Homologous recombination has been most studied and is best understood for Escherichia coli. Double-strand DNA breaks in bacteria are repaired by the RecBCD pathway of homologous recombination. Breaks that occur on only one of the two DNA strands, known as single-strand gaps, are thought to be repaired by the RecF pathway. Both the RecBCD and RecF pathways include a series of reactions known as branch migration, in which single DNA strands are exchanged between two intercrossed molecules of duplex DNA, and resolution, in which those two intercrossed molecules of DNA are cut apart and restored to their normal double-stranded state.
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Breaks that occur on only one of the two DNA strands, known as single-strand gaps, are thought to be repaired by the RecF pathway. Both the RecBCD and RecF pathways include a series of reactions known as branch migration, in which single DNA strands are exchanged between two intercrossed molecules of duplex DNA, and resolution, in which those two intercrossed molecules of DNA are cut apart and restored to their normal double-stranded state. Homologous recombination occurs in several groups of viruses. In DNA viruses such as herpesvirus, recombination occurs through a break-and-rejoin mechanism like in bacteria and eukaryotes.
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" He put great weight on the point that races graduate into each other, writing "But the most weighty of all the arguments against treating the races of man as distinct species, is that they graduate into each other, independently in many cases, as far as we can judge, of their having intercrossed", and concluded that the stronger evidence was that they were not different species. This conclusion on human unity was supported by monogenism, including John Bachman's evidence that intercrossed human races were fully fertile. Proponents of polygenism opposed unity, but the gradual transition from one race to another confused them when they tried to decide how many human races should count as species: Louis Agassiz said eight, but Morton said twenty-two. Darwin commented that the "question whether mankind consists of one or several species has of late years been much agitated by anthropologists, who are divided into two schools of monogenists and polygenists.
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The bead theory is a disproved hypothesis that genes are arranged on the chromosome like beads on a necklace. This theory was first proposed by Thomas Hunt Morgan after discovering genes through his work with breeding red and white eyed fruit flies. According to this theory, the existence of a gene as a unit of inheritance is recognized through its mutant alleles. A mutant allele affects a single phenotypic character, maps to one chromosome locus, gives a mutant phenotype when paired and shows a Mendelian ratio when intercrossed.
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In the wild, hairy and smooth forms of otherwise identical plants are seen together. They intercrossed the forms experimentally, “When therefore the well-grown mongrel plants are examined, they present just the same appearance of discontinuity which the wild plants at the Tosa Falls do. This discontinuity is, therefore, the outward sign of the fact that in heredity the two characters of smoothness and hairiness do not completely blend, and the offspring do not regress to one mean form, but to two distinct forms.” At about this time, Hugo de Vries and Carl Erich Correns began similar plant-breeding experiments.
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With orchid crossing coming to an end, Mead devoted more and more attention of the breeding of amaryllis (hippeastrum), being greatly helped by Nehrling, who had a fine collection and allowed Mead to take what he wanted in the way of pollen for his experiments. Specimens with just a hair line color around each petal were intercrossed to produce nearly white varieties with a narrow border around each petal. His hybrid red and white striped ‘Mead-strain’ amaryllis also became the bulb of choice for many American southern gardens in the 1940s and 1950s. The American Amaryllis Society's yearbook for 1935 was dedicated to Mead in recognition of his pioneer work with hybrid amaryllis plants, and in 1937 the International Bulb Society awarded Mead The Herbert Medal for his contributions in advancing the knowledge of bulbous plants.
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