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Quantitative inheritance is also a particular case of gene interaction. In this case, where the phenotypic differences of a given trait show no significant variations, the variations are slow and continuous and gradually change from a “minimum” phenotype to a “maximum” phenotype.
It is easy to conclude, therefore, that in quantitative (or polygenic) inheritance genes have an additive effect and are called polygenes.
Quantitative inheritance is very common in nature. Some characteristics of economic importance, such as meat production in beef cattle, corn production, etc., are examples of this type of inheritance. In men, height, skin color and even intelligence are cases of quantitative inheritance.
Inheritance of skin color in man
According to Davenport (1913), skin color in humans is the result of the action of two gene pairs (AaBb), without dominance. Thus, A and B determine the production of the same amount of melanin pigment and have additive effect. Therefore, it is concluded that there should be five shades of color in human skin, according to the amount of genes A and B.
|Aabb, aaBb||light mulatto|
|AAbb, aaBB, AaBb||medium mulatto|
|AABb, AaBB||dark mulatto|
Let us look at the genotypic and phenotypic results that would be obtained from the intersection of two double-heterozygous medium mulatto individuals:
Medium mulatto X medium mulatto
1/16 : 4/16 : 6/16 : 4/16 : 1/16
white light mulatto mulatto medium dark mulatto black
And the color of the eyes?
Every biology teacher has to answer, during genetics classes, the inevitable question about how eye color is inherited. However, many still erroneously treat this genetic trait as a kind of simple Mendelian inheritance, the occurrence of which is influenced by a single pair of genes associated with the production of dark and light eyes.
This simplistic explanation, however, does not show how the full range of eye color arises and does not clarify why brown-eyed parents can have children with brown, blue, green, or any other shade. Eye color is a characteristic whose heritage is polygenic, a type of continuous variation where the alleles of various genes influence the final coloration of the eyes. This occurs by producing proteins that drive the proportion of melanin deposited in the iris. Other genes produce spots, rays, rings and pigment diffusion patterns.
Normal or Gauss curve phenotype distribution
Typically, extreme phenotypes are those in smaller quantities, while intermediate phenotypes are observed at higher frequencies. The quantitative distribution of these phenotypes establishes a curve called normal (Gauss curve).
The number of phenotypes that can be found in a case of polygenic inheritance depends on the number of allele pairs involved, which we call no.
|Number of Phenotypes = 2n + 1|
If a trait is determined by three pairs of alleles, seven distinct phenotypes can be found. Each group of individuals expressing the same phenotype constitutes a phenotypic class.
Knowing the number of pairs involved in inheritance, we can estimate the expected frequency of individuals demonstrating extreme phenotypes, in which no is the number of gene pairs.
|Frequency of extreme phenotypes = 1/4no|