to white. Quantitative inheritance the multiple factor hypothesis Phenotypes can be from Two or more pairs of genes account for the additive effect on the phenotype. These theoretical distributions when compared with observed results, it could be shown that at least four or five gene pairs may actually be involved in the control of skin colour Fig. Variations in the size of tall plants is partly environmental and partly polygenic but single mutation as well can result in to dwarf plants. This is due to the presence of modifier genes for most of the quantitative characters. Likewise, those with the genotype aabbccdd will be smaller than the Sebright Bantam parent. It is clear from these ratios that in these crosses seed color was governed by one ratio in F2 , two in F2 or three ratio in F2 genes. Clearly speaking, transgressive segregants surpass the parental limits for a quantitative character and they are the result or effect of segregation. Davenport and iii corolla length in Nicotiana longiflora studied by E. Related Articles:. However, it is known now thatthere are three genes involved in kernel colour in wheat. However, on a close examination of the colored seeds; Nilsson-Ehle found that there were marked differences in the intensity of their color. This pointed out that more than 4 pair of genes are involved in determining the length of corolla in Nicotiana longiflora. When the F1 hybrids were self-fertilized the F2 progenies or offsprings showed a ratio of 15 red to 1 white.
A careful note at this place should be made of the fact that the ratios outlined above i. Since in a population of F? Quantitative characters: i It shows continuous variation. The data for the different lines are denoted by different symbols.
This is the reason for their name. In the real world, genes often come in many versions alleles.
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. It may not be put up in to clear cut classes. However, out of three red, one was as red as one of the parents and two were lighter and were comparable to F1 individuals. This variation, as in parents, could thus be due to environmental effect. The effects of genes are additive or cumulative. The problem of the inheritance of quantitative character was taken up by the Swedish botanist. However, it is known now thatthere are three genes involved in kernel colour in wheat. His assumption was that the Negroes differ from the whites in having 2 pairs of colour forming genes that do not show complete dominance. This is due to the presence of modifier genes for most of the quantitative characters. For example, in sweet peas tallness is controlled by polygenes. It is presumed that environmental factors are controlled and uniform. Sampling is essential and this can lead us only near the truth but never to the truth or reality. 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.
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