Assertion (A) : In humans the genotype with all the dominant alleles (AABBCC) will have the darkest skin color.

Reason (R) : In a polygenic trait, phenotype reflects the contribution of each allele.

The correct option is: Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).

Step-by-Step Explanation:

1. Understanding Polygenic Inheritance:
Human skin color is a classic example of a polygenic trait, which is controlled by three or more pairs of non-allelic genes (usually represented as A, B, and C). In polygenic inheritance, the phenotype (the observable physical trait) is determined by the cumulative effect of multiple genes, where each dominant allele contributes quantitatively to the trait. This is also known as quantitative inheritance.

2. Analyzing Assertion (A):
Let us assume skin color is controlled by three genes: A, B, and C. Each gene has a dominant allele (A, B, C) that contributes to melanin production (dark pigment) and a recessive allele (a, b, c) that does not.
A genotype with all dominant alleles, represented as $AABBCC$, has the maximum possible number of dominant alleles (six dominant alleles). Because each dominant allele adds to the amount of melanin pigment, individuals with the genotype $AABBCC$ will produce the maximum amount of melanin and therefore exhibit the darkest skin color. Conversely, a genotype with all recessive alleles ($aabbcc$) will result in the lightest skin color. Thus, Assertion (A) is true.

3. Analyzing Reason (R):
The reason states that in a polygenic trait, the phenotype reflects the contribution of each allele. In quantitative inheritance, each dominant allele has an additive effect. This means that the total number of dominant alleles present in the genotype, regardless of which specific locus they are on, determines the intensity of the phenotype. Since each allele adds a distinct unit of pigment to the skin color, the phenotype directly reflects the sum contribution of each individual allele. Thus, Reason (R) is also true.

4. Evaluating the Connection (Explanation):
Because the phenotype reflects the contribution of each individual allele (as stated in R), the accumulation of the maximum number of dominant alleles ($AABBCC$) leads to the expression of the darkest phenotype (as stated in A). Therefore, Reason (R) is the correct logical explanation for Assertion (A).