Reproduction and inheritance
Reproduction in plants
Asexual reproduction needs one parent and produces genetically identical offspring (clones), e.g. runners, tubers, cuttings: fast, but no variation. Sexual reproduction involves the fusion of male and female gametes (fertilisation), giving variation. In flowers, pollen (male gametes) from the anther lands on the stigma (pollination). A pollen tube grows down to the ovule in the ovary, where fertilisation happens; the ovule becomes a seed and the ovary a fruit. Insect-pollinated flowers are colourful with nectar and sticky pollen; wind-pollinated flowers are small with feathery stigmas and light pollen. Germination needs water, oxygen and warmth.
≈ 40 minChromosomes and cell division
The nucleus contains chromosomes, long strands of DNA. A gene is a length of DNA that codes for a specific protein; different versions of a gene are alleles. The base sequence is copied into mRNA, which carries it to ribosomes where amino acids are joined in the matching order. Human body cells are diploid (46 chromosomes in 23 pairs); gametes are haploid (23). Mitosis produces two genetically identical diploid cells for growth, repair and asexual reproduction. Meiosis produces four genetically different haploid gametes.
≈ 45 minHuman reproduction
Sperm are made in the testes; eggs are released from the ovaries. Fertilisation happens in the oviduct; the zygote divides into an embryo that implants in the uterus lining. The placenta lets oxygen and nutrients diffuse from mother to fetus and CO₂ and urea back, without the bloods mixing; amniotic fluid cushions the fetus. The menstrual cycle (about 28 days) is controlled by hormones: oestrogen rebuilds the uterus lining, LH triggers ovulation around day 14, progesterone maintains the lining. STIs such as HIV spread through sexual contact and can be prevented with condoms.
≈ 45 minMonohybrid inheritance
The genotype is the pair of alleles an organism has (e.g. Bb); the phenotype is the characteristic shown (brown eyes). A dominant allele (capital letter) shows whenever present; a recessive allele (lowercase) shows only when there are two (bb). Homozygous means two identical alleles (BB or bb); heterozygous, two different (Bb). A Punnett square predicts offspring: Bb × Bb gives 1 BB : 2 Bb : 1 bb, a 3 : 1 phenotype ratio. A test cross with a homozygous recessive reveals an unknown genotype. In codominance both alleles show (blood group AB).
≈ 45 minVariation, mutation and selection
Variation is the differences between individuals. Continuous variation has a range of values (height, mass) and is affected by many genes and the environment; discontinuous variation falls into distinct groups (blood group) and is mainly genetic. Mutations are random changes in DNA that create new alleles. In natural selection, individuals with features better suited to the environment survive, reproduce and pass on their alleles, so populations change over time: evolution. Artificial selection (selective breeding) is when humans choose which individuals breed.
≈ 40 min