Q.For Questions number 13 to 16, two statements are given — one labelled as Assertion (A) and the other labelled as Reason (R). Select the correct answer to these questions from the codes (a), (b),

Concept: Polygenic inheritance — skin colour in humans is controlled by multiple genes, each contributing additively to the phenotype.

Reasoning:

1. Assertion (A) is true: In polygenic inheritance of skin colour, each dominant allele (A, B, C) adds a unit of melanin. The genotype AABBCC has the maximum number of dominant alleles, producing the darkest skin.
2. Reason (R) is true: In a polygenic trait, the phenotype is the cumulative effect of all contributing alleles — more dominant alleles produce a more extreme phenotype. …

Concept and intuition:

Skin colour in humans is not controlled by a single gene with simple dominance. Instead, it is a polygenic trait — influenced by multiple genes (here, three genes: A, B, C). Each dominant allele (A, B, C) contributes a unit of melanin (dark pigment), while each recessive allele (a, b, c) contributes less. The total number of dominant alleles determines the skin shade: more dominant alleles → darker skin. The genotype AABBCC has six dominant alleles (the maximum possible), so it gives the darkest skin. Reason (R) states that in a polygenic trait, the phenotype reflects the additive contribution of each allele — which is exactly why AABBCC is darkest. So (R) correctly explains (A).

Step-by-step reasoning:

1. Identify the trait type:

   Skin colour in humans is a classic example of polygenic inheritance (also called quantitative inheritance). Unlike Mendelian traits with clear dominant/recessive pairs, polygenic traits show continuous variation (e.g., light to dark skin). The phenotype is determined by the total number of dominant alleles across multiple genes.

2. Understand the additive model:

   For three genes (A/a, B/b, C/c), each dominant allele adds a fixed amount of pigment. So:

   • Genotype aabbcc (0 dominant alleles) → lightest skin.
   • Genotype AABBCC (6 dominant alleles) → darkest skin.
   • Intermediate numbers (1 to 5 dominant alleles) produce intermediate shades.

3. Evaluate Assertion (A):

   "In humans the genotype with all the dominant alleles (AABBCC) will have the darkest skin colour."

   This is true — because it has the maximum possible number of dominant alleles (6), hence the maximum pigment.

4. Evaluate Reason (R):

   "In a polygenic trait, phenotype reflects the contribution of each allele."

   This is true — the additive effect of each dominant allele is exactly what produces the range of skin colours. The phenotype is a sum of individual allele contributions. …