Heterosis in chromosome behaviour

Abstract

Most examples and measurements of heterosis are concerned with the more familiar features of the phenotype, with such characters as size and growth rate of the body or its outer parts. This account is concerned with less accessible, though not less important, parts of the phenotype, and describes heterosis as it is shown in certain aspects of the behaviour of chromosomes. We can consider and investigate the chromosomes in two ways. First, in so far as they consist of genic material, they are genotype . Secondly, because chromo­somes themselves are subject to the control of the genes they bear, as well as to the effects of external conditions, they are phenotype ; and since heterosis is a roperty of the phenotype it is with this phenotypic aspect of the chromosomes we shall mainly be dealing.