Identifier: physicalbasisofhmorg (find matches)
Title: The physical basis of heredity
Year: 1919 (1910s)
Authors: Morgan, Thomas Hunt, 1866-1945
Subjects: Chromosomes Heredity Chromosomes Genetics, Medical
Publisher: Philadelphia London : Lippincott
Contributing Library: Francis A. Countway Library of Medicine
Digitizing Sponsor: Open Knowledge Commons and Harvard Medical School

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astids. Itis the segregation of the genes in the germ-cells of the Fjindividual that leads to the 3:1 ratio in Fo, and not thedistribution of the plastids as in the preceding case. The most peculiar case is that of Pelargonium de-scribed by Baur. Wliite leaves and branches, and greenleaves and branches occur on the same plant (Fig. 103).Self-fertilized seeds from each breed true to color ofbranch. White to green gives a different result, viz., 222 PHYSICAL BASIS OF HEREDITY mosaic seedlings with patches of green and white on stemsand leaves (Fig. 104). When these seedlings grow intoplants, the color of the leaves will depend on the colorof that part of the stem from which the terminal bud, andlateral buds grow out. If a bud lies in a green part of thestem the new part will be green (Fig. 104, a): if the newbud lies in a white part of the stem the new part will bewhite (Fig. 104, c): and if it lies in a partly green, partlywhite region the new part will have some white, some ^^ ^f^ ^^^
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Fig. 104.—Diagram to show how a sectorial chimera may be produced. If the ter-minal bud has come from a region of the seedling entirely green, all of the future leaveswill be green, a; if from a region without chlorophyll, all the future leaves will be white, c;but if the terminal bud lies partly in one, partly in the other region, some white and somegreen leaves will arise, b. (After Baur.) green parts (Fig. 104, h). The only explanation that issuggested by Baur is that in this plant the plastids aretransmitted both by the egg and by the pollen. The whiteplant with defective plastids contributes part of the plas-tids in the fertilized egg, the green plant with normalplastids the other part. The fertilized egg contains there-fore both kinds of plastids. During division of the egg andembryo, the granules become irregularly distributed inthe cells. Whenever a cell gets only defective granules,it and its descendants are white, producing white parts:when a cell gets mostly or only green

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