General laws of heredity. G. Mendel’s laws of inheritance and their essence
Heredity is the property of organisms to pass their traits to offspring. G. Mendel (1865) used the hybridological method on pea plants: he crossed pure lines, followed each trait separately and counted the offspring numerically. A cross for one pair of traits is called a monohybrid cross. The first law (uniformity of F1 hybrids): when two different pure (homozygous) parents are crossed, all F1 offspring are alike and show the dominant trait (AA × aa → all Aa). The second law (segregation): when F1 hybrids are crossed with each other, F2 segregates 3 : 1 by phenotype and 1 AA : 2 Aa : 1 aa by genotype. The reason is that gametes are pure: in meiosis only one allele of a pair enters a gamete (the rule of gamete purity). The genotype is the sum of all genes of an organism, the phenotype the whole set of its external and internal traits. A test cross (crossing an individual of unknown genotype with aa) shows whether a dominant-trait organism is homozygous or heterozygous. Dominance is not always complete: in incomplete dominance the heterozygote is intermediate and the F2 phenotypes are also 1 : 2 : 1 (e.g. cotton fibre: brown – AA, cream – Aa, white – aa); in human blood groups one gene has three alleles (multiple allelism), and in group AB both alleles show together (codominance).
“Gamete box”: each pair writes A or a on two coloured cards, builds a Punnett square (2×2) and calculates the F2 ratio.