Biology · Principles of Inheritance and Variation · NEET
It is Mendel's THIRD law. It says that when two pairs of characters (two genes) are inherited together in a dihybrid cross, the separation of one gene pair into gametes does not affect the separation of the other gene pair. Each gene pair assorts (sorts itself) independently. So an RrYy plant makes four gamete types — RY, Ry, rY, ry — in equal numbers, because R/r sorting is independent of Y/y sorting.
Take RrYy x RrYy. Because each gene follows the 3:1 rule on its own, seed shape gives 3 Round : 1 wrinkled, and seed colour gives 3 Yellow : 1 green. Since the two genes assort independently, you MULTIPLY the two ratios: (3 Round : 1 wrinkled) x (3 Yellow : 1 green) = 9 Round-Yellow : 3 Round-green : 3 wrinkled-Yellow : 1 wrinkled-green. That is the 9:3:3:1 F2 phenotypic ratio. NEET loves this multiplication trick — it saves you from drawing a 16-box Punnett square.
Segregation is about ONE gene: the two alleles of a single gene separate cleanly into different gametes (purity of gametes), giving a 3:1 monohybrid ratio. Independent Assortment is about TWO or more genes: it describes how different gene pairs sort independently of each other, giving the 9:3:3:1 dihybrid ratio. Simple line: segregation = alleles of the same gene separate; independent assortment = different genes separate independently.
No. It works only for genes on DIFFERENT chromosomes (or genes very far apart on the same chromosome). NCERT clearly states that the law does NOT hold true for genes located close together on the same chromosome — these are called linked genes. Linked genes tend to stay together, so you do not get a clean 9:3:3:1. This exact idea has been asked as an Assertion-Reason question in NEET 2022.
An RrYy plant makes 2^2 = 4 types of gametes (RY, Ry, rY, ry). When you self-cross RrYy x RrYy, the Punnett square is 4 x 4 = 16 boxes. Those 16 boxes collapse into 9 genotypes and 4 phenotypes in the 9:3:3:1 ratio. The general rule '2^n gamete types' (n = number of heterozygous gene pairs) is built directly on independent assortment.
During meiosis (Metaphase I), homologous chromosome pairs line up at the cell's middle. Each pair orients independently of the other pairs, so which chromosome goes to which pole is random for every pair. Genes on different chromosomes therefore end up mixed into gametes in all possible combinations. This is why Mendel's law later fit perfectly into the Chromosomal Theory of Inheritance.
Genes R and Y follow independent assortment. If RRYY produce round yellow seeds and rryy produce wrinkled green seeds, what will be the phenotypic ratio of the F2 generation?
Assertion (A): Mendel's law of Independent assortment does not hold good for the genes that are located closely on the same chromosome. Reason (R): Closely located genes assort independently.
In a testcross involving F1 dihybrid flies, more parental-type offspring were produced than the recombinant-type offspring. This indicates:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is often called Mendel's Third Law, after the Law of Dominance and the Law of Segregation. It comes from his dihybrid (two-trait) experiments.
A dihybrid cross — for example RrYy x RrYy (round yellow seeds x round yellow seeds) — which gives the 9:3:3:1 F2 phenotypic ratio.
9:3:3:1 for a normal dihybrid cross with complete dominance. If one gene shows incomplete dominance, the categories change (for example 2 x 3 = 6 phenotypes), as tested in NEET 2023.
During Metaphase I, each homologous chromosome pair lines up and separates independently of the others. Genes on different chromosomes therefore combine randomly in gametes — this is the physical basis of the law and part of the Chromosomal Theory of Inheritance.
It explains genetic variation and lets you quickly calculate gamete types (2^n) and F2 ratios by multiplying single-gene ratios. NEET regularly tests the 9:3:3:1 ratio and the linkage exception, so this concept appears almost every year.
