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THE POPULAR SCIENCE MONTHLY 



if it is a factor which differs in the ease of the two sexes it is a " sex- 

 determining factor." There are many parts of a germ cell, all of which 

 may be concerned in heredity and development, but the chromosomes 

 appear to be the seat of the differential factors for Mendelian char- 

 acters. 



pert ilization 

 Union of 

 Simplex Groups 



OCT 



ABCD 



Cleavage 

 Duplex Groups 

 ABCD abed 



aBCd 



Aa.Bb.Cc.Dd. 

 Synapsis. 



AbcD 

 Reduction Germ Cells 



Division Simplex Groups 



Somatic Divisions 

 Duplex Groups 



Fig. 59. Cellular Diagram Corresponding to Fig. 58, Showing the Union 

 op Maternal Chromosomes (ABCD) and Paternal Ones (abed) in Fertilization, 

 their distribution in cleavage, their union into 4 pairs (Aa, Bb, Cc, Dd), in synapsis 

 and the separation of the pairs in the reduction division. Only 2 of the 16 possible 

 types of germ cells are shown. (From Wilson.) 



2. Modifications of the Principle of Dominance. — A great number 

 of animal and plant hybrids show one contrasting character completely 

 dominant over the other one, as Mendel observed in the case of his peas. 

 But in a considerable number of cases this dominance is incomplete or 

 imperfect. When white-flowered strains of four-o'clocks are crossed 

 with red-flowered ones the F x plants bear neither white nor red flowers, 

 but pink ones, and the F 2 plants bear white, red and pink flowers. The 

 whites and reds are always homozygous, the pinks heterozygous ; pure 

 white and pure red are produced only when their factors are duplex 

 (WW), (RE) ; when they are simplex (WE) pink is produced. In this 

 case red is not completely dominant over white, but the hybrid is more 

 or less intermediate between the two parents (Fig. 56). 



It has long been known that the breed of fowls called Blue Anda- 

 lusian does not breed true, but in each generation produces a certain 

 number of blacks and whites as well as blues. Bateson found that the 

 blues are really hybrids between blacks and whites in which neither of 



