Chapter 18 — Chapter 18 — Inheritance

Mendel developed true-breeding pea varieties. In monohybrid crosses for seed shape, true-breeding round × wrinkled gave all round F1 seeds; round was dominant, wrinkled recessive. Self-pollinated F1 gave 1/4 wrinkled F2 seeds — a 3:1 ratio, really a disguised 1:2:1 (1/4 pure dominant, 2/4 non-pure dominant, 1/4 pure recessive). Mendel explained that traits are controlled by 'factors' (genes); each diploid individual has two genes for a character on chromosomes, and each gamete receives one. Alternative forms of a gene are alleles; same alleles = homozygous, different = heterozygous. A gene is 'a sequence of nucleotides as part of DNA which codes for a polypeptide'; its location is the locus. Each gamete receives just one allele, selected randomly — law of segregation. Genotype = allele combination; phenotype = observable trait. Testcross: cross dominant phenotype with recessive (rr): all round → RR; 1:1 → Rr.