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Galton pioneered studies of twins as a way to separate the effects of genotype and environment on phenotypic differences among individuals. Depending on whether they arise from one or from two egg cells, twins can be identical (monozygotic) twins or fraternal (dizygotic) twins. Identical twins arise from a single fertilized egg (the zygote), which, after several rounds of cell division, separates into two distinct but genetically identical embryos. Strikingly similar in overall appearance (Fig. 18.9a), identical twins have stimulated the imagination since antiquity, inspiring stories by the Roman playwright Plautus, William Shakespeare (himself the father of twins), Alexander Dumas, Mark Twain, and many others.

In contrast, fraternal twins result when two separate eggs, produced by a double ovulation, are fertilized by two different sperm. Whereas identical twins are genetically identical, fraternal twins are only as closely related as any other pair of siblings (Fig. 18.9b).