320 



GENETICS 



contain two chromosomes of that pair, others but one (fig- 

 ure 65). When a zygote is formed by union of two germ 

 cells, each of which contains two chromosomes of the given 

 pair, there is produced an individual that has four chromo- 

 somes to one of its pairs (figure 6^, G) ; its genes are thus 



D 



Parents 



Germ Cells Fertilized Egg^ 



Figure 65. Diagram of the formation of a tetrasomic (G) as a 

 result of germ cell formation by trisomies (A and D). The basic 

 number of chromosome pairs is here seven. The two trisomies A and 

 D each have three chromosomes for the second pair from the left. In 

 forming germ cells, two chromosomes of this pair pass into one set 

 of germ cells (C and E), only one into the other set (B and F). 

 By union of C and E, a zygote G is formed having four chromo- 

 somes in place of two, for the second pair ; this is a tetrasomic. 



in fours, so far as this chromosome is concerned. Such in- 

 dividuals are known as tetrasomics. They have different 

 characteristics from any of those spoken of above. 



There are in Datura twelve different varieties that can 

 be formed by giving four chromosomes in place of two, in 

 different cases, to each of the twelve chromosome pairs. 

 And 66 additional types can be formed by giving four chro- 



