Complete Question–Answer Solutions (Intext + End-of-Chapter Exercise)
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Q1. If a trait A exists in 10% of a population of an asexually reproducing species and a trait B exists in 60% of the same population, which trait is likely to have arisen earlier?
Answer: Trait B is likely to have arisen earlier. In asexual reproduction, offspring are almost identical copies of the parent, so a new variation spreads through the population only gradually, generation after generation, as it keeps getting copied into more individuals.
Since trait B is now present in a much larger fraction of the population (60%) than trait A (10%), it must have had a longer time to spread. A trait that has had more generations to propagate is the one that originated earlier, so B predates A.
Q2. How does the creation of variations in a species promote survival?
Answer: Variation means that not every individual in a population is identical — small differences exist in body features, physiology, and behaviour. If the environment changes suddenly (say, a rise in temperature or the appearance of a new disease), individuals whose variations happen to suit the new condition are more likely to survive and reproduce.
This is well illustrated by bacteria: in a population, a few individuals may already have a chance variation that makes them heat-tolerant. If the temperature of their surroundings rises sharply, the heat-sensitive majority may die out, but the heat-resistant variants survive and continue the population.
In short, variation acts as a safety net — it ensures that a species is not wiped out entirely by a changing environment, and it also supplies the raw material on which natural selection acts over time, driving evolution.
Q3. How do Mendel’s experiments show that traits may be dominant or recessive?
Answer: Mendel crossed a pure-bred tall pea plant (TT) with a pure-bred short pea plant (tt). All the plants of the first filial (F1) generation were tall — the short trait seemed to disappear completely.
When Mendel allowed these F1 tall plants (genotype Tt) to self-pollinate, the second filial (F2) generation showed both tall and short plants in a ratio of about 3:1. This proved that the short trait had not actually been lost in F1 — it was only masked, or hidden, by the tall trait.
This is exactly the definition of dominant and recessive traits: tallness, which expressed itself in the heterozygous F1 plants, is the dominant trait, while shortness, which stayed hidden in F1 but reappeared in F2, is the recessive trait.
Q4. How do Mendel’s experiments show that traits are inherited independently?
Answer: Mendel performed a dihybrid cross using pea plants with two different characters at once — pure round, yellow seeds (RRYY) crossed with pure wrinkled, green seeds (rryy). Every F1 plant produced round, yellow seeds (RrYy), showing that round and yellow are the dominant traits.
When these F1 plants were self-pollinated, the F2 generation did not just show the two original combinations. It also produced two completely new combinations — round green seeds and wrinkled yellow seeds — in the overall ratio of 9 round-yellow : 3 round-green : 3 wrinkled-yellow : 1 wrinkled-green.
The appearance of these new combinations proves that the gene for seed shape and the gene for seed colour separate and get distributed to gametes independently of each other, rather than always travelling together. This is the basis of Mendel’s Law of Independent Assortment.
Q5. 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?
Answer: No, this single family’s data is not sufficient on its own. Blood group O corresponds to the genotype ii, and since it is recessive, the mother (group O) must be ii. The father, who shows group A, could have either genotype IAIA or IAi — we cannot tell which just from his blood group.
If the father is IAi, he can pass on the i allele, and combined with the mother’s i allele, the daughter could indeed be ii (group O) purely by chance — this outcome is possible whether A is dominant or not, so one child’s blood group cannot settle the question.
To establish dominance reliably, we would need data from many such crosses across a larger population (or already-known genetic evidence, such as the fact that both IA and IB are dominant over i), not just the outcome for a single couple’s single child.
Q6. How is the sex of a child determined in human beings?
Answer: Human sex is determined chromosomally. Women have two X chromosomes (XX) while men have one X and one Y chromosome (XY). During gamete formation, every egg produced by the mother carries an X chromosome, since she only has X chromosomes to give.
The father, however, produces two kinds of sperm in equal numbers — half carry an X chromosome and half carry a Y chromosome. Which sperm happens to fertilise the egg decides the sex of the child: an X-bearing sperm gives an XX zygote, which develops into a girl, while a Y-bearing sperm gives an XY zygote, which develops into a boy.
So it is the father’s contribution (X or Y), not the mother’s, that determines the sex of the offspring in humans.
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
Answer: Correct option: (c) TtWW.
Flower colour: Every offspring is violet, even though the other parent is white (ww, since white — the recessive trait — must be homozygous). For the tall parent to guarantee a violet-flowered offspring no matter what allele the other parent gives, it must supply the dominant W allele every single time — which only happens if the tall parent is homozygous, WW.
Height: About half the offspring are short. If the tall parent were homozygous tall (TT), crossing with tt would give 100% tall offspring — no short plants at all. Since roughly half turned out short, the tall parent must be heterozygous, Tt; a Tt × tt cross gives tall and short offspring in a 1:1 ratio, matching what’s described.
Putting the two together, the tall, violet parent’s genotype is TtWW.
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?
Answer: This observation is suggestive of light eye colour being a recessive trait, but it is not conclusive proof by itself.
The reasoning: a recessive trait can only be physically expressed when an individual carries two copies of the recessive allele (is homozygous recessive). Two such parents can only pass on that recessive allele, so their children are very likely to also show the same recessive trait — which matches the pattern observed here. A dominant trait, by contrast, can be expressed even in heterozygous individuals, so two dominant-trait parents (if both are heterozygous) could occasionally have a child who does not show the trait at all — a pattern that isn’t what was described.
That said, a purely observational study of this kind cannot rule out other explanations and does not count as a controlled breeding experiment. To confirm the trait is genuinely recessive, geneticists would want pedigree analysis across several generations or controlled crosses, similar in spirit to what Mendel did with pea plants.
Q3. Outline a project which aims to find the dominant coat colour in dogs.
Answer: Step 1 — Choose the population: Select a group of dogs of the same breed that show two clearly contrasting coat colours (for example, black and white), and where possible, identify individuals that are likely to be pure-bred for each colour (i.e., have a family history of only that colour).
Step 2 — Set up controlled crosses: Breed pure black-coated dogs with pure white-coated dogs over one or more litters, keeping careful pedigree/breeding records of every parent and pup.
Step 3 — Observe the F1 generation: Note the coat colour of all the puppies born from this cross. Whichever colour appears in all (or nearly all) of these F1 pups is the dominant coat-colour trait; the colour that disappears is recessive.
Step 4 — Confirm with an F2 generation: Breed F1 dogs among themselves and observe the coat colours of their pups. If the trait follows simple Mendelian inheritance, the dominant and recessive colours should reappear in roughly a 3:1 ratio, confirming the result from Step 3.
Step 5 — Repeat and record: Repeat the crosses over multiple litters and, ideally, multiple dog families to get a statistically reliable result, and maintain records throughout to rule out the influence of any other coat-related genes.
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Q4. How is the equal genetic contribution of male and female parents ensured in the progeny?
Answer: Every normal body cell has a full (diploid, 2n) set of chromosomes, but the gametes — sperm from the father and the egg from the mother — are produced by meiosis and carry only a half (haploid, n) set each.
When fertilisation occurs, the sperm and egg fuse to form a zygote, which restores the full diploid (2n) number of chromosomes — exactly half contributed by the father’s sperm and exactly half by the mother’s egg.
So, although a sperm cell is physically much smaller than an egg cell, each still carries the same number of chromosomes (and hence genes). This is what mathematically guarantees an equal genetic contribution from both parents to the offspring, regardless of any difference in the size of the two gametes.
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