General laws of variation
Variation is the property of organisms to acquire new traits and to differ from one another. In modification (phenotypic) variation the genotype does not change; a trait changes with the environment within the norm of reaction and is not inherited (for example a leaf that receives much light is thicker). Hereditary variation is of two kinds: combinative – new combinations of parental genes (crossing over, independent assortment of chromosomes, random fusion of gametes) – and mutational – a change in the genetic material. Mutations are gene (the nucleotide sequence changes, e.g. substitution or loss), chromosomal (loss, doubling, inversion or transfer of a chromosome segment) and genomic (the number of chromosomes changes: polyploidy, aneuploidy, e.g. Down syndrome – trisomy of chromosome 21; G. D. Karpechenko doubled the chromosomes of a cabbage–radish hybrid and obtained a fertile allopolyploid). Mutations are increased by mutagens: physical (ionising radiation, ultraviolet), chemical (some chemicals, substances in tobacco smoke) and biological (some viruses). Mutations can be useful, harmful or neutral; most are harmful or neutral. N. Vavilov formulated the law of homologous series (1920): in genetically close species and families hereditary variation forms similar series.
“Mutation or modification?” – the teacher reads examples, students raise one of two coloured cards and give a reason.