HEREDITY 



583 



ents, the Mendelian ratio in the F2 genera- 

 tion is 3:1 (3 tall plants to 1 short plant, 

 Fig. 408; in a dihybrid the ratio is 9 : 3 : 

 3 : 1 (Fig. 409); and in a trihybrid the ratio 

 is 27 : 9 : 9 : 9 : 3 : 3 : 3': 1. Where 4 

 different pairs of genes are concerned, there 

 are 81 possible genotypes and 16 pheno- 

 types; 5 different pairs of genes give 243 pos- 

 sible genotypes and 31 phenot\pes. In man 

 the somatic number of chromosomes is 46 * 

 (Fig. 410), and the reduced number of chro- 

 mosomes in the mature germ cells is 23. If 

 these chromosomes are all different and each 



chromosome is often smaller than its mate, 

 which is known as the X chromosome; in 

 some species the Y chromosome is absent. 

 These X and Y chromosomes are called sex 

 chromosomes, in contrast to all the other 

 chromosomes, which are termed autosomes. 

 The X chromosome of the male resembles 

 the chromosomes of a homologous pair 

 possessed by the female. Thus the female 

 has one pair of X chromosomes; and the 

 male has one X chromosome, which may 



* The chromosome number in man is still con- 

 troversial, but 46 is probably correct. Several investi- 

 gations with new and improved technics strongly 

 favor the conclusion that there are 46 chromosomes. 

 Kodani reports that all individuals possess 46 chro- 

 mosomes, but some individuals have either 47 or 48. 

 The explanation for the 47 or 48 chromosome count 

 is the presence, either singly or in duplicate, of a 

 supernumerary chromosome. 



is the bearer of genes, as we have every rea- 

 son to suppose is true, the number of pos- 

 sible genotypes and phenotypes in human 

 offspring is very large indeed. 



Inheritance of sex 



Thus far we have considered the 2 mem- 

 bers of each pair of homologous chromo- 

 somes to be exactly alike; but in the males 

 of most species of animals studied, including 

 man, the members of one pair are dissimilar 

 (Figs 410 and 411). One known as the Y 



Figure 410. Chromosomes of man. TTiis photo- 

 graph shows the chromosomes of a normal male with 

 one X, a smaller Y chromosome, and 44 autosomes. 

 This is a metaphase stage when each chromosome is 

 split into two chromatids. The illustration demon- 

 strates clearly that the typical human chromosome 

 number is 46. (Courtesy of Doctors Tjio and Puck, 

 Dept. of Biophysics, Univ. of Colorado Medical 

 Center.) 



or may not have a Y chromosome as a mate. 

 Experiments have shown that the X chro- 

 mosome bears genes influential in determin- 

 ing sex. Figure 412 indicates how sex 

 determination may depend on the distribu- 

 tion of these X chromosomes. 



In Fig. 412, the primordial germ cells of 

 man are shown with one X chromosome and 

 one Y chromosome; and the primordial 

 germ cells of a woman are shown with 2 X 

 chromosomes. During maturation two types 

 of sperms are produced, one with an X 

 chromosome and the other with a Y chro- 

 mosome, but only one type of egg with an 

 X chromosome. Sex is determined at the 

 time of fertilization, since the egg that is 

 fertilized by the sperm with a Y chromosome 

 becomes a zygote with only one X chromo- 

 some and develops into a male, whereas the 



