The Organism as a Whole, from a Physicochemical Viewpoint

by Jacques Loeb

YRYR YRgw YRgR YRYw
gwYR gwgw gwgR gwYw
gRYR gRgw gRgR gRYw
YwYR Ywgw YwgR YwYw

Since w and g are recessives and therefore disappear when in combina­tion with their respective dominants Y and R the result will be 9 YR (yellow round), 3 Yw (yellow wrinkled), 3 Rg (round green), and 1 gw (green wrinkled) as Mendel actually observed and as all investigators since have confirmed.

Bateson made the discovery that these Mendelian ratios 9: 3: 3: 1 did not always occur when forms differing in two characters were crossed. He found typical and very constant devia­tions from this ratio in definite cases and these cases he interpreted as being due to “gametic coupling.”

These phenomena demonstrate the existence of a complex interrela­tion between the factorial units. This interrela­tion is such that certain combina­tions between factors may be more frequent than others. The circumstances in which this interrela­tion is developed and takes effect we cannot as yet distinguish, still less can we offer with confidence any positive concep­tion as to the mode in which it is exerted.[206]

Morgan has given an ingenious explana­tion of these devia­tions on the basis of the chromo­some theory of Mendelian heredity. He assumes that they occur in those cases where the two or more characters are contained in the same chromo­some. In that case the two factors lying in the same chromo­some should generally be found together. Such was the case for instance in the experi­ments with flies having red eyes and yellow body colour versus white eyes and grey body colour, the character for white eyes and yellow body being located in the X chromo­some (see preceding chapter), or in the experi­ments on Abraxas. These phenomena are called linkage, and the numerical results of linkage were given in the preceding chapter in connec­tion with the crossing of sex-linked characters.

We have already mentioned that before the matura­tion division occurs the homologous maternal and paternal chromo­somes fuse—the so-called synapsis of the cytologists—and afterward separate again. It had been observed by Janssens that in this stage of fusion and subsequent separa­tion a partial twisting and a partial exchange between two chromo­somes may take place. Morgan assumes that this exchange accounts for certain devia­tions in the ratio of linkage. If in Fig. 40 the white and black signify two homologous chromo­somes I and I₁ containing the two pairs of homologous factors AB and ab respectively, the synapsis state would be as in Fig. 41. If the separa­tion were complete, either I or its homologue I₁ might be lost in the matura­tion division of the egg. If, however, the synapsis is slightly irregular, as in Fig. 42, where the chromo­somes are slightly twisted, I and I₁ will not separate completely but an exchange will take place, part of I₁ and I becoming exchanged. This would result in the forma­tion of two mixed chromo­somes Ab and aB (Fig. 42). This partial exchange of homologous chromo­somes, which Morgan calls “crossing over,” occurs, as he found in Drosophila, in the egg only, not in the matura­tion division of the sperm. He informs me that in the silkworm moth Tanaka found that it occurs only in the male, while in Primula it takes place both in the ovules and in the pollen as shown by Gregory.

Fig. 40

Fig. 41

Fig. 42

Morgan and his fellow-workers have put this theory to numerous tests by breeding experi­ments and the results have fully supported it. According to the chromo­some theory linkage should occur only when factors lie in the same chromo­some. Hence it should be possible, on the basis of this linkage theory, to foretell how many linkage groups there may occur in a species; namely, as many as there are chromo­somes. In Drosophila there are four pairs of chromo­somes, and Morgan and his fellow-workers found only four groups of linked characters.[207] This agreement can be no mere accident.

Carrying the assump­tion still farther, these authors were able to show that each individual character has in all probability a definite loca­tion in the chromo­some, so that it seems as if each individual chromo­some consisted of a series of smaller chromo­somes, each of which may be a factor in the determina­tion of a hereditary character which is transmitted according to Mendel’s law of segrega­tion. Biology has thus reached in the chromo­some theory of Mendelian heredity an atomistic concep­tion, according to which independent material determiners for hereditary characters exist in a linear arrangement in the chromo­somes.

II