Biology · Principles of Inheritance and Variation · NEET
A dihybrid cross studies TWO different traits at once. In Mendel's classic example the two traits were seed colour (yellow or green) and seed shape (round or wrinkled). 'Di' means two, so a dihybrid is a hybrid for two genes. He crossed a pure round-yellow plant (RRYY) with a pure wrinkled-green plant (rryy). This is different from a monohybrid cross, which follows only ONE trait.
Because the two genes assort independently, each trait behaves like its own monohybrid 3:1 cross. Seed shape gives 3 round : 1 wrinkled, and seed colour gives 3 yellow : 1 green. You simply multiply them: (3 round : 1 wrinkled) x (3 yellow : 1 green) = 9 round-yellow : 3 round-green : 3 wrinkled-yellow : 1 wrinkled-green. That product is the 9:3:3:1 ratio. NEET loves this exact reasoning.
The parents are RRYY (round, yellow) and rryy (wrinkled, green). Each parent makes only ONE type of gamete: RY from the first, ry from the second. These unite to give the F1 = RrYy, which is round and yellow because R and Y are dominant. All F1 look alike.
Four gamete types: RY, Ry, rY, and ry. Use the formula 2 to the power n, where n = number of heterozygous gene pairs. Here n = 2, so 2 squared = 4 gamete types. This is why the Punnett square for a dihybrid cross has 4 rows and 4 columns.
It is a 4 x 4 grid = 16 boxes, because the F1 makes 4 gamete types on each side (4 x 4 = 16). Those 16 boxes sort into 4 phenotype classes in the ratio 9:3:3:1. If you count carefully: 9 show both dominant traits, 3 show one dominant, 3 show the other dominant, and 1 shows both recessive traits.
9:3:3:1 is the PHENOTYPE (appearance) ratio. The genotype ratio is more detailed: 9 different genotypes appear (like 1 RRYY, 2 RRYy, 2 RrYY, 4 RrYy, and so on) but many different genotypes can give the same appearance. For NEET, remember 9:3:3:1 is about how they LOOK, not their exact gene combination.
They are: 9 round-yellow (both dominant), 3 round-green, 3 wrinkled-yellow, and 1 wrinkled-green (both recessive). Notice that round-green and wrinkled-yellow are NEW combinations not seen in the parents — these are recombinant types produced by independent assortment.
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?
Match List-I (Type of cross) with List-II (Phenotypic ratio). A. Monohybrid Cross, B. Dihybrid Cross, C. Incomplete dominance, D. Test Cross | I. 1:1, II. 1:2:1, III. 3:1, IV. 9:3:3:1.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is the PHENOTYPE ratio (how offspring look). The genotype pattern is more detailed, giving 9 genotypes. For NEET remember 9:3:3:1 describes appearance.
Multiply two monohybrid 3:1 ratios: (3:1) x (3:1) = 9:3:3:1. This works only when the two genes assort independently.
A monohybrid cross follows ONE trait and gives a 3:1 F2 ratio. A dihybrid cross follows TWO traits and gives a 9:3:3:1 F2 ratio.
The F1 (RrYy) makes 4 gamete types (2 to the power 2), so the Punnett square is 4 x 4 = 16 boxes.
The Law of Independent Assortment — each pair of alleles separates into gametes independently of the other pair, which is why new trait combinations appear in F2.
