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A TEXTBOOK OF BOTANY [Ch. VI, S 



Fig. 219. — A diagram to illustrate the principle of the chromosome 

 mechanism of heredity. 



The triangular masses of cells are adult individual plants, or animals, 

 male and female, developed from the parental germ cells shown below, and 

 forming above their own germ cells, which are uniting in pairs into fertilized 

 egg cells. In the nuclei of the individuals are the chromosomes, reduced 

 for simplicity to two, and composed of determiners, reduced for simplicity 

 to four, a black determiner being assumed to be always dominant to a 

 white one. For example, we may take a triangle to mean height of stem, 

 black meaning taller and white shorter ; circle, color of corolla, black darker, 

 white lighter; square, shape of leaf, black longer, white rounder; diamond, 

 texture of stem, black rougher, white smoother. Thus the two individuals 

 would be taller, longer-leaved, darker-flowered, rougher-stemmed, though 

 having both the capacity to transmit the other qualities, as shown in 

 their germ cells. 



Two such individuals as here pictured, being externally alike though 

 difTerently constituted in their chromosomes, are described in the technical 

 language of genetics as phenotypically identical but gcnotypically different ; 

 and, having both dominant and recessive determiners, are heterozygous for 

 all characters. They can, however, as the diagram shows, produce offspring 

 which contain only the dominant or the recessive determiners for certain 

 characters, that is, are homozygous for those characters. 



