HEREDITY 



573 



eye color of the son may not be the same as 

 that of the father. From time immemorial, 

 humans have taken note of the resemblance 

 of children to their parents and tried to de- 

 termine whether a child had its father's 

 nose or its mother's, and the same for other 

 features. The characteristics of the son may 

 be like those of neither the father nor the 

 mother, and thus we may discover that 

 heredity involves diflferences as well as like- 

 nesses between parents and offspring. 



The study of heredity deals with the 

 origin of similarities and the differences be- 

 tween parent and offspring. It is concerned 

 with the nature of these similarities and 

 differences, their sources, and how they de- 

 velop. Briefly stated, heredity is a study of 

 the transmission, from generation to genera- 

 tion, of developmental potentialities (genes) 

 and how they come to expression. 



Genes 



The gene is the chemical unit of heredity, 

 which is transmitted in the chromosome, 

 and which by interaction with the internal 

 and external environment may control the 

 development of a trait. Although the exact 

 physical nature of the gene is not known, 

 there is some evidence for regarding it as a 

 complex chemical substance; the essential 

 genie material is desoxyribonucleic acid 

 (abbreviated DNA). Investigations on the 

 nature of genes are difficult because they 

 cannot be isolated for study or seen with a 

 microscope. One of the most important 

 properties of a gene is its ability to reproduce 

 itself, for each gene must be duplicated in 

 each cell division. The way in which a gene 

 duplicates itself is unknown, but it prob- 

 ably depends on enzyme activities. 



The fertilized egg in the case of man and 

 of other higher animals and the unfertilized 

 egg in parthenogenetic species contain the 

 sum total of the genes that largely consti- 

 tute the heredity of the individual. We have 

 already discussed in Chapters 8 and 34, the 

 origin of the egg and sperms, fertilization, 



chromosome behavior, and embryonic de- 

 velopment. The study of genetics requires 

 further study of these phenomena, since the 

 distribution of the genes which are responsi- 

 ble for the heredity of any individual is 

 accomplished during the processes that oc- 

 cur before and at the time of fertilization. 

 Of particular interest are the chromosomes 

 and their behavior, since the genes are con- 

 tained in the chromosomes. We shall there- 

 fore begin our discussion of genetics with a 

 description of what at present biologists be- 

 lieve to be the structure of the chromo- 

 somes. 



Chromosome structure 



The chromosomes are deeply staining 

 structures found in the nuclei of cells. Their 

 morphology and behavior are described in 

 the accounts of mitosis in Chapter 2, the 

 origin of eggs and sperms on page 80, and 

 illustrated in Fig. 404. Figure 10 illustrates 

 the structure of a chromosome in different 

 phases of mitosis. 



Chromosomal constitution 

 of somatic cells 



During the life of the individual, from 

 the fertilized egg (zygote) to the period of 

 sexual reproduction, every cell in the body 

 contains the same number of chromosomes, 

 a number which is usually constant for and 

 characteristic of the species. In sexually re- 

 producing organisms, since the chromosomes 

 of the zygote result from a union of sperm 

 and egg, the zygote chromosomes consist 

 of a paternal and a maternal set. The chro- 

 mosomes composing the paternal set gen- 

 erally differ from each other in length, shape, 

 and in the genes they contain. Similariy, 

 the maternal set is composed of different 

 chromosomes. However, for each chromo- 

 some of the paternal set, there is a chromo- 

 some in the maternal set which is similar to 

 it in form and genie content. Chromosomes 

 which show such close resemblance to each 



