NCERT Solutions Class 10 Science Chapter 8 Heredity

In asexual reproduction the offspring is almost an exact copy of the single parent. A new trait can only show up when a small variation (a copying error in the DNA) occurs. Once a variation appears, it is passed on copy after copy, so it slowly spreads to more and more individuals over many generations.

• Compare how common the two traits are. Trait B is present in 60% of the population, while trait A is present in only 10%.
• A trait found in more individuals has had more generations to copy itself and spread; a trait found in very few individuals is likely a recent change.
• Trait B (60%) is far more widespread than trait A (10%), so trait B must have appeared earlier and trait A later.

Answer: Trait B is likely to have arisen earlier, because a higher frequency (60%) means it has had more generations to copy and spread through the population.

Variations are the small differences between individuals of the same species. The environment is not fixed: temperature, food, water and diseases keep changing. Natural selection means that when conditions change, the individuals whose variation suits the new conditions survive and reproduce, while others may die out.

• No two individuals are exactly alike; reproduction keeps adding small differences.
• When the environment changes suddenly, for example a heat wave, some individuals already carry a variation that lets them tolerate heat.
• Those heat-tolerant individuals survive and pass the helpful variation to their offspring, while individuals without it may not survive.
• Even if many individuals die, the species as a whole continues because at least some variants were suited to the new conditions.

For example, bacteria that can withstand heat survive better in a heat wave. If all the bacteria were identical and none could tolerate heat, a single heat wave could wipe out the entire population.

Answer: Variation gives a species a range of forms, so that when the environment changes, at least some individuals are suited to the new conditions and survive, keeping the species alive.

A trait is a visible feature such as plant height. Each plant carries two copies of the gene for a trait, one from each parent. A dominant trait shows up even when only one copy is present; a recessive trait shows up only when both copies are of the recessive type. Mendel found this by crossing tall and short pea plants and counting the offspring across two generations.

• Parents (P): Mendel crossed a pure tall plant (TT) with a pure short plant (tt).
• First generation (F₁): every offspring was tall (Tt). No plant was of medium height, so tallness appeared even though a shortness copy (t) was present. This makes tall the dominant trait.
• Second generation (F₂): Mendel let the F₁ tall plants self-pollinate. One quarter of the offspring were short. Shortness had been hidden in F₁ and reappeared in F₂, which makes short the recessive trait.

The F₂ shows a 3 tall : 1 short visible ratio. Tall stays visible with one or two T copies; short needs two t copies. That is exactly what dominant and recessive mean.

Answer: In F₁, tall plants (Tt) appeared even though a shortness copy was present, so tall is dominant. In F₂, the hidden shortness reappeared in one quarter of plants (3 : 1 ratio), so short is recessive.

Independent inheritance means that the gene for one trait is passed on without being tied to the gene for another trait. Mendel tested this with a dihybrid cross, where two different traits (here height and seed shape) are followed at the same time. If the traits travel together, no new combinations appear; if they travel independently, new mixed types show up in F₂.

• Parents: cross a tall plant with round seeds (TTRR) with a short plant with wrinkled seeds (ttrr).
• F₁: all offspring are tall with round seeds (TtRr), showing tall and round are dominant.
• F₂: self-pollinate the F₁ plants. Besides the two parental types, new combinations appear: tall plants with wrinkled seeds, and short plants with round seeds.
• Tallness did not stay tied to round seeds, so the two traits separated and recombined freely. They are inherited independently.

Answer: In the dihybrid cross, F₂ produced new combinations (tall-wrinkled and short-round) in a 9 : 3 : 3 : 1 ratio, showing that each trait is passed on independently of the other.

Blood group is decided by two copies of a gene. Blood group A can be one of two genotypes: AA (two A copies) or AO (one A and one O copy). Blood group O has only one genotype: OO (two O copies). To call a trait dominant or recessive, we must see how it behaves when both copies are present, and one child does not give us enough of that information.

• The daughter is O, so she is OO. She must get one O from each parent, so the father, though group A, carries a hidden O; his genotype is AO.
• Work out the cross: father AO × mother OO. The children can be AO (group A) or OO (group O), in a 1 : 1 ratio.
• The daughter is OO, so she shows blood group O whether O is dominant or recessive. Her single result fits both possibilities.
• From the father we can reason that A behaves as dominant and O as recessive (his A copy masks his O copy), but the daughter alone does not prove it; we would need to see more children.

Answer: No, this single daughter is not enough. She is OO and shows group O in either case. The cross AO × OO can give both A and O children, so one child cannot fix which trait is dominant.

Humans have 23 pairs of chromosomes. Of these, one pair is the sex chromosomes. Females have two X chromosomes (XX) and males have one X and one Y (XY). Each germ cell (egg or sperm) carries only one sex chromosome, and the combination formed at fertilisation decides the sex of the child.

