Reproduction, Inheritance and Variation
Reproduction
Asexual reproduction produces genetically identical offspring from one parent (for example, runners in plants). Sexual reproduction involves gametes and produces variation. In humans, sperm and egg fuse at fertilisation to form a zygote; the placenta exchanges substances between mother and fetus. In flowering plants, pollination transfers pollen to the stigma, fertilisation forms a seed, and fruit dispersal follows.
Genes and Inheritance
Chromosomes carry genes made of DNA; humans have 23 pairs. Alleles are different versions of a gene; a dominant allele is expressed even when only one copy is present, while a recessive allele needs two copies. Genotype is the allele combination, phenotype is the visible characteristic. A monohybrid cross can be shown in a Punnett square.
Variation and Selection
Variation arises from genes and environment. Continuous variation (height) shows a smooth range; discontinuous variation (blood group) falls into distinct categories. Natural selection acts on variation: individuals better suited to their environment survive and reproduce, passing on advantageous alleles.
Traps to Watch
- Writing phenotype ratios when the question asks for genotype ratios.
- Forgetting that a recessive phenotype requires two recessive alleles.
- Confusing pollination (transfer of pollen) with fertilisation (fusion of gametes).
Worked Example
Cross two heterozygous parents (Bb × Bb) for brown eyes (B dominant) and blue eyes (b recessive). Offspring genotypes are BB, Bb, Bb, bb: 1 BB : 2 Bb : 1 bb, so phenotype ratio is 3 brown : 1 blue, and the probability of a blue-eyed child is 1/4.