Unit 2- exam style questions

Q: Explain why sexual reproduction is considered costly compared to asexual reproduction.

A: It requires two parents, males cannot produce offspring directly, and only half of each parent’s genome is passed on, disrupting successful genomes.

Q: Why do the benefits of sexual reproduction outweigh its costs?

A: Because increased genetic variation provides the raw material for natural selection, allowing adaptation and survival under changing selection pressures.

Q: How does the Red Queen Hypothesis explain the persistence of sexual reproduction?

A: Host–parasite coevolution creates a constant “arms race.” Sexual reproduction produces genetically variable offspring, reducing the likelihood that all are susceptible to parasites.

Q: Why is parthenogenesis more common in colder climates or areas of low parasite density?

A: In these environments, selection pressure for genetic variability is lower, so asexual strategies are advantageous.

Q: Explain why vegetative cloning is advantageous for plants in succession.

A: It allows rapid and successful recolonisation of disturbed habitats without needing pollinators or gametes.

Q: Why are asexually reproducing populations more vulnerable to environmental change?

A: Low genetic variation limits their ability to adapt to new selection pressures.

Q: How can prokaryotes evolve quickly despite reproducing asexually?

A: Horizontal gene transfer (e.g., plasmids in bacteria/yeast) introduces new genetic material between individuals, speeding up evolution.

Q: Why must gametes be haploid?

A: To ensure chromosome number is restored to diploid at fertilisation, preventing doubling each generation.

Q: Explain the difference between homologous chromosomes and sister chromatids.

A: Homologous chromosomes are matching pairs with the same loci but potentially different alleles, while sister chromatids are identical copies of a single chromosome joined at a centromere.

Q: Why does meiosis increase genetic variation compared to mitosis?

A: Because it involves recombination and independent assortment, producing genetically unique gametes.

Q: How does independent assortment create variation?

A: Homologous chromosomes align randomly during meiosis I, so each gamete receives a random mix of maternal and paternal chromosomes.

Q: Calculate how many gamete combinations can be produced if an organism’s haploid number is 4.

A: 2ⁿ = 2⁴ = 16 different gamete combinations.

Q: Describe how crossing over increases variation.

A: Non-sister chromatids exchange DNA at chiasmata, producing recombinant chromatids with new allele combinations.

Q: Why are linked genes less likely to assort independently?

A: They are located close together on the same chromosome, so they are usually inherited together unless separated by crossing over.

Q: How does recombination frequency provide information about gene location?

A: Higher frequency indicates greater distance between linked genes, as crossing over is more likely.

Q: What is the evolutionary advantage of hermaphroditism in animals like earthworms?

A: Increases the chance of successful reproduction when encounters with mates are rare, since any two individuals can exchange gametes.

Q: Why are males described as heterogametic?

A: They have XY sex chromosomes and therefore produce two types of gametes (X-bearing and Y-bearing).

Q: What role does the SRY gene play in mammals?

A: It triggers the development of testes and male traits.

Q: Explain why X-inactivation is essential in female mammals.

A: To prevent a double dose of X-linked gene products, which could harm cells.

Q: Why are female carriers of X-linked disorders often unaffected?

A: Random X-inactivation ensures about half of cells express the working allele, reducing the impact of the mutation.

Q: Why do males show sex-linked traits more often than females?

A: They lack homologous alleles on the Y chromosome to mask recessive mutations on the X chromosome.

Q: How does environmental sex determination differ from genetic sex determination?

A: Sex is influenced by external factors (e.g., temperature, size, parasitic infection, competition, or sex ratio), not fixed chromosomes.

Q: How does temperature-dependent sex determination affect reptile populations?

A: Incubation temperature during a sensitive period determines sex (e.g., cooler = males, warmer = females in turtles).

Q: Why might environmental sex determination be adaptive in some species?

A: It allows adjustment of offspring sex ratio in response to resource availability and population conditions.