Exercise — Heredity

1. 12 marksNCERT Cl-10 Science, Exercise Q1

   A Mendelian experiment consisted of breeding tall pea plants bearing violet flowers with short pea plants bearing white flowers. The progeny all bore violet flowers, but almost half of them were short. This suggests that the genetic make-up of the tall parent can be depicted as (a) TTWW (b) TTww (c) TtWW (d) TtWw

   Hint. Work out the height genotype and the flower-colour genotype separately, using what '100% violet' and 'almost half short' each tell you.

   Step 1 — Work out the height genotype. The short parent must be tt (recessive). Since the progeny are 'almost half short,' this is a 1:1 ratio — the signature of a Tt × tt cross, not a TT × tt cross (which would give 100% tall). So the tall parent must be Tt.

   Step 2 — Work out the flower-colour genotype. The white parent must be ww (recessive). Since the progeny are ALL violet, with no white at all, the tall parent cannot be Ww (which would give roughly half white progeny) — it must be WW, guaranteeing every offspring gets at least one W.

   Step 3 — Combine the two. The tall parent's full genotype is Tt (height) combined with WW (flower colour) — TtWW.

   ✦ Answer: (c) TtWW.

   Where students slip. Picking TTWW — that genotype would make all progeny tall (no short plants at all), which contradicts 'almost half of them were short.'

2. 22 marksNCERT Cl-10 Science, Exercise Q2

   A study found that children with light-coloured eyes are likely to have parents with light-coloured eyes. On this basis, can we say anything about whether the light eye colour trait is dominant or recessive? Why or why not?

   Hint. Check whether this same pattern would show up if light eyes were dominant, and separately check whether it would show up if light eyes were recessive.

   Step 1 — Check the 'light eyes are dominant' case. If light eye colour were dominant, a light-eyed parent would carry at least one copy of that dominant version and could readily pass it to children, who would then show light eyes too — matching the observed pattern.

   Step 2 — Check the 'light eyes are recessive' case. If light eye colour were instead recessive, a light-eyed parent must have two copies of that recessive version (it's the only way to show a recessive trait), so every child inheriting a copy from that parent would carry it — and if both parents have light eyes, all children would show it too, again matching the observed pattern.

   Step 3 — Compare the two cases. Since the same observation follows whether the trait is dominant or recessive, this study alone cannot distinguish between the two possibilities.

   ✦ Answer: No — the pattern (light-eyed children tend to have light-eyed parents) would appear whether light eyes are dominant or recessive, so this observation alone can't tell us which one it is.

   Where students slip. Assuming a trait that 'runs in families' must be dominant — a recessive trait can run in families too, especially if both parents happen to show it; family resemblance alone doesn't establish dominance either way.

3. 33 marksNCERT Cl-10 Science, Exercise Q3

   Outline a project which aims to find the dominant coat colour in dogs.

   Hint. Copy Mendel's own method: pick contrasting pure-breeding parents, cross them, then self-breed (or sibling-breed) the F1 and count the F2 carefully.

   Step 1 — Choose starting dogs. Find dogs from two contrasting, pure-breeding coat colours (e.g. black and white) — lines that reliably produce only their own colour when bred among themselves.

   Step 2 — Cross them and record F1. Breed a pure black-coated dog with a pure white-coated dog, and record the coat colour of every F1 puppy.

   Step 3 — Identify the dominant colour. If all F1 puppies show one colour (say, black), that colour is likely dominant, and the other (white) is recessive.

   Step 4 — Confirm with F2. Breed the F1 (black) dogs among themselves and record the coat colours of a large number of F2 puppies. If the 'missing' colour (white) reappears in roughly one quarter of them, with an approximate 3:1 ratio of black to white, this confirms black as dominant.

   Step 5 — Keep careful counts. Record the exact number of puppies of each colour at every generation, since it was precisely this careful counting that let Mendel establish his rules of inheritance in the first place.

   ✦ Answer: Cross pure-breeding dogs of two contrasting coat colours, record the F1 colour (the dominant one, if uniform), then breed F1 dogs together and count F2 colours — a roughly 3:1 ratio confirms which colour is dominant, mirroring Mendel's own pea-plant method.

   Where students slip. Skipping the F2 generation and concluding from F1 alone — F1 uniformity only suggests which colour is dominant; the 3:1 reappearance in F2 is what actually confirms it, the same way it did for tallness in peas.

4. 42 marksNCERT Cl-10 Science, Exercise Q4

   How is the equal genetic contribution of male and female parents ensured in the progeny?

   Hint. Think about what would happen to the DNA amount each generation if germ cells carried a full, unreduced set of chromosomes.

   Step 1 — State the problem this solves. If germ cells carried the full, normal number of chromosomes, combining two of them at fertilisation would double the DNA amount every generation.

   Step 2 — Describe the actual mechanism. Each germ cell (sperm or egg) instead carries only one chromosome from each pair — half the normal number — rather than a complete matching set.

   Step 3 — Explain how this restores balance. When two germ cells combine at fertilisation, the normal two-copies-per-pair number is restored, since exactly one full set is contributed by each parent.

   ✦ Answer: Germ cells carry only half the normal chromosome number (one from each pair); combining two germ cells at fertilisation restores the full number, with each parent contributing exactly one complete set — keeping the DNA amount constant across generations.

   Where students slip. Saying each parent 'contributes half their DNA at random' without explaining the actual mechanism — the key point is that germ cells are specifically built to carry just one chromosome from each pair, which is what guarantees equal, non-doubling contribution from each parent.

Solutions written by the tuition.in editorial team and checked against the NCERT Class 10 Science textbook, Reprint 2026-27 (jesc108.pdf) — a 6-page chapter with two in-text question sets (6 questions total) plus one end-of-chapter Exercise (4 questions, not 12 — a stale figure carried over from the old combined 'Heredity and Evolution' chapter). The Evolution half is gone entirely, and the chapter never uses 'allele', 'genotype', 'phenotype', 'Punnett square', or the formal law names — it teaches through Mendel's own TT/Tt/tt notation and narrative.. Questions are referenced from the NCERT textbook for identification.