GCSE Biology · AQA 8461 · Topic 4.6.1
Reproduction
Key points
- Meiosis halves the chromosome number to make four genetically different gametes; fertilisation restores the full number.
- Asexual reproduction needs one parent and makes identical offspring; sexual reproduction makes varied offspring that may survive change better.
- DNA has two strands in a double helix. Its monomers are nucleotides (a sugar, a phosphate and one of the bases A, T, C, G). A pairs with T, C with G.
- Three bases code for one amino acid. Ribosomes join amino acids to make proteins, using a template copied from the gene.
- Key terms: allele, dominant, recessive, homozygous, heterozygous, genotype, phenotype.
- Genetic disorders: polydactyly (dominant allele) and cystic fibrosis (recessive allele). Embryo screening raises ethical issues.
- Sex is set by XX (female) and XY (male) chromosomes; every pregnancy has a 50% chance of each.
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Living things pass genetic information to their offspring in two ways: sexually or asexually. This sub-topic links meiosis, DNA, protein synthesis and the rules of inheritance.
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