Question

Characters are determined by two or more gene pairs and they have additive or cumulative effect. What are such genes called as?

• A. Polygenes.
• B. Multiple alleles.
• C. Pleiotropic genes.
• D. Qualitative genes.

Answer 1

Answer: (A)

Polygenic inheritance is an example of quantitative inheritance as it is controlled by two or more genes and dominant alleles of each have a cumulative effect on the character. The genes involved in such inheritance are called polygenes. The full trait is shown only when all the dominant alleles are present.
For example, kernel color in wheat. It is controlled by two pairs of genes (A and B).
Cross= AABB (Dark red) X aabb (White)
F1= AaBb ( intermediate red)
F2 genotypic ratio= $1:4:6:4:1$ ( five phenotypes observed).
Multiple alleles are more than two alternatives of the same gene. For example, human populations has four blood groups: A, B, Ab and O. It is controlled by three alleles I^A, I^B, and i.
Pleiotropic gene control expression of more than one character. For example, in pea plant, one gene controls the synthesis of starch in seed and the shape of the seeds. It has two alleles B and b. The three genotypes show following phenotypes:

• BB= effective starch synthesis and round seeds.
• Bb= intermediate starch synthesis and round seeds.
• bb= lesser efficient in starch synthesis and wrinkled seeds.

Quantitative gene- In this expression of dominant trait depends on presence or absence of dominant allele. For example - polygenic inheritance.