Heredity – NCERT Exercises

Solution: In an asexually reproducing species, variations appear only through occasional DNA copying errors and are passed directly to the offspring. A variation that has been present in the population for a longer time gets more opportunities to spread and accumulate. Therefore trait B, found in 60 % of the individuals, is likely to have arisen earlier, and trait A (only 10 %) must have appeared more recently.

Solution: Environmental conditions — temperature, water availability, diseases, predators — keep changing. Variations produce individuals that are slightly different from one another. If a sudden change (say, a rise in temperature or a new disease) occurs:

• Most individuals may perish, but a few with a useful variation survive.
• The survivors reproduce and pass the useful variation on.
• Thus the species as a whole survives, even though many individuals die.

Without variation, a single unfavourable change could wipe out the entire population. Variation therefore acts as an "insurance policy" for the species.

Solution: Mendel crossed a pure tall pea plant (TT) with a pure dwarf pea plant (tt).

• All plants of the F₁ generation were tall (Tt) — the dwarf character had disappeared.
• When F₁ plants were self-pollinated, the F₂ generation contained both tall and dwarf plants in the ratio 3 : 1.

The dwarf trait had been hidden in F₁ but reappeared in F₂, showing that it was not lost. A trait that is expressed even when only one copy of the allele is present is called dominant (T); the one that stays hidden in the presence of a dominant allele and is expressed only when both alleles are of this type is called recessive (t). Thus Mendel's experiment demonstrated the concept of dominant and recessive traits.

Solution: Mendel performed a dihybrid cross: he crossed plants bearing round-yellow seeds (RRYY) with plants bearing wrinkled-green seeds (rryy).

• All F₁ plants produced round-yellow seeds (RrYy).
• When F₁ plants were self-pollinated, the F₂ generation showed four phenotypes in the ratio 9 round-yellow : 3 round-green : 3 wrinkled-yellow : 1 wrinkled-green.

The appearance of two new combinations — round-green and wrinkled-yellow — proved that the two pairs of traits (shape and colour of seed) did not travel together. They were inherited independently of each other. This is Mendel's Law of Independent Assortment.

Solution: No, the information is not enough. For the daughter to have blood group O, she must be homozygous (OO). She gets one O allele from her mother (who is OO, blood group O) and the other O from the father. The father therefore must be heterozygous (AO) — he has an A and an O allele.

The fact that the father shows blood group A even though he carries one O allele, shows that A is dominant over O — the daughter could only become O when she received O from both parents. So:

• Father: A O (blood group A)
• Mother: O O (blood group O)
• Daughter: O O (blood group O)

Since the father's O allele is masked in his own body but expressed in the daughter (OO), A is dominant and O is recessive. Thus the given information is actually sufficient to infer that A is dominant over O.

Note: NCERT accepts that the dominance of A over O can indeed be concluded here — once one works out the parents' genotypes, the dominance relationship becomes clear.

Solution: Humans have 23 pairs of chromosomes, of which one pair is the sex chromosome pair:

• Female: XX — produces only X-carrying eggs.
• Male: XY — produces two types of sperm: X-carrying and Y-carrying, in equal numbers.

At fertilisation:

• If an X-sperm fuses with the egg → zygote is XX → girl.
• If a Y-sperm fuses with the egg → zygote is XY → boy.

Since the mother can only give an X chromosome, the sex of the child is decided by the type of sperm (X or Y) contributed by the father. The probability of a boy or a girl is therefore about 50 : 50. Blaming the mother for the sex of the child is scientifically incorrect.

Solution: Not entirely. Certainly, variations that give an advantage in the current environment help their carriers survive and reproduce, so they tend to spread. However:

• A variation that is neutral (neither helpful nor harmful) may also be carried on in the population for a long time.
• A variation that is currently neutral or even slightly unfavourable may become advantageous if the environment changes (e.g., heat tolerance becomes useful when global temperature rises).
• Some variations survive just by chance — especially in small populations.

Thus it is better to say that variations useful under the present or a future environment are selected for. It is the environment, not the individual, that decides which variation is advantageous. Hence the statement is only partly correct.

Solution: When the number of surviving individuals of a species falls to a very low level, the genetic variation in the population is sharply reduced. For tigers this creates several problems:

• The few tigers left carry only a small fraction of the species' gene pool.
• Inbreeding (mating among close relatives) becomes unavoidable, increasing the chance that harmful recessive alleles come together and cause genetic disorders.
• If a new disease or environmental change strikes, there are unlikely to be tigers with a useful variation to survive it — the whole species could be wiped out.
• Lost genetic variation cannot be recovered — it is gone forever.

Thus small surviving populations lose adaptability and face a high risk of extinction. This is why tiger conservation is urgently important.

Answer: (c) TtWW.

Reasoning: Let T = tall (dominant), t = dwarf, W = violet (dominant), w = white.

• Short white parent must be tt ww (homozygous recessive for both).
• Progeny = all violet ⇒ every offspring received at least one W allele. This is possible only if the tall parent was homozygous for violet, i.e. WW (not Ww — a Ww × ww cross would give half white).
• Progeny = about half tall, half short ⇒ the tall parent must have been heterozygous for height (Tt). A Tt × tt cross gives 1 tall : 1 dwarf.

Combining the two conclusions, the tall parent's genotype is Tt WW, which matches option (c).