Biology · Principles of Inheritance and Variation · NEET
You write GAMETES, not the parents. A gamete carries only ONE allele of the pair. So a Tt parent makes two kinds of gametes: T and t. A TT parent makes only one kind: T. Write one parent's gametes across the top and the other parent's gametes down the left column. Then each inside box gets the top letter plus the side letter joined together.
Look at the parent's genotype. If it is homozygous (TT or tt), it makes only ONE kind of gamete. If it is heterozygous (Tt), it makes TWO kinds (T and t) in equal numbers. For one gene, this gives a small 2x2 grid at most. Tip: number of gamete types = 2 raised to the number of heterozygous gene pairs.
Both parents make T and t gametes. Fill the 2x2 grid: the four boxes are TT, Tt, Tt, tt. Three boxes show the tall trait (TT, Tt, Tt) and one box shows dwarf (tt). So the phenotype ratio is 3 tall : 1 dwarf. This is the classic Mendelian monohybrid F2 ratio.
From Tt x Tt the four boxes are TT, Tt, Tt, tt. The GENOTYPE ratio counts the letter combinations: 1 TT : 2 Tt : 1 tt = 1:2:1. The PHENOTYPE ratio counts how they LOOK: TT and Tt both look tall, so 3 tall : 1 dwarf = 3:1. Same square, two different ratios. NEET often asks which one, so read the question carefully.
When a pure tall parent (TT) is crossed with a pure dwarf parent (tt), every box in the grid becomes Tt. All offspring are heterozygous and, because T is dominant, they all look tall. That is why the F1 generation is uniform. The hidden dwarf allele reappears only when F1 plants are selfed (Tt x Tt).
Yes. Each box is one equally likely outcome. In a Tt x Tt cross the tt box is 1 out of 4, so the chance of a dwarf (or a recessive-disease child) is 1/4 = 25%. For a test cross Tt x tt the boxes are Tt, Tt, tt, tt, giving 1:1, so 50% show the dominant trait. NEET disease questions (like sickle-cell carriers) use exactly this 25% logic.
The production of gametes by the parents, the formation of zygotes, the F1 and F2 plants can be understood from a diagram called
A tall true breeding garden pea plant is crossed with a dwarf true breeding garden pea plant. When the F1 plants were selfed the resulting genotypes were in the ratio
A heterozygous pea plant with violet flowers was crossed with a homozygous pea plant with white flowers. Violet is dominant over white. Which one represents the expected combinations among 40 progenies?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The British geneticist Reginald C. Punnett developed it. NEET has directly asked this, so remember the name Punnett (not Mendel, who did the crosses).
Four boxes (a 2x2 grid), because each parent makes at most two kinds of gametes for one gene. A dihybrid cross (two genes) needs a 4x4 grid with 16 boxes.
It shows the genotypes in the boxes. You then translate those genotypes into phenotypes using the dominance rule. So one square gives you both a genotype ratio (like 1:2:1) and a phenotype ratio (like 3:1).
Yes. The boxes are filled the same way, but the heterozygote (like Rr in snapdragon) shows a NEW phenotype (pink). So Rr x Rr gives 1 red : 2 pink : 1 white, where the genotype ratio 1:2:1 equals the phenotype ratio.
Set up the carrier cross, for example HbA HbS x HbA HbS. The four boxes give 1 HbA HbA : 2 HbA HbS : 1 HbS HbS. Only the homozygous recessive box (1 of 4) is diseased, so the chance is 25%. NEET 2021 asked exactly this.
