Assertion (A): Mendel’s law of Independent assortment does not hold good for genes located closely. Reason (R): Closely located genes assort independently.

(A) is correct but (R) in not correct.

(A) is not correct but (R) is correct.

Both (A) and (R) are correct and (R) is the correct explanation of (A)

Bothe (A) and (R) are correct but (R) is not the correct explanation of (A)

Law of Independent Assortment

சுயாதீன வரிசைமுறை விதி

NCERT Class 12 · Principles of Inheritance and Variation · 1 NEET question

Most students confuse Independent Assortment with Segregation. Remember: Segregation happens in every monohybrid cross, but Independent Assortment is exclusive to dihybrid crosses. If you miss this distinction, you lose 4 marks on a basic concept.

The idea

Mendel’s Law of Independent Assortment states that when two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair. During gamete formation, the alleles of different genes segregate independently into the gametes. This results in the production of four types of gametes in equal frequency in an F2 generation of a dihybrid cross. The phenotypic ratio observed is 9:3:3:1.

Why it matters

It exists because non-homologous chromosomes align independently during meiosis, allowing for new combinations of alleles in gametes.

NEET lens

NEET focuses on the 9:3:3:1 ratio and the gamete calculation formula 2^n. Always check if the genes are linked; if genes are linked on the same chromosome, they violate this law. Do not memorize the ratio without understanding that it only applies when genes are on different chromosomes.

Clinical significance

Independent assortment is the biological basis for genetic diversity in humans. When this process is disrupted during meiosis, it leads to aneuploidy or chromosomal nondisjunction. Such errors result in conditions like Down syndrome or Turner syndrome, where the random distribution of chromosomes fails.

Introduction

Mendel's Law of Independent Assortment states that alleles of different genes (on different chromosomes) assort independently during gamete formation — each gene pair segregates independently of every other gene pair.

Core concept

Dihybrid cross: P: RRYY (round yellow) × rryy (wrinkled green). F1: RrYy (all round yellow — both dominant). F1 × F1: RrYy × RrYy. Gametes: RY, Ry, rY, ry (each in 1/4 proportion). F2: 4×4 Punnett square → 16 combinations. F2 phenotypic ratio: 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green = 9:3:3:1. Interpretation: R and Y assort independently → each trait follows 3:1 independently → combined: (3R:1r) × (3Y:1y) = 9:3:3:1. Molecular basis: homologous pairs of different chromosomes orient independently at metaphase I of meiosis → independent segregation. Chromosome 1 can go to either pole independently of chromosome 2.

Go deeper

Condition for independent assortment: genes must be on different chromosomes (or very far apart on the same chromosome → effectively unlinked). If genes are on the same chromosome (linked): deviates from 9:3:3:1 → linkage (Morgan, 1910). Probability approach: P(round yellow F2) = P(round) × P(yellow) = 3/4 × 3/4 = 9/16. Independent assortment = basis of genetic recombination between parental allele combinations.

From Class 10

When Mendel crossed plants differing in two traits (like seed colour and seed shape), he found that the two traits were inherited independently of each other — each pair of alleles sorted into gametes without influencing the other.