NATURE OF THE GENETIC EFFECTS 355 



one set of chromosomes, a condition referred to as haploid, instead of two 

 sets, and only one instead of the two genes of each type previously present. 

 For a condensed account of meiosis and its associated genetic phenomena, 

 with diagrams, see MuUer, Little, and Snyder (1947), pp. 40-56. 



The clue to the complexities of meiosis lies in the remarkable process of 

 synapsis, which initiates it. In synapsis each gene in the nucleus some- 

 how finds and becomes temporarily adherent to its homologous gene, 

 usually of identical composition, which was originally derived from the 

 other parent and is still present in the same cell. Since the correspond- 

 ing genes are usually arranged in the same sequence in homologous chro- 

 mosomes, this process brings every two homologous chromosomes together 

 in pairs, adhering side by side along the whole of their lengths, with like 

 genes apposed. Each of the members of these pairs now reproduces 

 itself, i.e., it synthesizes a daughter chromatid, so that tetrads, of four 

 chromatids each, are formed in the place of the original pairs. The two 

 meiotic cell divisions follow without any further chromatid formation 

 intervening. Thereby the four members of each tetrad become pulled 

 apart, at their centromeres, to give two and two at the first meiotic 

 division and one and one at the second. Thus they are distributed 

 singly among the four granddaughter nuclei derived from each nucleus 

 that underwent meiosis. 



In this way each gamete comes to have just one chromatid — now to be 

 termed a "chromosome" — out of each tetrad, and thus has just one set 

 of chromosomes instead of the original two sets. Accordingly, for every 

 gene of which the premeiotic cell had two representatives, a maternal 

 and a paternal (to be referred to as "a pair" of genes), the gamete carries 

 but one, a maternal or a paternal, as the case may be. This is the basis 

 for Mendel's first law, that of segregation, whereby 50 per cent of the 

 gametes of any individual receive any given gene inherited from one of 

 the parents, while the rest receive the homologous gene (called an allele 

 if it is not identical in composition with the first one) inherited from the 

 other parent. 



But a gamete receiving a given gene of one pair does not necessarily 

 receive, as its quota from any other pair, that gene which was derived 

 from the same parent. If the second pair of genes in question lies in 

 another pair of chromosomes, the chance is just 50 per cent that the gene 

 of the second pair which is received by any given gamete is from the other 

 parent than that which furnished that gamete with the gene of the first 

 pair. This is expressed by saying that there is among the gametes a 

 50 per cent frequency of recombination of genes that lie in different pairs 

 of chromosomes. This result is due to the fact that it is a matter of 

 indifference, in the orientation of the tetrads on the meiotic division 

 figures, whether an element of paternal or maternal origin happens to be 

 placed so as to be pulled to a given pole of the division figure, and that 



