appearance, internal anatomy, physiology, and behavior. Similarly, the term genotype can refer to an organism’s entire genetic makeup, not just its alleles for a single genetic locus. In most cases, a gene’s impact on phenotype is affected by other genes and by the environment. In this integrated view of heredity and variation, an organism’s phenotype reflects its overall genotype and unique environmental history.
Considering all that can occur in the pathway from geno- type to phenotype, it is indeed impressive that Mendel could uncover the fundamental principles governing the transmis- sion of individual genes from parents to offspring. Mendel’s laws of segregation and of independent assortment explain heritable variations in terms of alternative forms of genes (hereditary “particles,” now known as the alleles of genes) that are passed along, generation after generation, accord- ing to simple rules of probability. This theory of inheritance is equally valid for peas, flies, fishes, birds, and human beings—indeed, for any organism with a sexual life cycle.