In sexual reproduction, DNA comes from two parents, combining their genetic material. This mixing produces new combinations of traits in offspring. In asexual reproduction, only one parent is involved and offspring arise mainly from DNA copying, introducing only minor errors. Thus, sexual reproduction generates far greater variation.

Source: Chapter 8, Section 8.1 — Accumulation of Variation During Reproduction

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The key point examiners look for is two parents → mixing/combining of DNA → more variation. Contrast this with asexual reproduction where variation arises only from small copying errors. Mentioning both sides strengthens the answer for full 2 marks. Avoid vague statements like "more diversity"; be specific about why (combining genetic material from two individuals).

Sexual reproduction produces greater variation among offspring.

Reasons:

1. Sexual reproduction involves two individuals, so the offspring inherits DNA from both parents, creating new combinations of traits.
2. Traits can be inherited separately, giving rise to new combinations not present in either parent.
3. During asexual reproduction, offspring are nearly identical to the parent (e.g., sugarcane field shows very little variation), whereas sexually reproducing organisms like human beings show quite distinct variations among individuals.

Thus, modes of sexual reproduction allow greater variation to be generated, which helps in survival of the species.

Source: Chapter 7 (What you have learnt), Chapter 8 (Introduction)

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• Examiners expect you to name the type (sexual) and give at least 2 clear reasons for full marks.
• Key points to include: involvement of two parents → DNA from both → new trait combinations; contrast with asexual reproduction (near-identical offspring).
• The sugarcane vs. humans example from the textbook introduction is a great supporting point — use it to show you've read the chapter.
• Avoid vague statements; say why two parents lead to more variation (new DNA combinations, separate inheritance of traits).

Environmental conditions determine which variations are preserved. Individuals with variations suited to the current environment survive and reproduce, passing those traits to the next generation — a process called natural selection.

Over time, this leads to accumulation of beneficial variations, gradually changing the species. For example, heat-resistant bacteria survive a heat wave and multiply, while others die out.

Source: Chapter 8, Section 8.1 — Accumulation of Variation During Reproduction

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• The key phrase examiners look for is "selection of variants by environmental factors" — this is directly from the textbook.
• Mention a concrete example (heat-resistant bacteria) to secure both marks.
• Avoid over-explaining heredity or DNA copying — the question is about which variations are preserved and the long-term effect on the species.
• Two clear points = 2 marks: (1) environment selects variants, (2) effect on species over time.

The reappearance of short plants in F2 shows that the 'shortness' trait was not lost or destroyed in the F1 generation — it was merely hidden (unexpressed). The F1 plants inherited both the tallness and shortness factors, but only tallness was expressed. When F1 plants self-pollinated, the shortness factor was transmitted to the next generation and reappeared in ¼ of F2 plants.

• The trait that gets expressed (tallness) is called the dominant trait.
• The trait that remains hidden (shortness) is called the recessive trait.

Source: Chapter 8, Section 8.2.2 – Rules for the Inheritance of Traits

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• The key idea examiners expect: the recessive trait is inherited but not expressed in F1 — it is carried in the Tt genotype. Students often incorrectly say the trait "disappeared" or "mixed."
• Always name both terms (dominant and recessive) with examples — this question specifically asks for both, so leaving either out loses marks.
• The 3-mark split is roughly: 1 mark for explaining shortness was hidden (not lost), 1 mark for it being carried in F1 and reappearing in F2, 1 mark for correctly naming dominant and recessive with the correct trait matched to each term.

The passing of traits from parents to their offspring is called heredity (inheritance).

Source: Heredity, Section 8.2

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The examiner expects the single key term heredity (also accept inheritance). The passage explicitly states: "The rules of heredity determine the process by which traits and characteristics are reliably inherited." No further elaboration is needed for a 1-mark question.

This shows that the tall trait is dominant over the short trait — only one copy of the tall factor is enough to express tallness, masking the recessive short trait completely.

Source: Chapter 8, Section 8.2.2

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Examiners expect the key terms dominant and recessive here. The F1 result (all tall, no medium) proves one trait completely masks the other — that is the definition of dominance. Avoid saying the short trait "disappeared"; it was inherited but not expressed.

The reappearance of short plants in F₂ proves that the F₁ tall plants were not pure tall — they had inherited both the tallness and shortness traits. The shortness trait was present but not expressed (recessive) in F₁. This shows that two copies of a factor (gene) controlling a trait are present, where 'T' (tallness) is dominant and 't' (shortness) is recessive, giving F₁ genotype Tt.

Source: Chapter 8, Section 8.2.2

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• The key point examiners want: F₁ plants carried both traits (Tt), but only expressed the dominant one.
• Second key point: shortness is recessive — it reappears only when both copies are 't' (tt) in F₂.
• Mention dominant/recessive terminology and the idea of two copies of genes — these fetch marks.
• Avoid saying "the trait disappeared in F₁" without explaining it was hidden, not gone.

