HEREDITY 325 



acter, as TT for example, the individual is termed homozygous. 

 Since the heterozygous type may result in two ways, either by union 

 of an egg with T and a sperm with t, or by an egg with t and a 

 sperm with T, it occurs twice as often as either of the homozygous 

 types, tt and TT. This accounts for the ratio of genotypes in the 

 Fi' generation of a monohybrid cross, i TT : 2 Tt : i tt. The domi- 

 nance of Tallness over Dwarfness accounts for the phenotype ratio 

 of 3 Tall : I Dwarf (Fig. 204). These ratios are always obtained 

 from a simple monohybrid cross if the number of progeny is large 

 enough to constitute a fair sample. 



It is thus possible to refer the segregation of alternative char- 

 acters in inheritance to the processes which take place during the 

 reduction of chromosome number in the maturing of the gametes 

 and to account for the Mendelian ratios on the basis of chromosomal 

 behavior during mitosis, maturation, and fertilization. The involved 

 processes leading to equal distribution of chromatin and to the 

 maintenance of chromosome number and integrity, as well as the 

 various steps in maturation, now have an important significance 

 and their functions are clear. But Mendel knew nothing of chromo- 

 somes; his interpretations were wholly independent of any struc- 

 tural basis. 



The case, which we have given, of a monohybrid cross with the 

 complementary genes truly dominant and recessive, located on 

 members of a single pair of homologous chromosomes, is com- 

 paratively simple. In fact, Mendel's laws are in themselves simple. 

 But, like so many natural laws that are simple in their funda- 

 mentals, their application in some cases becomes exceedingly com- 

 plicated and abstruse. Each chromosome in the nuclear complex in 

 the organism contains a great number of genes for alternative char- 

 acters located on its synaptic mate. In Man, with forty-eight chromo- 

 somes, it may be assumed that there are twenty-four such groups 

 of pairs of alternative characters; the number in each group, that 



