Genetics and Inheritance
Explain why males are more likely than females to express X-linked recessive traits.
Males have only one X chromosome, so a single recessive allele is expressed because there is no second X allele to mask it.
Two species show a 4 % difference in DNA sequence. If the mutation rate is 1 % per million years, estimate the time since divergence.
Each lineage accumulates 2 %; 2 ÷ 1 % = 2 million years since divergence.
State the four key principles of natural selection.
(1) Variation exists; (2) more offspring are produced than survive; (3) competition occurs; (4) advantageous traits increase survival and are inherited.
Explain why energy transfer between trophic levels is inefficient.
Most energy is lost as heat, movement, or waste; only ~10 % is converted to biomass and passed to the next level.
Describe the role of DNA polymerase during DNA replication.
DNA polymerase adds complementary nucleotides to the exposed bases on the template strand, forming new daughter strands in the 5′ → 3′ direction.
A carrier mother (XᴴXʰ) and a normal father (XᴴY) have children.
State the probability that a son will have haemophilia.
50 % of sons (XʰY) will have haemophilia.
Explain how twin studies help distinguish between genetic and environmental influences.
Identical twins share DNA, so differences in phenotype between them must result from environmental or epigenetic factors.
Differentiate between incomplete dominance and codominance.
Incomplete dominance = blended phenotype (red × white → pink).
Codominance = both alleles fully expressed (AB blood group).
Describe the role of decomposers in ecosystems.
They break down dead matter into inorganic nutrients (e.g. nitrates, phosphates), recycling materials for producers.
Explain the role of the enzyme helicase in DNA replication.
Helicase unwinds and separates the two strands of the DNA double helix by breaking hydrogen bonds between complementary bases, creating replication forks for DNA polymerase to add nucleotides.
Explain how crossing-over during meiosis I increases genetic diversity.
Homologous chromosomes exchange corresponding DNA segments, creating recombinant chromatids with new allele combinations.
Define adaptive radiation and give one example.
Adaptive radiation is the rapid diversification of one ancestral species into many adapted to different niches, e.g. Darwin’s finches on the Galápagos Islands.
Explain what is meant by phenotypic selection.
It is the process where individuals with certain observable traits survive or reproduce more successfully, changing allele frequencies over time.
Outline two key steps of the nitrogen cycle, naming the microorganisms involved.
(1) Nitrogen fixation – Rhizobium converts N₂ to ammonia.
(2) Nitrification – Nitrosomonas/Nitrobacter convert ammonia → nitrates.
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Compare spermatogenesis and oogenesis in terms of timing and number of gametes produced.
Spermatogenesis occurs continuously after puberty, producing four viable sperm per meiosis; oogenesis begins before birth and produces one ovum and polar bodies.
Describe how DNA methylation can alter gene expression without changing base sequence.
Methyl groups added to cytosine bases prevent transcription factors binding, silencing gene expression.
A population of beetles changes from mostly green to mostly brown after a drought.
Explain this change using natural selection.
Brown beetles were better camouflaged, survived predators, reproduced more, and passed on brown alleles, increasing their frequency.
Define a keystone species and describe one ecological role.
A species that has a disproportionately large effect on ecosystem stability; e.g. sea otters control urchins, maintaining kelp forests.
Describe the function of tRNA in protein synthesis.
tRNA carries specific amino acids to the ribosome, matching its anticodon to the complementary mRNA codon. This ensures the correct sequence of amino acids is assembled into a polypeptide during translation.
Describe how recombinant DNA technology can be used to produce a human protein such as insulin.
The insulin gene is inserted into a bacterial plasmid using restriction enzymes and ligase; bacteria then express the gene to produce insulin.
Explain how comparing amino-acid sequences of proteins can provide evidence for evolutionary relationships.
Species with more similar sequences share more recent common ancestors because fewer mutations have accumulated over time.
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Explain how energy-flow limitations influence the number of trophic levels in an ecosystem.
Because energy decreases at each level, there is insufficient energy to support many higher levels, limiting food-chain length.
Describe the key events that occur during meiosis I and explain how they contribute to genetic variation.
During meiosis I, homologous chromosomes pair and exchange genetic material through crossing-over in prophase I, then segregate independently during anaphase I. This results in daughter cells with unique combinations of maternal and paternal chromosomes, increasing genetic diversity.