This demonstrates Mendel's Law of Independent Assortment. When F1 tall round-seeded plants were self-pollinated, F2 offspring showed new combinations — tall wrinkled and short round — because the traits for height and seed shape are inherited independently of each other, allowing factors (genes) to recombine freely during reproduction.

Source: Chapter 8, Section 8.2.2

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• The key phrase in the question is "combinations not seen in either parent" — this is the hallmark of Independent Assortment, not just Dominance/Recessiveness.
• Examiners expect you to name the law, explain it means traits are inherited separately, and give the new combinations (tall wrinkled / short round) as evidence.
• Don't confuse this with the Law of Segregation (which explains dominant/recessive in monohybrid crosses). Dihybrid = Independent Assortment.

(a) Sexually reproducing organisms have two copies of each gene (one from each parent). During fertilisation, two germ cells (gametes) fuse to form the new individual. If each germ cell carried two copies, the offspring would have four copies, and copy number would keep doubling each generation. Therefore, each germ cell must carry only one copy of each gene so that the offspring, after fusion of two gametes, has the correct two copies.

(b) The allele for tallness (T) is dominant over the allele for shortness (t). Even though the plant carries the recessive allele (t), the dominant allele (T) expresses itself and masks the effect of the recessive allele. Hence the plant appears tall.

Source: Chapter 8, Section 8.2 Heredity

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• Part (a) tests understanding of why meiosis halves the chromosome number — so fertilisation restores the correct diploid number. The key logic is: germ cell (n) + germ cell (n) → offspring (2n).
• Part (b) tests the concept of dominance vs. recessiveness. Always use the correct terms "dominant" and "recessive" — examiners look for these specific words.
• Budget roughly 1 mark for (a)'s logic and 1 mark for the conclusion; 1 mark for (b)'s explanation of dominance.

A gene is a functional unit of DNA on a chromosome that carries information for inheritance of a specific trait from parents to offspring.

Source: Chapter 8, Section 8.2 Heredity; Chapter 7, Section 7.1

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• Examiners expect you to link gene → DNA → trait/inheritance in one line.
• The passage states that chromosomes contain DNA, which is the "information source" for making proteins and inheriting features — a gene is the specific unit of that information for one trait.
• Avoid vague answers like "genes are in cells." Always mention DNA, chromosome, and trait/inheritance for full credit.

A gene is a segment of DNA that controls the production of a specific protein, which in turn determines a physical trait.

In pea plants, the gene for height exists in two forms (alleles): T (tall) and t (short). The allele T directs the production of a protein (enzyme) that promotes normal growth, making the plant tall. Allele t does not produce this functional protein, so the plant remains short.

Since T is dominant, even one copy (Tt) makes the plant tall. Both copies must be tt for the plant to be short. Thus, the gene controls the characteristic by determining which protein is made.

Source: Chapter 8, Section 8.2.2

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• Examiners expect three clear points for 3 marks: (1) what a gene does (controls protein production), (2) how the specific alleles T and t work, and (3) the dominant/recessive relationship explaining the final trait.
• Avoid writing a full essay — keep it tight and use the terms dominant, recessive, allele for full credit.
• The phrase "protein that promotes growth" is key — it links the gene directly to the physical characteristic, which is what the question asks.

Germ cells are formed by meiosis, a special cell division that halves the chromosome number. So each germ cell carries only one copy (half set) of each gene.

If germ cells had the same number of gene copies as body cells, then after fertilisation (fusion of two germ cells), the new individual would have twice the DNA of the previous generation. This would keep doubling each generation, disrupting the control of the cellular apparatus by DNA.

Source: Chapter 7, Section 7.3.1 — Why the Sexual Mode of Reproduction?

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• The key term to use is meiosis — examiners expect it.
• Two points carry 1 mark each: (1) why germ cells have half the DNA, and (2) the consequence if they didn't (doubling of DNA each generation).
• Avoid vague answers like "to maintain balance" — be specific: fusion of two germ cells would double the DNA content each generation.

A typical human cell has 23 pairs of chromosomes (46 total). Of these, 1 pair consists of sex chromosomes (XX in females, XY in males).

The passage states humans have 22 pairs of autosomes plus one pair of sex chromosomes, totalling 23 pairs. Examiners expect both numbers clearly stated. This is a common 1-mark question — one crisp sentence suffices.

Source: Chapter 8, Section 8.2.4 Sex Determination

The father has two sex chromosomes: X and Y. During reproduction, he can pass either an X or a Y chromosome to the child. Since the mother always contributes an X chromosome, if the father contributes an X chromosome, the child will be a girl (XX). If the father contributes a Y chromosome, the child will be a boy (XY). Thus, the father's contribution determines the sex of the child.

Source: Chapter 8, Section 8.2.4 – Sex Determination

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• The examiner wants you to clearly state that the father is either XY and can pass X or Y, while the mother always passes X.
• Mention both outcomes (XX = girl, XY = boy) explicitly — each is worth marks.
• Do not say the mother determines sex; the passage clearly states sex is determined by the paternal chromosome.
• The phrase "father's contribution determines the sex" should appear in your answer to directly address the question.