In-text Questions — Mendel and Sex Determination — Heredity

1. 13 marksNCERT Cl-10 Science, In-text Qs after §8.2.4, Q1

   How do Mendel's experiments show that traits may be dominant or recessive?

   Hint. Look at what happens to the 'missing' trait in F1 — does it vanish, or does it just stay hidden and reappear later?

   Step 1 — Look at the F1 generation. Crossing a tall pea plant with a short one gave an F1 generation that was entirely tall — no trace of shortness was visible at all.

   Step 2 — Look at the F2 generation. When these F1 tall plants were self-pollinated, one quarter of the F2 plants turned out to be short.

   Step 3 — Interpret this pattern. Since shortness reappeared in F2, it must have been inherited (carried) in the F1 plants all along, just not expressed — this hidden-then-reappearing behaviour is exactly what defines a recessive trait, while tallness, which showed up whenever present, is the dominant trait.

   ✦ Answer: F1 plants were all tall (showing only one trait, not a blend), but shortness reappeared in a quarter of F2 — proving shortness had been silently carried in F1. This pattern of 'expressed vs. hidden-then-reappearing' is what reveals tallness as dominant and shortness as recessive.

   Where students slip. Concluding that shortness 'disappeared' in F1 and then somehow came back — it never disappeared; it was present but unexpressed, which is the whole point of the recessive-trait concept.

2. 23 marksNCERT Cl-10 Science, In-text Qs after §8.2.4, Q2

   How do Mendel's experiments show that traits are inherited independently?

   Hint. Look for offspring with combinations of traits that neither original parent had.

   Step 1 — Recall the two-trait cross. Crossing a tall, round-seeded plant with a short, wrinkled-seeded plant gave an F1 that was all tall with round seeds.

   Step 2 — Look at the F2 generation. Self-pollinating this F1 gave an F2 with new combinations not seen in either original parent — some plants were tall with wrinkled seeds, and others were short with round seeds, alongside the original parental combinations.

   Step 3 — Interpret this. Since height and seed shape could recombine into new pairings in F2, the gene for height and the gene for seed shape don't have to travel together — each is inherited independently of the other.

   ✦ Answer: New trait combinations (tall/wrinkled and short/round) appeared in F2 that weren't present in either parent — showing that height and seed shape are inherited independently rather than being locked together.

   Where students slip. Expecting F2 to show only the two original parental combinations — the appearance of new mixed combinations is precisely the evidence for independent inheritance; if the traits weren't independent, only the original combinations would ever reappear.

3. 33 marksNCERT Cl-10 Science, In-text Qs after §8.2.4, Q3

   A man with blood group A marries a woman with blood group O and their daughter has blood group O. Is this information enough to tell you which of the traits – blood group A or O – is dominant? Why or why not?

   Hint. Try assuming A is dominant, and check the outcome is possible — then try assuming O is dominant instead, and check that too.

   Step 1 — Check whether 'A is dominant' fits the data. If A were dominant, the mother (group O, the recessive trait) must carry two copies of O. The father, being group A, could be a carrier of O alongside his A copy. If he passed his hidden O copy to the daughter, she would receive O from both parents and show group O — this fits.

   Step 2 — Check whether 'O is dominant' also fits the data. If O were instead dominant, the father (group A, now the recessive trait) must carry two copies of the A-causing version, so he could only ever pass that version on — never an O copy. The daughter would still end up with one copy from each parent and could still show group O under this alternative assumption, too.

   Step 3 — Compare the two cases. Since the observed outcome (daughter is group O) is consistent with either A being dominant or O being dominant, this one family's result can't distinguish between the two possibilities.

   ✦ Answer: No, it isn't enough — a daughter with blood group O is possible whether A is dominant over O, or O is dominant over A. Telling the two apart would need the kind of generation-tracking, large-sample cross Mendel used, not just one family's outcome.

   Where students slip. Concluding 'O must be recessive since the daughter shows it and one parent doesn't' — that reasoning only rules out some genotype combinations; it doesn't rule out the alternative where O is actually the dominant trait, since that scenario can produce the exact same result.

4. 42 marksNCERT Cl-10 Science, In-text Qs after §8.2.4, Q4

   How is the sex of the child determined in human beings?

   Hint. Both parents give one sex chromosome each — but only one parent actually has a choice of which one to give.

   Step 1 — Recall the two sex chromosomes. Women have two X chromosomes (XX); men have one X and one shorter Y chromosome (XY).

   Step 2 — Note what the mother always contributes. Every child inherits an X chromosome from the mother, regardless of the child's eventual sex, since both of her chromosomes are X.

   Step 3 — Note what the father contributes. The father can pass on either his X or his Y chromosome — an X from the father gives a girl (XX), a Y from the father gives a boy (XY).

   ✦ Answer: The mother always contributes an X; the father contributes either an X (resulting in a girl) or a Y (resulting in a boy) — so it's specifically what the child inherits from the father that determines its sex.

   Where students slip. Saying the mother's contribution decides the child's sex — she always gives the same X chromosome regardless of outcome; it's the father's X-or-Y choice that actually determines it.

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.