Basics of genetics and Mendel’s laws

G. Mendel (1865) crossed pea plants differing in one pair of traits (monohybrid cross) and counted the results quantitatively over generations; his laws were rediscovered in 1900 by H. de Vries, K. Correns and E. Tschermak. First law (uniformity of F1, dominance): crossing two different homozygotes gives an F1 that is all alike and shows only one parental trait: AA × aa → all Aa. Second law (segregation): crossing F1 heterozygotes gives in F2 a 3:1 phenotype ratio (¾ dominant, ¼ recessive) and a 1:2:1 genotype ratio (AA : Aa : aa); Aa × Aa → ¼ AA, ½ Aa, ¼ aa. In incomplete dominance the heterozygote shows an intermediate trait (four-o’clock red × white → pink), so in F2 both phenotype and genotype ratios are 1:2:1. An organism with the dominant phenotype may be AA or Aa; the test cross (× aa) tells: AA × aa → all dominant; Aa × aa → 1 : 1 (Aa : aa). Gamete purity hypothesis: in a hybrid the factors do not blend, each gamete receives only one of a pair of alleles (homologous chromosomes separate in meiosis); allelic genes are genes at the same position of homologous chromosomes that determine alternative forms of one trait.