Richard Goldschmidt and the Crossing-Over Controversy

Abstract

ONE of the basic tenets linking the Mendelian laws to the chromosomal theory of heredity was the assumption of “crossing over.” On the basis of F. A. Jannsen's description of the “physical twisting or inter-twining that occurred between paired homologous chromosomes during the early stages of meiosis” (Jannsen 1909), the notion of crossing over was first employed by Thomas Hunt Morgan in 1911 as a satisfactory mechanical explanation for the factorial recombination found in Mendelian crosses. Crossing over, as Garland Allen noted, thus became a foundational assumption in the subsequent articulation of the Mendelian chromosome theory of heredity. Morgan and his co-workers proposed a “beads-on-a-string” model of the chromosome, with genes represented as particulate beads strung together linearly like pearls on a necklace. Crossing over was invoked to explain the linked traits in terms of the proximity of these particulate genes along a chromosome. Genes that were farther apart on a chromosome had a greater chance of recombination as a result of the exchange between homologous chromosomes. This relationship between frequency of recombination and the arrangement of genes on a chromosome allowed the first chromosome maps to be constructed (Allen 1978). The central place of crossing over in the conceptual arsenal of the Morgan group was evident in The Mechanism of Mendelian Heredity (Morgan et al. 1915). Indeed, it formed the basis for the Mendelian chromosome theory of heredity and ultimately the theory of the gene.

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