Inheritance is analogous to the mixing of fluids, such that offspring show a smooth blend of miscible characteristics transmitted from their parents. From routine crosses of domesticated animals and plants, as well as from personal human family experiences, people throughout the ages had always accepted what seemed obvious: that some form of blending inheritance (such as a “mixing of the bloods”) must account for the general trend toward offspring intermediacy in phenotypic traits (such as body shape or facial features) between the sire and dam. Whatever fluid-like medium might govern heredity seemed to be thoroughly blendable during offspring production.
blending inheritance; non-miscible particles
Inheritance is analogous to the mixing of fluids, such that offspring show a smooth blend of miscible characteristics transmitted from their parents. From routine crosses of domesticated animals and plants, as well as from personal human family experiences, people throughout the ages had always accepted what seemed obvious: that some form of blending inheritance (such as a “mixing of the bloods”) must account for the general trend toward offspring intermediacy in phenotypic traits (such as body shape or facial features) between the sire and dam. Whatever fluid-like medium might govern heredity seemed to be thoroughly blendable during offspring production.
By tallying numbers of offspring displaying alternative traits in experimental crosses involving true-breeding inbred strains of pea plants, Gregor Mendel deduced the particulate nature of inheritance. The non-miscible particles that Mendel uncovered would later – in 1909 – be named “genes” (see Chapter 12). Mendel’s findings revealed two of the most fundamental rules of heredity for diploid organisms: (1) the law of segregation, which states that the two alleles (alternative forms of a gene) segregate from one another during gametogenesis (the production of gametes); and (2) the law of independent assortment, which states that alleles at separate loci normally segregate independently of one another during gamete formation. Later discoveries (see Chapters 13 and 14) would identify exceptions to both of these Mendelian laws, but such exceptions did more to highlight the generality of Mendel’s rules than to dishonor them.
This perfect score is merited by the fact that Mendel’s discoveries about heredity have withstood the test of time, proved generalizable to all kinds of multicellular organisms, and become a solid foundation for the entire field of genetics. Indeed, if inheritance truly were miscible rather than particulate, the population genetic variation that is prerequisite for evolution would be rapidly lost (halved) in each successive generation of sexual reproduction. But Mendel in effect showed that genetic variation could be maintained indefinitely in populations because the particles of heredity tend to maintain their separate identities across the generations. Mendel’s elucidation of the particulate nature of inheritance ranks second on the all-time list of the most important conceptual breakthroughs in evolutionary genetics, trailing only Darwin’s elucidation of natural selection as the principal shaping force of adaptive evolution (see Chapter 1).
1. Mendel GJ. Versuche über pflanzenhybriden [Experiments on plant hybridization]. Verhandlungen des Naturforschenden Vereins (Bruenn). 1865;4:3–47.