Genes are the basic unit of heredity and continuous stretch of DNA which are linearly arranged on chromosomes. Genotype consists of the sum total of all recessive and dominant genes. The observable attributes of organisms, as determined by genotype, are termed as phenotype. Mendel proposed the presence of unit factors for each trait as heredity material that is faithfully transmitted from one generation to next. He called them “factors”. Morgan provided experimental proof of “chromosomal theory of inheritance”, first proposed by Sutton and Boveri, by demonstrating correlated inheritance of genes for eye color, miniature wing and yellow body and X-chromosome. William Bateson coined the term “genetics” for the branch of science that study inheritance and variations in organisms. Danish geneticist W. Johannsen coined the term gene to refer the mendelian factors. While working on pure lines of bean plants, he introduced the terms genotype and phenotype to distinguish between the units of heredity that determine the traits and the form of the trait determined by genotype. Thus, the correct answer is option A.