Key facts & rules

• Chromosome: structure made of DNA; humans have 46 (23 pairs); gametes have 23 (haploid).
• Gene: section of DNA that codes for a protein/characteristic.
• Allele: a version of a gene. Dominant allele (capital letter) masks recessive (lower case).
• Genotype: genetic makeup (e.g. Tt). Phenotype: observable characteristics.
• Homozygous: both alleles the same (TT or tt). Heterozygous: different alleles (Tt).
• Monohybrid cross: Punnett square — 3:1 ratio (Aa × Aa) or 1:2:1 (genotype ratio).
• Sex determination: XX = female; XY = male. Father determines sex of offspring.
• Mutation: change in DNA sequence — can be caused by radiation or chemicals.
• Meiosis: produces 4 haploid gametes with genetic variation (crossing over + independent assortment).