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Topic 7 of 8 · Form 4

Reproduction, inheritance and evolution

Asexual (one parent, genetically identical offspring) and sexual (gametes fuse, variation). Flowering plants: pollination (insect and wind), fertilisation, seed and fruit formation, germination conditions. Humans: reproductive systems, menstrual cycle, fertilisation, pregnancy, placenta, birth; sexually transmitted infections.

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Reproduction

Asexual (one parent, genetically identical offspring) and sexual (gametes fuse, variation). Flowering plants: pollination (insect and wind), fertilisation, seed and fruit formation, germination conditions. Humans: reproductive systems, menstrual cycle, fertilisation, pregnancy, placenta, birth; sexually transmitted infections.

Inheritance

  • Chromosomes, genes and alleles; DNA codes for proteins.
  • Mitosis (growth, repair, asexual reproduction – identical diploid cells) and meiosis (gametes – haploid, variation).
  • Monohybrid crosses with dominant and recessive alleles; homozygous and heterozygous; codominance (ABO blood groups); sex inheritance (XX/XY); sex-linked characteristics.

Worked example: Two heterozygous parents (Tt) are crossed. Give the expected phenotype ratio.

TT : Tt : tt = 1 : 2 : 1 → 3 tall : 1 short

Variation and selection

Continuous and discontinuous variation; mutation; natural selection (variation, competition, survival of the fittest, passing on alleles); adaptations (e.g. xerophytes); selective breeding; antibiotic resistance.

Genetic modification (introduction)

Inserting genes into other organisms (e.g. bacteria producing human insulin; pest-resistant crops), with benefits and concerns.

Exam tip: Lay out genetic crosses with parent phenotypes, genotypes, gametes, offspring genotypes and phenotypes, and the ratio.

Check yourself

  1. What is a heterozygous genotype?
  2. Give one difference between mitosis and meiosis.
  3. Why does natural selection lead to antibiotic resistance?
Show answers
  1. Two different alleles of a gene (e.g. Tt).
  2. Mitosis gives two identical diploid cells; meiosis gives four genetically different haploid cells.
  3. Resistant bacteria survive treatment and reproduce, passing on resistance alleles.
Next topicEcology and human influences on ecosystems

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