34 Plant Genetics 



chromosome. In some cases also there are more Men- 

 delian determiners than there are chromosomes. 



The situation is represented in fig. 3. This shows 

 a somatic cell with the diploid or 2X number of chromo- 

 somes. In the formation of gametes this number is 

 reduced to the haploid, or x number, which in this case 

 is two. The diagram shows that the reduction separates 

 (segregates) the two chromosomes carrying the char- 

 acter 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 characters, and these two char- 

 acters 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 its behavior in breeding. The 

 result of inbreeding such hybrids is indicated in the ac- 

 companying diagram (fig. 4), 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 Men- 

 del'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 determiner it is §sii4 to receive a single dose. 



