9 min readPathway sheet1-mark MCQ on the ratios; a 3-mark question on the F1 and F2 results; sometimes a 5-mark answer with the 16-box checkerboard or a 4-mark case

Mendel's dihybrid cross and independent inheritance

When Mendel followed seed shape and seed colour together, the F2 ratio was 9:3:3:1, showing that the two traits are inherited independently.

ParentsRound yellow (RRYY) × wrinkled green (rryy)

F1All round yellow (RrYy)

F2 ratio9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green

Dominant formsRound over wrinkled; yellow over green

The dihybrid cross

1
Parents: round yellow seeds (RRYY) × wrinkled green seeds (rryy).
2
Gametes: RY from the first parent; ry from the second.
3★
F1: all RrYy, all round and yellow. So round and yellow are dominant.
4
F1 self-pollinated: each F1 plant makes four kinds of gametes in equal numbers: RY, Ry, rY, ry.
5★
F2: a 4 × 4 checkerboard gives 16 boxes: 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green.
6
New combinations, round green and wrinkled yellow, appear in F2, though neither parent had them.

★ key step

F2 checkerboard (RrYy × RrYy)

Gametes	RY	Ry	rY	ry
RY	RRYY	RRYy	RrYY	RrYy
Ry	RRYy	RRyy	RrYy	Rryy
rY	RrYY	RrYy	rrYY	rrYy
ry	RrYy	Rryy	rrYy	rryy

Reading the F2 result

At least one R and at least one Y: round yellow, 9 boxes.
At least one R, but yy: round green, 3 boxes.
rr, with at least one Y: wrinkled yellow, 3 boxes.
rr and yy: wrinkled green, 1 box.
Shape alone: 12 round : 4 wrinkled = 3 : 1. Colour alone: 12 yellow : 4 green = 3 : 1. Each trait still follows the monohybrid ratio.

Monohybrid vs dihybrid cross

MonohybridFollows one pair of traits (tall/short). F2 ratio 3 : 1. Checkerboard 2 × 2 (4 boxes).

DihybridFollows two pairs of traits (seed shape and colour). F2 ratio 9 : 3 : 3 : 1. Checkerboard 4 × 4 (16 boxes).

Independent inheritance: the gene for seed shape and the gene for seed colour are passed on separately, so they can mix into new combinations. Your textbook also describes tall plants with round seeds crossed with short plants with wrinkled seeds; the pattern is the same. Some questions, including in the NCERT Exemplar, start instead from round green (RRyy) × wrinkled yellow (rrYY) seeds. F1 and F2 come out the same, but then round yellow and wrinkled green are the new combinations. New combinations are always the ones neither parent had.

How to draw the dihybrid cross

Write Parents: RRYY (round yellow) × rryy (wrinkled green).
Write the gametes, RY and ry, and then F1: RrYy (round yellow).
List the four kinds of F1 gametes: RY, Ry, rY, ry.
Draw a 4 × 4 grid, write the gametes along the top and down the side, and fill all 16 boxes.
Mark the boxes of each kind, then write the ratio 9 : 3 : 3 : 1 with the names of the four kinds of seed.

In real life

Because genes for different traits are inherited independently, breeders can bring together a useful trait from one variety and a different trait from another, for example in new kinds of peas, tomatoes or rice.

Exam angle

3 marks: “Pea plants with round yellow seeds (RRYY) are crossed with plants with wrinkled green seeds (rryy). What are the F1 and F2 results?” F1: all RrYy, round yellow. F2: 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green.

2 marks: “What does the appearance of round green and wrinkled yellow seeds in the F2 generation show?” That the genes for seed shape and seed colour are inherited independently of each other.

1 marks: “Write the F2 ratio of a dihybrid cross.” 9 : 3 : 3 : 1.

Remember it

9:3:3:1 adds up to 16 boxes: a big 9 with both dominant, two 3s with one dominant each, and a lonely 1 with both recessive.