Question

The weight of fruit in a plant is determined by the number of dominant alleles of a certain number of genes. If seven weight categories are noticed, how many gene sites would be involved?
A. Two
B. Three
C. Four
D. Five

Answer 1

Hint: Dominant alleles are those alleles which always show their expression when present in any homozygous or heterozygous genotype. Certain genes like weight are polygenic genes, which are expressed quantitatively. Their quantity is directly proportional to the expression obtained.

Formula used: Number of phenotypes for polygenes \[= 2n + 1\]
Here, \[n\] denotes the number of gene sites or the allelic pairs.

Complete answer: Polygenic inheritance is also known as quantitative inheritance. This is because the quantity of the polygenes determines the degree of the expression of the genotype.
In polygenic inheritance, the dominant genes have a cumulative effect. Each dominant allele expresses a part of the functional polypeptide. The full trait is only shown when all the dominant alleles are present or it is a dominant heterozygous condition. The weight of fruits in the plant follows the polygenic inheritance. Since there are seven weight categories, this means that there are seven different kinds of phenotypes. Therefore, the number of gene sites can be found from the formula for the number of phenotypes for polygenes.
Using the formula for the number of phenotypes for polygenes, the number of gene sites will be
\(\begin{array}{c}2n + 1 = 7\\2n = 7 - 1\\2n = 6\\n = 3\end{array}\)
Therefore, there are three gene sites for seven weight categories. Thus, based on the above information we can conclude that if seven weight categories are noticed then three gene sites would be involved.
Hence, the correct answer to this question is option (B).

Note: The segregation and the assortment of the genes controlling the quantitative traits were first demonstrated by H Nilsson-Ehle in the year 1910 and then by East in the year 1910. They demonstrated the kernel color in wheat and the corolla length in the tobacco plant. After this Davenport studied the skin color in humans.