Dominant and recessive traits

Mendel crossed tall pea plants with short ones. Every plant in the first generation, F1, was tall. There were no medium-height plants. Then he let the F1 plants self-pollinate. In the second generation, F2, one quarter of the plants were short.

So the F1 plants carried the shortness trait without showing it. Each plant has two copies of the gene for height. TT and Tt plants are tall, and only tt plants are short. One T is enough for tallness, so tallness is dominant. Shortness needs both copies to be t, so it is recessive.

A small example

Cross two Tt plants. The F2 comes out as 1 TT : 2 Tt : 1 tt. The first 3 are tall and the last 1 is short. So 3 in 4 plants, or 75%, are tall.

Where marks go

Question 15 of CBSE's 2026-27 Science sample paper crosses violet-flowered (VV) and white-flowered (vv) pea plants. Part A asks for the share of violet flowers in F1 and F2, with a reason. The board's Science marking scheme for 2026-27 gives 1 mark for three things together. They are 100% in F1, 75% in F2, and the reason: violet is dominant.

Part C asks about medium-height plants with light violet flowers, for 2 marks. The scheme's answer is zero percent. There are no halfway traits, since one copy of T or V acts the same as two. See how step marking works.

About the author

Deepti Sardana

Founder Member & Academic Lead

An IIT alumna (M.Sc., M.Phil.) with over 25 years in teaching, child psychology and learning assessment. As Academic Lead at EduBrahma, she leads the method TRUtesting uses to find where and why a child loses marks.

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