272 



GENERAL ZOOLOGY 



Chromosome Basis of Mendelian Inheritance. — A knowledge 

 of the principles of segregation and independent assortment 

 enables one to predict the result of hybridizing one or more pairs 

 of allelomorphic conditions. The operation of both of these 

 principles can be understood if one assumes that the genes for 

 hereditary characters are located in the chromosomes, since the 

 disjunction of homologous chromosomes in the reduction division 

 of maturation meets all of the requirements of a mechanism to 

 bring about segregation and independent assortment of genes. 



Gameies YS 



YS 



Ys 



ys 



ys 



Ys 



ys 



ys 



Fig. 163. — Diagram showing composition of the F2 in a dihybrid cross. The 

 light circles represent yellow smooth peas; the heavy circles green smooth; the 

 light irregular circles yellow wrinkled; the heavy irregular circles green wrinkled. 

 YSYS (yellow smooth) is an extracted dominant; ysys (green wrinkled) an 

 extracted recessive; these with YsYs and ySyS are homozygous; the remainder 

 are heterozygous. Summary: Nine yellow smooth; three yellow wrinkled; 

 three green smooth; one green wrinkled. 



In the diagram (Fig. 164) this mechanism of segregation is illus- 

 trated by using a single pair of homologous chromosomes (synap- 

 tic mates), each of which carry a gene for character A or its 

 allelomorph a. To illustrate the law of independent assortment 

 for two pairs of characters, it would be necessary to use two 

 pairs of chromosomes, since there must be as many pairs of 

 chromosomes as there are independently assortable pairs of 

 characters. Thus, if we are dealing with two pairs of characters, 

 A, a, and B, b, the zygotic composition of the Fi would be AaBb. 

 In the formation of the gametes of the F u A separates from a, and 



