If heritability analysis exhausted the techniques available to it for explanation, genetics would be in a sorry state. Luckily, the pursuit of genetic explanation has available an array of reliable techniques that underscore the irrelevance of heritability analysis to modern genetics. These techniques use known rules about genes, especially their transmission, to infer which traits can be explained from a genetic basis. The basic pattern for this type of explanation was set by Mendel (1866). Mendel hypothesized that many traits in his pea plants were controlled by pairs of inherited factors, now called “genes” or, more accurately “alleles.” Each factor in a pair was one of two possible types. These factors obeyed definite rules during their transmission from one generation to the next: each member of a pair was transmitted independently of the other, and each pair was transmitted independently of other pairs. “Dominant” traits were those controlled by pairs of similar or different factors; “recessive” traits were controlled by pairs of similar factors. Mendel's rules for the transmission of factors established definite expectations for the inheritance of both dominant and recessive traits. In this sense, the factors explained those traits that satisfied these expectations. But they were silent about the mechanisms of phenogenesis, that is, how the factors brought about the traits.

Modern genetic explanation follows the same pattern. Accepting Mendel's laws (though with an important exception that will be discussed in detail in § 5.1), genes are said to be able to explain the origin of a trait if its pattern of inheritance is one that is predicted from those laws.