. Genetics in relation to agriculture. Livestock; Heredity; Variation (Biology); Plant breeding. THE PHYSICAL BASIS OF MENDELISM 63 chromosomes. In the mitoses which have preceded this particular divi- sion, the chromosomes were each time conceived to be reformed from the identical group of chromomeres wliich they contained originally. In synapsis, however, as shown at B there is a certain amount of intertwin- ing of the paired threads and in the unraveling of the chromosomes after the contraction stage there is likewise a twisting of the filaments about each other. The indications are, theref
. Genetics in relation to agriculture. Livestock; Heredity; Variation (Biology); Plant breeding. THE PHYSICAL BASIS OF MENDELISM 63 chromosomes. In the mitoses which have preceded this particular divi- sion, the chromosomes were each time conceived to be reformed from the identical group of chromomeres wliich they contained originally. In synapsis, however, as shown at B there is a certain amount of intertwin- ing of the paired threads and in the unraveling of the chromosomes after the contraction stage there is likewise a twisting of the filaments about each other. The indications are, therefore, that in synapsis there is a possibihty of interchange of chromatin material between the members of a pair of homologous chromosomes. In all cases, however, in order to uphold our conception of the definite organization of the chromosomes with respect to the chromomeres which they contain, this interchange of material must involve exactly eciuivalent portions of the two chromo-. I Fia. 30.—Diagram of chromatin interchange between homologous members of a pair of chromosomes. (After Midler.) somes. The chromosomes of the reduction division shown at F may not, therefore, be identical with the four originally present in A, but may represent various combinations of portions of both members of a par- ticular pair of chromosomes. The results of such interchange between members of homologous pairs of chromosomes is shown in Fig. 30. At the left is shown a pair of chromosomes one in outline the other in full black. In the middle the steps in chromatin interchange are diagrammed and finally at the right this interchange results in a pair of chromosomes each of which is made up of parts of both members of the original pair of chromosomes. Various comVjinations may result depending on the points at which interchange takes place, but in every case the exchange involves corresponding portions of the two chromosomes. Independent Distribution of Chromosomes.—In Fig. 31 are illus- trate
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