Question

How can you find the number of pairs of alleles involved in polygenic inheritance by using the number of phenotypic forms of the trait they condition?

Answer 1

Polygenetic inheritance is also called as quantitative inheritance which refers to a single inherited polygenetic trait that is controlled by two or more different genes. Monogenetic traits are determined by the different alleles of a single gene. Polygenetic traits may display a range of possible phenotypes, determined by a number of different genes.  Polygenic traits exhibit incomplete dominance so the phenotype displayed in offspring is a mixture of the phenotypes displayed in the parents. Each of the genes that contributes to a polygenic trait, has an equal influence and each of the alleles has an additive effect on the phenotype outcome. The physical traits that are controlled by polygenic inheritance, such as hair color, height and skin color, as well as the non-visible traits such as blood pressure, intelligence, longevity etc occur on a continuous gradient, with many variations of quantifiable increments.  

Polygenic inheritance should not be confused with the effects caused by multiple alleles. In the case of multiple alleles, a gene contains several different allele variants on the same locus of each chromosome, for example the three different alleles which control for blood type – A, B & O.  There may be large numbers of different genes controlling for a single phenotype trait, so it becomes difficult to present or demonstrate. Luckily, the distribution of phenotypes determined through polygenic inheritance usually fits into a normal distribution of probabilities, with most offspring displaying an intermediate phenotype of the two parents.