. Genetics in relation to agriculture. Livestock; Heredity; Variation (Biology); Plant breeding. Fig. 72.—Three types of Paratettix, BB, CC, II, and two of the hybrids between them. {After Nabours.) The Presence and Absence Hypothesis.—The foregoing accounts of the relations existing in the expression of the hybrid characters as compared with the two parental characters serves as an adequate introduction for a brief consideration of the presence and absence hypothesis. Accord- ing to the presence and absence hypothesis as advanced by Bateson and Punnett, the only relations which can exist with
. Genetics in relation to agriculture. Livestock; Heredity; Variation (Biology); Plant breeding. Fig. 72.—Three types of Paratettix, BB, CC, II, and two of the hybrids between them. {After Nabours.) The Presence and Absence Hypothesis.—The foregoing accounts of the relations existing in the expression of the hybrid characters as compared with the two parental characters serves as an adequate introduction for a brief consideration of the presence and absence hypothesis. Accord- ing to the presence and absence hypothesis as advanced by Bateson and Punnett, the only relations which can exist with respect to a certain fac- tor depend on its presence or absence from the hereditary material. Thus if we consider the factor R for round shape in peas, and its allelo- morph r for wrinkled shape, according to the presence and absence hypothesis the r of the genetic formula of the wrinkled' pea is not itself a factor as we have assumed throughout the discussion in this text, but merely represents the absence of the factor R. The wrinkled character, therefore, is merely an expression of the action of the set of genetic fac- tors in peas when the factor R has been taken away from the system. In this text we have throughout assumed that the recessive symbols stand for factors just as truly as do the dominant ones, and we have regarded the difference between a recessive factor and its corresponding dominant allelomorph as dependent upon some change in a dominant factor sometimes profound and sometimes less profound so that all Digitized by Microsoft®. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Babcock, E. B. (Ernest Brown), b. 1877; Clausen, Roy Elwood, 1891-. New York, McGraw-Hill; [etc. , etc. ]
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