Author Description

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1919 Excerpt:...factors of environment nor association in particular combinations in the germplasm serves to change their individuality or constitution with a significant frequency. Heredity, then, is the distribution of genes, and the genes have been located definitely in the chromosomes. Fortunately, chromosome distribution has been standardized in a remarkable manner in the majority of plants and animals; hence, the greater part of the phenomena found in breeding experiments may be described by a comparatively few simple mathematical formulae. It is to this orderly chromosome distribution that one must impute the utility of the Mendelian nomenclature, for to it in large measure is due the regularity with which certain ratios recur. There are irregularities in chromosome distribution, it is true. They have even furnished some of the critical tests of the modern theory of heredity taken as a whole. But because they curtail the practical value of the theory through limiting the possibilities of prediction, it is well that they are rare. The standard chromosome mechanism for distribution of genes is that in which homologous chromosomes mate at synapsis, and homologous genes, one froneither parent, pass by chance to either pole of the mitotic figure, in the formation of the mature gametes. The chromosomes may separate without having exchanged genes, presumably; or, genes may be exchanged. Just how this interchange occurs is not wholly clear. MorGan has assumed that the genes have a linear arrangement, and that there must be transverse breaks in the chromosomes. Castle (1918) believes the arrangement is not linear, and that breaks may occur in many ways. It is possible that neither assumption is correct. The writer has felt for some time that possibly the genes are arranged s...