Reproduction, Inheritance and Variation
Plant reproduction
A flower has sepals, petals, stamens (anther and filament) and carpels (stigma, style and ovary). Pollination is the transfer of pollen from anther to stigma; fertilisation is the fusion of a male gamete with an egg inside the ovule. The ovule becomes the seed and the ovary becomes the fruit. Wind-pollinated flowers are small, dull and produce light pollen; insect-pollinated flowers are colourful and produce sticky pollen.
Human reproduction
Sperm are made in the testes and eggs in the ovaries. Fertilisation normally occurs in the oviduct, and the placenta allows exchange of substances between mother and fetus. The amniotic fluid protects the fetus.
Inheritance
DNA is organised into chromosomes; a gene codes for a protein, and different forms of a gene are alleles. A dominant allele is expressed even when only one copy is present; a recessive allele needs two copies. Genotype describes alleles, phenotype describes appearance. Human sex is determined by XX (female) and XY (male). Mitosis produces identical cells for growth; meiosis produces gametes with half the chromosome number.
Variation and selection
Variation comes from genetic differences and the environment. Mutations create new alleles, and natural selection favours individuals best adapted to their surroundings.
Worked example
Cross two heterozygous tall plants, Bb × Bb. The Punnett square gives BB, Bb, Bb and bb. The genotype ratio is 1 BB : 2 Bb : 1 bb, and because B is dominant the phenotype ratio is 3 tall : 1 short.
Traps that cost marks
Pollination is not fertilisation. Meiosis halves the chromosome number; mitosis does not. Genotype and phenotype are not interchangeable. A recessive phenotype requires two recessive alleles.