Chapter 2 

 CHROMOSOMES AND GENES 



As usually observed, chromosomes appear as thick, homoge- 

 neous bodies during metaphase and anaphase, and as long, thin 

 threads at prophase. The prophase threads do not visually show 

 any differentiations except for the centromeres and numerous 

 granules, or chromomeres, which are most noticeable during the 

 prophase of the first of the two divisions that form spores in 

 most plants and germ cells in animals. Although a chromosome 

 shows little differentiation visually, it consists of a large num- 

 ber of submicroscopic structures, or genes, spaced along the 

 thread in a linear order but not an equal distance apart from 

 one another. The chromonema is a series of such genes separated 

 by inert regions and has therefore been referred to as a gene 

 string. These genes, or factors as they are sometimes called, are 

 too small to be observed with the photomicroscope, but they are 

 regions of great physiological activity. Exerting their effect 

 during development and in conjunction with one another and 

 with the environment, the genes determine the various physical 

 and mental characteristics which make up the mature plant or 

 animal. It is very difficult to obtain a reasonably accurate esti- 

 mate of the size of a gene, but it has been suggested that it is 

 about the size of some viruses and that its maximum dimen- 

 sions are roughly about 100 X 20 X 20 mix.* 



The number of genes in a plant or animal is apparently large. 

 Not many organisms have been studied intensively but in Droso- 

 phila melanog aster, over 500 genes have been discovered, and in 

 Zea mays, about 400. These are by no means the total numbers 

 of genes in these organisms but only the numbers found and 

 identified; they are probably only small fractions of the total 

 numbers. Muller has estimated that at least 1150 genes are 



* One fi, or micron, is one one-thousandth of a millimeter. A m/i is a 

 millimicron and is one one-thousandth of a micron. 



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