Reproduction, Genetics, and Variation
Reproduction
Asexual reproduction produces genetically identical offspring from one parent (e.g. binary fission, runners, tubers, cuttings). Sexual reproduction involves gametes (sperm and egg) fusing at fertilisation to form a zygote, producing genetic variation. In humans, the male reproductive system produces sperm in the testes (testosterone); the female system produces eggs in the ovaries (oestrogen) and supports pregnancy in the uterus. The placenta exchanges materials between mother and fetus; the amniotic fluid protects it. Know the menstrual cycle roles of oestrogen and progesterone.
Genetics
DNA carries genetic information in genes on chromosomes. Humans have 23 pairs of chromosomes; gametes have 23 single chromosomes (haploid). Alleles are different versions of a gene; dominant alleles mask recessive ones. Genotype is the allele combination (e.g. Bb); phenotype is the observable characteristic. Homozygous means two identical alleles; heterozygous means two different alleles. A Punnett square predicts offspring ratios. Sex is determined by X and Y chromosomes: XX female, XY male.
Variation and mutation
Variation can be genetic (inherited alleles, mutation) or environmental (diet, exercise, sunlight). Continuous variation shows a range (height); discontinuous variation falls into categories (blood group). Mutations are random changes in DNA and are the source of new alleles; most are neutral or harmful, some beneficial. Natural selection acts on variation: individuals better adapted survive and reproduce, passing on alleles.
Worked example
Cross two heterozygous tall pea plants (Tt × Tt). Punnett square gives TT, Tt, Tt, tt - so 3 tall : 1 short, or 75% tall. If both parents were tt, all offspring would be short.