Heredity — NCERT Solutions, AI Tutor & Practice

Understand: Heredity is the passing on of characters (traits) from one generation to the next, i.e., from parents to offspring.

Analyse:

• Traits are controlled by units called genes, which are located on chromosomes inside the nucleus.
• During sexual reproduction, the male and female gametes each carry a set of chromosomes.
• Fertilisation combines the DNA from both parents, so the offspring inherits genes from both.

Conclude: Traits are transferred through genes present in the DNA carried by gametes. The offspring therefore shows a mixture of characteristics inherited from both parents.

Understand: Mendel crossed pure-breeding pea plants differing in a single character.

Analyse:

• A dominant trait is one that expresses itself in the offspring even when only one copy of its gene is present.
  • Example: In a cross between tall (TT) and short (tt) pea plants, all F₁ plants were tall, so tallness is dominant.
• A recessive trait is one that does not express in the presence of the dominant allele; it appears only when both alleles are recessive.
  • Example: Shortness (tt) is the recessive trait, hidden in the F₁ generation but reappearing in F₂.

Conclude: Dominant = tall (T), Recessive = short (t).

Parents (P): TT (tall) × tt (short)

Gametes: T and t

F₁ generation: All offspring are Tt (tall) — 100% tall.

F₁ self-cross: Tt × Tt

Punnett square:

| | T | t | |---|---|---| | T | TT | Tt | | t | Tt | tt |

F₂ genotypes: $1\,TT : 2\,Tt : 1\,tt$

F₂ phenotypes: 3 tall : 1 short

Phenotypic ratio = $3:1$ (Genotypic ratio = $1:2:1$).

Understand: Mendel crossed pure round-yellow (RRYY) seeds with pure wrinkled-green (rryy) seeds.

Analyse:

• F₁ generation: All seeds were round and yellow (RrYy) — both dominant traits appeared.
• On self-pollinating the F₁ (RrYy × RrYy), four types of seeds appeared in F₂:
  • Round-yellow
  • Round-green
  • Wrinkled-yellow
  • Wrinkled-green

Conclude:

• F₂ ratio = $9 : 3 : 3 : 1$
  • 9 round-yellow : 3 round-green : 3 wrinkled-yellow : 1 wrinkled-green.
• The appearance of new combinations (round-green, wrinkled-yellow) shows that the two traits are inherited independently of each other.

Understand: Humans have 23 pairs of chromosomes, of which one pair is the sex chromosomes.

Analyse:

• Females have two X chromosomes (XX); all their eggs carry an X.
• Males have one X and one Y chromosome (XY); their sperm carry either an X or a Y.

Cross:

• If an X-carrying sperm fertilises the egg → XX → girl.
• If a Y-carrying sperm fertilises the egg → XY → boy.

Conclude: The sex of the child is determined by the type of sperm (X or Y) contributed by the father. The mother's egg always contributes an X, so she does not decide the child's sex.

Understand: Blood group is controlled by three alleles: $I^A$, $I^B$ (both dominant) and $i$ (recessive, group O).

Analyse:

• For the child to be group O (genotype ii), it must have received an i allele from each parent.
• So the father (group A) must be $I^A i$ and the mother (group B) must be $I^B i$.

Cross: $I^A i \times I^B i$

| | $I^A$ | i | |---|---|---| | $I^B$ | $I^A I^B$ (AB) | $I^B i$ (B) | | i | $I^A i$ (A) | $ii$ (O) |

Conclude: Since both parents carry the recessive i allele, a child of genotype ii (blood group O) is possible. This shows how recessive traits can appear in offspring.

Understand:

• Inherited traits are those that are controlled by genes present in the DNA and are passed from parents to offspring (e.g., eye colour, blood group).
• Acquired traits are those developed during an individual's lifetime due to environment, use, or practice (e.g., a person's low body weight due to starvation, or muscles built by exercise).

Analyse:

• Acquired traits affect only the somatic (body) cells, not the germ cells (gametes).
• Since only the DNA of gametes is passed to the next generation, changes in body cells do not reach the offspring.

Conclude: Acquired traits are not inherited because they do not bring about any change in the genes/DNA of the reproductive cells.

Law of Dominance:

• In a pair of contrasting characters (alleles), one allele is dominant and expresses itself, while the other is recessive and remains hidden.
• Example: In Tt, only the tall (T) trait is seen.

Law of Segregation (Purity of Gametes):

• The two alleles of a character separate (segregate) during gamete formation, so that each gamete carries only one allele of the pair.
• At fertilisation, the alleles pair up again.
• This is why recessive traits reappear in the F₂ generation.

Conclusion: These laws together explain the $3:1$ phenotypic ratio in the F₂ of a monohybrid cross.

Understand: Mendel needed an organism that gave quick, clear, and controllable results.

Reasons for choosing the pea plant:

1. It has several easily observable contrasting traits (e.g., tall/short, round/wrinkled seeds, green/yellow pods).
2. It has a short life cycle, giving many generations quickly.
3. It produces a large number of offspring, allowing statistical (ratio-based) analysis.
4. It is normally self-pollinating, but can also be cross-pollinated artificially, so breeding could be controlled.
5. The traits were clear-cut with no intermediate forms.

Conclude: These features made the pea plant ideal for studying inheritance and obtaining reliable ratios.

Understand: Rolling (R) is dominant; non-rolling (r) is recessive. Both parents are Rr.

Cross: Rr × Rr

| | R | r | |---|---|---| | R | RR | Rr | | r | Rr | rr |

Genotypic ratio: $1\,RR : 2\,Rr : 1\,rr$

Phenotypes:

• Rollers (RR + Rr) = $3/4$
• Non-rollers (rr) = $1/4$

Conclude: The fraction of children expected to be non-rollers = $\dfrac{1}{4}$ (25%).

Understand: Consider flower colour where violet (V) is dominant over white (v).

Homozygous organism:

• Has two identical alleles for a trait.
• Examples: VV (pure violet) or vv (pure white).
• Breeds true — always produces offspring of the same type when self-crossed.

Heterozygous organism:

• Has two different alleles for a trait.
• Example: Vv — appears violet (dominant expressed) but carries the recessive white allele.
• Does not breed true — produces mixed offspring when self-crossed.

Conclude: Homozygous = identical alleles (VV or vv); Heterozygous = different alleles (Vv).

Understand: In Mendel's monohybrid crosses, the F₂ generation always showed the $3:1$ ratio regardless of which parent contributed the dominant or recessive trait.

Analyse:

• When the dominant tall (TT) parent was the male, and also when it was the female, the results were the same.
• The recessive trait reappeared in F₂ in both cases in equal proportion.
• This means each parent contributes one full set of alleles to the offspring through gametes.

Conclude: Since the offspring's traits do not depend on which parent supplied a particular allele, both parents contribute equally to the genetic material (DNA) of the offspring. This is why each offspring has one copy of every gene from the mother and one from the father.