• Mother's eggs: since the mother is XX, every egg she makes carries an X chromosome.
• Father's sperm: since the father is XY, half his sperm carry an X and half carry a Y.
• Fertilisation: if an X-sperm fertilises the egg, the child is XX (a girl); if a Y-sperm fertilises the egg, the child is XY (a boy).
• The egg always gives an X, so it is the father's sperm that decides the sex. The chances of a boy or a girl are equal (50% : 50%).

Answer: The mother always passes an X chromosome. The child becomes a girl (XX) if the father's sperm carries X, or a boy (XY) if it carries Y. So the father's chromosome decides the sex, with a 50 : 50 chance of each.

We read two clues from the offspring. For each trait, the offspring ratio tells us the parent's genotype. If a cross gives all dominant offspring for a trait, the dominant parent was homozygous (two same copies). If a cross gives a 1 : 1 split for a trait, the dominant parent was heterozygous (one of each copy). Here T = tall, t = short, W = violet, w = white.

• Flower colour clue: all progeny are violet (none white). The short parent is white (ww) and can only give a w, so for every offspring to be violet the tall parent must supply a W to each one, which means it is WW.
• Height clue: almost half the progeny are short. The short parent is tt, giving only t. To get short (tt) offspring, the tall parent must also be able to give a t. A 1 : 1 tall-to-short split means the tall parent is Tt.
• Combine the two clues: tall parent is Tt for height and WW for colour, so its genotype is TtWW, which matches option (c).

Answer: Correct option: (c) TtWW. The all-violet progeny forces WW for colour, and the half-short progeny forces Tt for height.

Dominance describes how a trait behaves when both forms of the gene are present in the same individual, not how common the trait is in a family or population. A trait can run in a family whether it is dominant or recessive, so seeing that light-eyed children have light-eyed parents does not reveal dominance.

• The study shows a correlation, a tendency for light-eyed parents to have light-eyed children. It does not tell us the genotypes of the family members.
• If light eye colour were recessive, light-eyed parents would both be homozygous recessive and could only have light-eyed children, which matches the observation.
• If light eye colour were dominant, light-eyed parents could still pass it on and have many light-eyed children, which also fits the observation.
• Since the data agree with both possibilities, the study is not enough to decide dominance. We would need to track how the trait passes across more crosses and generations.

Answer: No. The study only shows that the trait runs in families (a correlation), which can happen for either a dominant or a recessive trait. Deciding dominance needs data on how the trait segregates across generations.

To find which coat colour is dominant, we cross dogs of two pure (true-breeding) colours and read the colour of the F₁ offspring. The colour that appears in all the F₁ puppies is the dominant one; the colour that disappears in F₁ and returns in F₂ is recessive. This is the same logic Mendel used for peas.

• Pick pure-breeding parents: choose dogs that breed true for two colours, for example a pure black dog and a pure brown dog.
• Make the first cross (P): mate the black dog with the brown dog and record the coat colour of every puppy in the F₁ litter.
• Read the F₁ result: if all F₁ puppies are black, black is dominant; if all are brown, brown is dominant.
• Confirm with the F₂: mate two F₁ dogs. If the recessive colour reappears in about one quarter of the F₂ puppies (a 3 : 1 ratio), it confirms the conclusion.
• Repeat for reliability: use several litters and count the puppies so the result is based on numbers, not one chance litter.

Answer: Cross pure dogs of two coat colours; the colour seen in all F₁ puppies is dominant. Confirm by mating F₁ dogs and checking that the recessive colour returns in about 1 of every 4 F₂ puppies (3 : 1).

Body cells carry chromosomes in pairs, one of each pair from the mother and one from the father. Special cells called germ cells (the egg and the sperm) are made by a division that halves the chromosome number, so each germ cell carries only one chromosome from every pair. When egg and sperm join at fertilisation, the pairs are restored, half from each parent.

• Start with the parent's body cells, which have two copies of every chromosome (one maternal, one paternal).
• During the making of germ cells, this number is halved, so each egg and each sperm receives exactly one chromosome from every pair.
• At fertilisation, the egg (with one set) fuses with the sperm (with one set), and the child's cells now have two sets again, one full set from each parent.
• Because each parent supplies one complete set of chromosomes, and chromosomes carry the genes, both parents contribute equally to the genetic material of the child.

If germ cells kept the full double set, the child would end up with four sets of chromosomes. Halving the number in germ cells keeps the chromosome count of the species stable from one generation to the next.

Answer: Each germ cell carries only one chromosome from every pair, so it holds half the genetic material. At fertilisation, the egg and sperm combine to give the child two sets, one full set from each parent, ensuring an equal contribution.