348 



EVOLUTION, GENETICS, AND EUGENICS 



(segregates) the two chromosomes carrying the character for tallness, 

 so that each gamete contains one. This occurs for the other characters 

 as well as for that of tallness. From the tall plant, therefore, all the 

 gametes will contain the character for tallness, and from a dwarf plant 

 all of the gametes would contain the character for dwarfness. When 

 these two individuals are crossed the zygote will contain both charac- 

 ters, and these two characters will be transmitted together in the 

 succeeding cell generations. The individual from such a zygote of 

 course would be tall, but at the same time it would be carrying a 

 recessive determiner for dwarfness, and this fact would be shown by 



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Fig. 58. — Diagram illustrating behavior of first hybrid generation (Fi) when 

 inbred. Illustrates meaning of "segregation" and "purity of gametes" and how 

 chance matings of Fi gametes result in 3:1 ratio in F, generation; dwarf indi- 

 vidual produced only by zygote in lower right-hand corner. {Frotn Coulter and 

 Coulter.) 



its behavior in breeding. The result of inbreeding such hybrids is 

 indicated in the accompanying diagram (Fig. 58), which represents 

 the chance matings of two kinds of gametes. The obvious results are 

 three tall individuals and one dwarf. This is the so-called monohybrid 

 ratio, which means the ratio when a single pair of allelomorphs is 

 considered. 



Before discussing the further development of Mendel's law it will 

 be necessary to explain some of the terminology of genetics. When 

 each gamete carries the same kind of determiner the zygote is said to 

 receive a double dose; when a zygote receives only a single such deter- 

 miner it is said to receive a single dose. In Fig. 58 one zygote receives 

 a double dose of tallness and two others a single dose. These phrases 

 are more or less common in the literature of the subject, but the more 



