Meiosis and variation
Meiosis makes four genetically different haploid cells from one diploid cell, by two divisions. Meiosis I separates homologous chromosomes (pairs carrying the same genes): in prophase I homologues pair as bivalents and cross over at chiasmata, swapping sections of chromatids; in metaphase I bivalents line up randomly (independent assortment); in anaphase I homologues separate. Meiosis II separates sister chromatids, like mitosis. Variation comes from crossing over, independent assortment ( combinations) and random fertilisation. Non-disjunction (failure to separate) gives gametes with an extra or missing chromosome, e.g. Down syndrome (trisomy 21). Meiosis halves the chromosome number so fertilisation restores diploidy.
≈ 50 minMonohybrid and dihybrid inheritance
Monohybrid: one gene. Two heterozygotes (Aa × Aa) give 3 : 1 dominant : recessive. Codominance: both alleles expressed (e.g. Iᴬ Iᴮ blood group AB). Multiple alleles: more than two alleles in the population (Iᴬ, Iᴮ, Iᴼ). Sex linkage: gene on the X chromosome, so males (XY) show recessive traits more often (e.g. haemophilia). Dihybrid: two genes on different chromosomes; AaBb × AaBb gives 9 : 3 : 3 : 1; a test cross with aabb gives 1 : 1 : 1 : 1. Autosomal linkage: genes on the same chromosome are inherited together, giving more parental types and fewer recombinants than expected. Epistasis: one gene affects the expression of another, modifying ratios (e.g. 9 : 3 : 4). The chi-squared test checks whether differences from expected ratios are due to chance.
≈ 55 min