 Our understanding of heredity today is vastly more sophisticated than Mendel’s, but the key elements of Mendel’s theory—discrete hereditary particles that come in different types, dominance and recessiveness, and the law of segregation—have turned out to be essentially correct. Mendel’s ‘factors’ are the genes of population genetics, and the alternative forms that a factor can take (e.g., \(R\) versus \(W\) in the pea plant example) are known as the alleles of a gene. The law of segregation is explained by the fact that during gametogenesis, each gamete (sex cell) receives only one of each chromosome pair from its parent organism. Other aspects of Mendel’s theory have been modified in the light of later discoveries. Mendel thought that most phenotypic traits were controlled by a single pair of factors, like seed shape in his pea plants, but it is now known that most traits are affected by many pairs of genes, not just one. Mendel believed that the pairs of factors responsible for different traits (e.g., seed shape and flower colour) segregated independently of each other, but we now know that this need not be so (see section 3.5, ‘Two-Locus Models and Linkage’, below). Despite these points, Mendel’s theory marks a turning point in our understanding of inheritance.