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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, and that these two kinds are transmitted only throughthe egg-cell. The green or white color of the leaves of agiven branch is an index of the kind of chlorophyll bodythat the ovaries will probably contain. At each divisionof the body-cells the chlorophyll grains present in it aresorted out more or less at random—^hence from a cell that CYTOPLASMIC INHERITANCE 221 contains both kinds, more white granules than green onesmay at times get into a cell, and at other times only whitegranules will get into one daughter cell, so that a whitebranch arises. In other species of plants that have white leaves andbranches and green leaves and branches, the cross maygive a different result. Thus in Melandrium and Antirrhi-num, green by white gives green F^ (whichever way thecross is made), in F2 there are 3 green to 1 white plant.In this case the results can be explained as due to theaction of genes in the chromosome on the production ofchlorophyll in the cytoplasm—an action of such a kind that
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Fig. 103.—Pelargonium that gave rise to a white branch. (After Baur.) the granules do not develop green color unless the (nor-mal) gene is present, in single dose at least. In this case,even if the eggs only transmit plastids, the jP^ individualfrom a white-leaved mother by a green-leaved father isgreen, because the paternal nucleus introduces a genethat causes the green color to develop in the plastids. 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,

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