Examples of Quantitative Inheritance

  1. 1.Kernel Colour in Wheat
    •  Nilsson-Eble (1909) and East (1910, 1916) gave first significant clue of quantitative inheritance by their individual works on wheat.
    •  They crossed a strain of red kernel wheat plant with another strain of white kernel.
    •  Grain from the F1 was uniformly red, but of a shade intermediate between the red and white of the parental generation.
    •  This might suggest incomplete dominance, but when F1 offsprings were crossed among themselves, the F2 zygotes showed five different phenotypic classes in a. ratio of 1 : 4 : 6 : 4:1.
    •  Noting that 1/16 of the F2 was an extreme in colour as either of the parental plants (red or white), they theorized that two pairs of genes controlling production of red pigment while operating in this cross.
    •  Each gene was supposed to contain two alleles.
    •  One allele produces a given quantity of the red pigment, while its counterpart did not produced any pigment.
    •  All alleles were equally potent in the production or lack of production of pigment.
    •  If we symbolize the genes for red with the capital letters A and B and their, alleles resulting in lack of pigment production by a and b: We can illustrate the results of this cross as follows: or 1/16 Red: 4/16 Dark: 6/16 Medium: 4/16 Light: 1/16 White.

1.Extreme in colour as either of the parental plants

  •  Noting that 1/16 of the F2 was an extreme in colour as either of the parental plants (red or white), they theorized that two pairs of genes controlling production of red pigment while operating in this cross.
  •  Each gene was supposed to contain two alleles.
  •  One allele produces a given quantity of the red pigment, while its counterpart did not produced any pigment.
  •  All alleles were equally potent in the production or lack of production of pigment.
  •  If we symbolize the genes for red with the capital letters A and B and their, alleles resulting in lack of pigment production by a and b: We can illustrate the results of this cross as follows: or 1/16 Red: 4/16 Dark: 6/16 Medium: 4/16 Light: 1/16 White.

1.Skin Colour in Man

  •  Another classical example of polygenic inheritance was given by Davenport (1913) in Jamaica.
  •  He found that two pairs of genes, A-a and B-b cause the difference in skin pigmentation between Negro and Caucasian people.
  •  These genes were found to affect the character in additive fashion.
  •  Thus, a true Negro has four dominant genes, AABB, and a white has four recessive genes aabb.
  •  The F1 offspring of mating of aabb with AABB, are all AaBb and have an intermediate skin colour termed mulatto.
  •  A mating of two such mulattoes produces a wide variety of skin colour in the offspring, ranging from skins as dark as the original Negro parent to as white as the original white parent.

The results of this cross are as follows :

  •  These results are clearly showing that A and B genes produce about the same amount of darkening of the skin; and therefore, the increase or decrease of A and B genes cause variable phenotypes in F2 in the ratio of 1 Negro: 4 dark : 6 intermediate: 4 light : 1 white.

1.Height in Man

  •  Skin colour in man is a rather simple example of polygenic inheritance because only two pairs of genes are involved.
  •  The inheritance of height in man is a more complex phenomenon involving perhaps ten or more pairs of genes.
  •  The character of tallness is recessive to shortness, thus, an individual having the genotype of more dominant genes will have the phenotype of shortness.
  •   Because, this quantitative trait is controlled by multiple pairs of genes and is variously influenced by a variety of environmental conditions.
  •  The heights of adults range from 140 cm to 203 cm.
  •  If one measured the height of a thousand adult men and the height of each is plotted against height in centimeters and the points connected, a bell-shaped curve is produced which is called curve of normal distribution and is characteristic of quantitative inheritance.

6.Other Examples

  •  Likewise, if one measures the length of thousand sea shells the same species, or counts the number of kernels per ear in a thousand ears of corn, or the number of pigs per litter in a thousand litters, or weighs one thousand hen’s egg, one will find a normal curve of distribution in each case.
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