The phenotypic differences among individual organisms can be due partly to direct effects of the environment and partly to genetic differences. The genes an individual passes on to its offspring are not affected by environmental effects on the phenotype of that individual. In other words, acquired characteristics are not inherited. However, the environment may affect the expression of an organism's genes. Hereditary variations are based on particles called genes.
Genes retain their identity as they pass from one generation to the next.
They do not blend with other genes. Variation in continuously varying traits is largely based on several discrete genes, sometimes many, each of which affects the trait slightly. This is called polygenetic inheritance. Genes sometimes change to alternative forms. These changes are called mutations and the alternative forms are called alleles.