10 min readPathway sheet1-mark MCQ and assertion-reason; a 3-mark question with a cross; often a 5-mark answer with the checkerboard

Mendel's monohybrid cross: dominant and recessive traits

Mendel crossed tall and short pea plants. F1 plants were all tall, but F2 gave tall and short in a 3:1 ratio: tall is dominant, short is recessive.

Mendel's plantGarden pea

F2 ratio of what we see3 tall : 1 short

F2 ratio of gene make-up1 TT : 2 Tt : 1 tt

Dominant traitShows even when only one copy is present

Mendel and his experiments

Gregor Johann Mendel worked with the garden pea. He chose traits with two clear, contrasting forms, such as tall/short plants, round/wrinkled seeds and violet/white flowers. He counted the plants of each kind in every generation, which helped him find the rules of inheritance. A child gets equal genetic material from both parents, so every trait is controlled by two copies, one from the mother and one from the father.

The monohybrid cross: tall × short

1
Parents: pure tall (TT) × pure short (tt).
2
Gametes: the tall parent gives T; the short parent gives t.
3★
F1 generation: all plants are Tt, and all are tall. The short trait is hidden, not lost.
4
F1 plants self-pollinated: Tt × Tt. Each plant gives gametes T and t in equal numbers.
5★
F2 generation: TT, Tt, Tt, tt, that is 3 tall : 1 short in what we see, and 1 TT : 2 Tt : 1 tt in gene make-up.
6
Conclusion: the short trait comes back in F2, so F1 plants carried both traits, but only the dominant one, tallness, showed.

★ key step

F2 checkerboard (Tt × Tt)

Gametes	T	t
T	TT (tall)	Tt (tall)
t	Tt (tall)	tt (short)

Dominant vs recessive

Dominant traitShows even when only one copy is present (TT or Tt). Written with a capital letter, e.g. T for tall.

Recessive traitShows only when both copies are recessive (tt). Written with a small letter, e.g. t for short.

Genotype means the gene make-up (TT, Tt or tt); phenotype means the trait we can see (tall or short). TT and Tt plants look the same: both are tall. These two words are not used in your textbook, but they are correct.

How to draw the monohybrid cross

Write Parents: TT (tall) × tt (short).
Below, write the gametes in circles: T and t.
Draw an arrow down to F1: Tt (all tall).
Write F1 × F1 (selfing): Tt × Tt.
Draw a 2 × 2 checkerboard with the gametes T and t along the top and down the side, and fill in TT, Tt, Tt, tt.
Under it, write both ratios: 3 tall : 1 short and 1 TT : 2 Tt : 1 tt.

In real life

Plant breeders in India use Mendel's rules when they cross two plants to bring together useful traits, such as a sturdy short stem and good grain, in new rice and wheat varieties.

Exam angle

3 marks: “A pure tall pea plant (TT) is crossed with a pure short pea plant (tt). Show the F1 and F2 generations and give the F2 ratios.” F1: all Tt, tall. F1 selfed: TT, Tt, Tt, tt; 3 tall : 1 short; 1 TT : 2 Tt : 1 tt.

2 marks: “What are dominant and recessive traits? Give one example of each from Mendel's work.” Dominant: shows with one copy (tall, T). Recessive: shows only with two copies (short, tt).

5 marks: “Describe Mendel's monohybrid cross with a checkerboard. What did he conclude from it?” TT × tt → F1 all Tt tall; selfing; 2 × 2 checkerboard; 3:1 and 1:2:1; two copies per trait, one from each parent; tall dominant; recessive hidden in F1 but not lost.

Remember it

Capital letters win: one capital is enough to show the trait. One trait gives 3:1; two traits give 9:3:3:1.