HEREDITY 141 



Fifty per cent of the germ-cells will contain the dominant 

 character and fifty per cent the recessive character. This 

 is true of both male and female germ-cells. 



134. Unit characters. Mendel's theory presupposes 

 the existence of unit characters, so-called because they 

 are transmitted as independent units. There exist in 

 every organic being a very large number of unit char- 

 acters, and these may be determined by experimental 

 breeding. Up to the present time a relatively small 

 number of unit characters have been definitely differ- 

 entiated and described, but our knowledge in this direc- 

 tion is being rapidly extended. 



135. Gametic purity. In one stage of maturation 

 of the male and female germ-cells, the nucleus of each 

 contains but half the normal number of chromosomes 

 characteristic of the species. This is the final stage in 

 the maturation process before fertilization takes place. 

 The germ-cells in this stage are called gametes. The 

 fertilized egg-cell which results from the union of a ma- 

 ture sperm and egg-cell (gametes) is called a zygote 

 (fertilized egg-cell). The zygote, therefore, contains the 

 normal number of chromosomes characteristic of the 

 species, one-half derived from the egg and one-half from 

 the sperm-cell. Gametic purity is a term used to desig- 

 nate the discontinuous nature of unit characters. The 

 gamete in Mendel's pea contains the factor necessary for 

 the production of yellow seeds, or it does not. 1 



The terms homozygous and heterozygous were pro- 

 posed by Bateson to designate the fundamental consti- 

 tution of the germ-cells in respect to inherited characters. 

 " An individual is said to be homozygous for a given 



1 Darbishire, "Breeding and the Mendelian Discovery," 

 p. 217. 



