HERITAGE OF THE INDIVIDUAL 291 



characters (genes) are in the simplex condition in the gametes 

 and duplex in the zygote. This close parallelism of gene 

 and chromosome behavior affords the most cogent evidence 

 that the chromosomes supply the physical basis of inheritance, 

 and that Mendelian segregation and related phenomena are 

 facts. For all practical purposes A, B, C, D, and a, b, c, d, 

 in figures 146 and 147 may be interpreted either as chromo- 

 somes or as characters. 



Turning now to the inheritance of characters whose genes 

 are borne by the same chromosome: these would seem to 

 be indissolvably linked together; and since the chromosome 

 number is usually not large there are twenty-three or 

 twenty-four in the gametes of Man compared with that of 

 heritable characters, we would expect sometimes to find 

 characters linked together. That is, not separately in- 

 herited as are yellow and round in our example. In reality 

 many cases are known in which characters are inherited 

 in groups. The inheritance of sex and sex-linked characters 

 will make the main point clear, and at the same time serve 

 to bring before us the essential facts in regard to the 

 determination of sex. 



1 . Sex Determination 



The reader will recall that in the general description of cell 

 structure it was stated that every cell of an organism contains 

 a definite even number of chromosomes. As a matter of fact, 

 in most instances the body cells of one sex, usually the male, 

 have one more functional chromosome than the 'regular' set, 

 and therefore an odd number. This extra chromosome, 

 which is commonly designated the X, or SEX CHROMOSOME, 

 has no mate at synapsis, remains undivided in the reducing 

 maturation division, and passes entire to one of the daughter 

 cells. Thus two classes of sperm are formed, one with and 



