Heredity — Genetics 681 



a definite location in a particular chromosome. The various genes are 

 arranged in a linear series within the chromosome. The position of a 

 gene in the chromosome bears no relation to the location of the result- 

 ing characteristic developed in the body of the organism. For example, 

 the genes for the determination of characteristics of the anterior end of 

 an organism are not necessarily all located at one end of a chromosome, 

 nor are all the genes for the traits of the posterior part of an organism 

 located in the opposite end of the chromosome. They are, however, 

 scattered promiscuously, yet specifically, throughout the chromosome. 



Each zygote (fertilized egg) contains two genes for each hereditary 

 character, one coming from each parent. This does not mean that the 

 two are necessarily identical, although they may be. Thus, all body 

 cells arising by mitosis from this zygote have duplicate genes for each 

 specific hereditary character (Figs. 335, 350, and 351). 



Genes recently have been photographed so that their form, at least, 

 has been somewhat demonstrated. The size of certain genes is thought 

 to approximate the size of a large organic molecule with a maximum 

 dimension of about 40,000 millimicrons.* 



A great majority of genes are stable and usually very resistant to en- 

 vironmental influences. X-rays and similar radiations are known causes 

 of changes in genes. A few genes apparently are unstable, being changed 

 frequently under ordinary conditions. 



Over 1,000 genes have been determined so far in the chromosomes of 

 the fruit fly {Drosophila melanogaster) (Fig, 332) and about 400 in 

 corn. Undoubtedly, the twenty-four pairs of human chromosomes con- 

 tain even greater numbers of genes. The total number of human heredi- 

 tary characters has never been stated or even approximated. 



The cells of an organism before maturation of the germ cells retain 

 the duplicate set of chromosomes and genes. During maturation, those 

 chromosomes which carry equivalent genes (hence, homologous chromo- 

 somes) unite temporarily in pairs. Later, during reduction division, one 

 of each pair of chromosomes (and hence, one of each pair of genes) goes 

 to each of the two new daughter cells. Therefore, each resulting gamete 

 or sex cell has only a single set of chromosomes (and genes). Thus, 

 when two sex cells are united during fertilization, the resulting cell 

 (zygote) again has its duplicate supply, one of each pair having been 

 contributed by each parent. 



Since chromosomes are usually found in pairs in typical organisms 

 (Figs. 332 and 333), genes must also be present in pairs. The pairs of 



*1 milllcrom is one-thousandth part of a micron, and a micron is one-thousandth part of a 

 millimeter. 



