Gene

The DNA in a cell is the source of information for making proteins. Each protein has its own gene: the stretch of DNA that carries the instructions for it. Mendel called these units factors. Today we call them genes.

Every body cell has two copies of each gene, one from each parent. The two copies may be the same or different. A germ-cell carries only one copy. When two germ-cells join, the pair is complete again.

A small example

NCERT uses plant height to show how a gene controls a trait:

• Plant height depends on the amount of a particular plant hormone.
• The amount of hormone depends on how well an enzyme works.
• The enzyme is a protein, made from the information in a gene.
• If the gene changes and the enzyme works less well, less hormone is made. The plant is short.

Letters stand for the two copies. A pea plant can be TT, Tt or tt for height, and a germ-cell carries 1 letter.

Where marks go

Question 15 of CBSE's 2026-27 Science sample paper uses V and v for flower colour and T and t for height. The board's Science marking scheme for 2026-27 writes the genetic make-up beside its answers. It puts Vv beside the violet F1 plants, and vvtt beside the short white plants. Using the letters shows the working behind each percentage. 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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