. The physical basis of heredity . the subsequent opening out of tlie tetrad, <<. maternal or the paternal chromosomes, the secondary orequational split. Only in very special cases is it possibleto be able to say which is the primary and which is thesecondary split. In fact, whenever crossing over takesplace in the four-strand stage this distinction fails tohave much meaning. 102 PHYSICAL BASIS OF HEREDITY There are certain questions connected with crossingover that are illustrated by the following models (,42,43). In these models of tetrads the dotted rod, splitlengthwise, stands
. The physical basis of heredity . the subsequent opening out of tlie tetrad, <<. maternal or the paternal chromosomes, the secondary orequational split. Only in very special cases is it possibleto be able to say which is the primary and which is thesecondary split. In fact, whenever crossing over takesplace in the four-strand stage this distinction fails tohave much meaning. 102 PHYSICAL BASIS OF HEREDITY There are certain questions connected with crossingover that are illustrated by the following models (,42,43). In these models of tetrads the dotted rod, splitlengthwise, stands for a maternal chromosome, and eachof its halves! may be called a strand. The split in the rodis the secondary (or equational) split. The black rod,also split lengthwise, stands for the paternal chromosome. In Fig. 41, a, the two split rods are represented astwisted about each other. If the two inner strands breakand the cords interchange at the levels, where they firstcome into contact with each other (Fig. 41, &), and then. Fig. 42.—Scheme showing the opening out of the strands of the tetrad, a, in two planes,6, according to Robertson and Wenrich. later the four strands come to lie side by side, , ■ fuse,the result will be that shown in Fig. 41, c. Two ofthe strands represent crossovers in the sense that aninterchange has taken place between a maternal and apaternal strand; and if at the first spermatocyte division,when the threads begin to pull apart, the maternal sepa-rate from the paternal threads, two threads may be seenactually crossing each other (Fig. 41, d). They are herethe two non-crossover strands, but if the two strandsthrown to the left had been thrown to the right the twocrossover strands would cross over. The scheme is essen-tially the same as the chiasma of Janssens, but the strandsthat cross may or may not (as here) represent the cross-over strands. CROSSING OVEE AND CHROMOSOMES 103 The next two figures (Fig. 42, a,h) show how Robert-son and Wenrich
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