Q.State and explain Mendel's Law of Independent Assortment with reference to a dihybrid cross involving pea plant. Consider the traits of seed shape (round vs. wrinkled) and seed colour (yellow vs. green). OR Given below is a diagram of the Lac operon in the presence of an inducer. Carefully observe the diagram and answer the following questions :

Imagine you are packing two different suitcases for a trip. One suitcase is for clothes, the other for books. How you arrange the shirts inside the clothes suitcase has nothing to do with how you stack the books in the book suitcase. The two packing jobs happen independently.

That is the core intuition behind Mendel's Law of Independent Assortment. It says that when an organism passes on its genes to its offspring, the inheritance of one trait (like seed colour) does not influence the inheritance of another, separate trait (like seed shape). Each trait gets its own "lottery ticket" during reproduction, and the tickets are drawn separately.

The precise meaning

Gregor Mendel, working with pea plants in the 19th century, noticed that certain pairs of characteristics — for example, seed colour (yellow or green) and seed shape (round or wrinkled) — were inherited completely independently of each other. A plant could pass on a yellow colour gene and a wrinkled shape gene together, or a green colour and a round shape, or any combination. The chance of getting a particular colour was always 50-50 (in a certain cross), and the chance of getting a particular shape was also 50-50, and these chances did not affect each other.

Why it matters

This law explains the enormous variety we see in living things. Because traits are shuffled independently, a single pair of parents can produce offspring with many different combinations of characteristics. For a human student, think of it this way: your eye colour and your hair colour are not tied together by fate. You could have your mother's eyes and your father's hair, or vice versa, or a mix. Independent assortment is one of the main reasons siblings (except identical twins) look different from each other.

In the NCERT Class 12 Biology textbook, Mendel's Law of Independent Assortment is stated as:

"When two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters."

This means that during the formation of eggs and sperm (gametes), the alleles (versions of a gene) for one trait separate into gametes without being influenced by the alleles for another trait.

A few key points to remember

• It is the second law Mendel discovered (the first is the Law of Segregation, which says each organism has two copies of each gene and passes one copy randomly to offspring).
• It explains why a dihybrid cross (a cross involving two traits) yields a 9:3:3:1 ratio of offspring types — but you do not need to memorise that ratio for a prose subject. …