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Heredity is one of the shortest chapters in Class 10 science, but it is the one where a little arithmetic earns full marks. If you can draw a Punnett square and explain what it means, you can answer almost any question in the chapter. This post works through Mendel's crosses with his real numbers, explains sex determination, shows the mistakes examiners see, and gives you a practice routine.
What does the syllabus include this year?
The CBSE Class X Science syllabus for 2026-27 lists, under Unit II: "Heredity; Mendel's contribution- Laws for inheritance of traits: Sex determination; brief introduction." The evolution topics that used to share this chapter sit in a box marked for formative assessment only. The syllabus's note for teachers is worded loosely about "Heredity and Evolution", so confirm with your teacher what your school will test. The content below is from Chapter 8 of the NCERT Class 10 Science textbook.
What did Mendel do?
Gregor Mendel grew garden peas and studied contrasting traits: round or wrinkled seeds, tall or short plants, violet or white flowers. NCERT notes that he was the first to keep count of how many individuals showed each trait in each generation, and that counting is what led him to his rules.
The one-trait cross. Mendel crossed a tall plant (TT) with a short plant (tt).
F1: every offspring gets T from one parent and t from the other, so all are Tt. All are tall; there are no medium plants.
F2: F1 plants self-pollinate, so Tt × Tt. Each parent passes T or t with equal chance.
Punnett square: TT, Tt, Tt, tt. That is 1 TT : 2 Tt : 1 tt.
What you see: TT and Tt are tall; only tt is short. So 3 tall : 1 short, and one quarter of F2 plants are short.
T is called dominant because one copy is enough to show the trait. t is recessive: it shows only when both copies are t.
Do Mendel's two-trait numbers really give 9:3:3:1?
NCERT's Figure 8.5 crosses plants with round, green seeds (RRyy) and wrinkled, yellow seeds (rrYY). All F1 seeds are round and yellow (RrYy). In F2 the four types appear in the counts Mendel reported: 315 round yellow, 108 round green, 101 wrinkled yellow and 32 wrinkled green, out of 556 seeds.
Check them against 9:3:3:1. There are 16 parts, so one part is 556 ÷ 16 = 34.75 seeds.
Round yellow: expected 9 × 34.75 ≈ 313; observed 315.
Round green: expected 3 × 34.75 ≈ 104; observed 108.
Wrinkled yellow: expected ≈ 104; observed 101.
Wrinkled green: expected ≈ 35; observed 32.
The fit is close. The new combinations (round green, wrinkled yellow) show that seed shape and seed colour are inherited independently. NCERT explains this through chromosomes: each gene set comes as separate pieces, and each germ cell takes one chromosome from each pair.
A board-style question. NCERT's first exercise crosses tall plants with violet flowers and short plants with white flowers. All offspring have violet flowers, but about half are short. What is the tall parent? All violet means the tall parent gave W every time, so it is WW. Half short means it gave t half the time, so it is Tt. The answer is TtWW.
How is sex determined in humans?
NCERT notes that species differ. In some reptiles, the temperature at which eggs are kept decides the sex; some snails can change sex. In humans, sex is largely genetic. We have 22 pairs of ordinary chromosomes and one pair of sex chromosomes. Women are XX; men are XY.
Every egg carries an X. A sperm carries either X or Y. A Punnett square of XX × XY gives XX, XX, XY, XY: half girls and half boys on average. So the child's sex depends on whether the father's sperm carries X or Y. This is a fact of biology, and it is also why blaming a mother for a child's sex makes no scientific sense.
Which mistakes come up most often?
Students struggle with the same ideas around the world. When Mills Shaw and colleagues (2008) analysed 500 of 2,443 essays from a US high-school genetics contest, they found students often could not correctly describe simple dominant and recessive patterns of inheritance. In Class 10 answers, that looks like:
Thinking "dominant" means "more common". Dominance is about which trait shows in Tt, not how many people have it.
Writing the F2 ratio as 1:2:1 when asked for appearance. 1:2:1 is the gene combinations; 3:1 is what you see.
Giving germ cells two copies. Each germ cell carries one copy of each gene.
Saying the mother decides the child's sex. She always passes an X.
A practice routine for this chapter
Draw the one-trait and two-trait crosses from memory, including the 16-box square.
Do six crosses with different parents, such as Tt × tt, which gives 1 tall : 1 short.
Redo the 9:3:3:1 check above with your own numbers, such as 800 seeds.
Explain sex determination aloud in under a minute.
When a cross does not come out right and you cannot see why, ask what went wrong at each step. Fermi's AI tutor notices where you are stuck and decides whether to explain, ask or show, so you can find the step you missed. For a revision plan across the whole subject, see our Class 10 science revision plan.
Tonight, draw the Tt × Tt square and write why only one box in four is short.
