Practice question

The term gene was introduced by Danish botanist Wilhelm Johannsen in 1909 to describe the Mendelian unit of heredity. Mendel referred to factors, but Johannsen clarified genotype as inherited constitution and phenotype as observable expression. He emphasised that genes are discrete entities at loci, with allelic variants influencing characters, segregating intact across generations. This terminology replaced vague concepts like gemmules and pangenes, providing precise vocabulary for linkage, mutation, complementation and molecular definitions still central to modern genetics and medical literature. Conceptual clarity supports solving numerical problems involving segregation ratios, recombination frequencies and probability calculations in crosses.