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 said to receive a single dose. 



