Chap. 20 THE PHYSICAL BASIS OF HEREDITY 395 



breeding has proven that the genes responsible for characters are segregated 

 in separate sex cells. 



The number of chromosomes is normally constant for each species, but 

 varies in different ones. Although each species has its characteristic number, 

 other species may have the same number; man and tobacco plants both have 

 24 pairs of chromosomes. There are 100 pairs of chromosomes in crayfishes 

 and 24 pairs in man. This tells plainly that there is no relation between an 

 animal's place in evolution and the abundance of its chromosomes. 



Chromosomes occur in pairs, except in mature sex cells. One member of 

 each pair is contributed by the egg and the other by the sperm cell of the 

 parents (Fig. 20.5). In the body cells the only chromosomes which may not 

 be paired are those which determine sex and in many species these are also 

 paired but of different shape and size as in man. In other species, usually in 

 the male parent, e.g., grasshoppers, half the sperm cells contain a sex 

 chromosomes and produce females, and half are without one and produce 

 males (Fig. 20.6). 



During development of human sex cells, the double number of chromo- 

 somes is reduced to the single or haploid number, 24 in the human sperm 

 and 24 in the egg (Figs. 20.5, 20.6). Each time a developing egg divides, 

 one member of each pair of chromosomes is segregated in the egg or in 

 the polar body, and likewise for the sperms, a result that is very significant 

 in the ancestry of all of us, whether mouse or man. 



Genes. Genes are the units of heredity, probably molecules of nucleoprotein 

 about five millionths of an inch long. By interaction with other genes, with 

 the cell content surrounding them, and the whole environment of the animal, 

 a gene or combination of genes controls the inheritance of such diverse 

 qualities as brown eyes, a soprano voice, and a way of walking. They are 

 contained and transmitted in chromosomes, hundreds of them being located 

 along the cross bands that are visible when certain chromosomes, as in the 

 fruit fly, are highly magnified (Figs. 20.7, 20.8). Although genes have not 

 been clearly seen, their places on a given chromosome have been located 

 exactly. 



Genes of fruit flies can be "knocked out" of chromosomes by treating the 

 animals with radium. When the sex organs of such flies are examined micro- 

 scopically, empty or damaged places may be found on the chromosomes of 

 the sex cells. In such flies, some part of the body may be changed, a new 

 wrinkle in the wings, or some action may be different. Treatments and ex- 

 aminations are repeated over and over again until the changed structure or 

 action of the fly is correlated with the particular spot on the chromosome. 

 Thus, the gene is located. Maps of chromosomes of fruit flies on which genes 

 are located are the results of the combination of experimental breeding and 

 microscopic examination of chromosomes (Fig. 20.8). 



