ABSTRACT
This chapter suggests that the chromosomes are the important carriers of the hereditary factors which give rise during development to the characters of the individual. J. Muller has more recently shown that translocations or "breakage with reattachment of a fragment to a nonhomologous chromosome" frequently occur following exposure of germ cells to the X-ray. The chromosomes are composed of genes, the genetic elements. The number of linkage groups corresponds in a given animal species with the number of chromosomes. A second peculiar occurrence may arise during the synaptic pairing of the chromosomes. One of the pairs may fail to have its members separate and so remain double. In this case, the two fused chromosomes must both pass into the same cell at division, instead of each unit of the pair going into one of the two sister cells. To this phenomenon the term nondisjunction is applied.
