512 THE PRINCIPLES OF HEREDITY 



These results show the independent action of each gene. The genes 

 for hair color and hair length do not stay together as they were found 

 in the original parents. We have a 3 : 1 ratio for each character, but 

 there is an independent assortment which produces new combinations 

 as well as the old combinations. This explains why children show 

 various mixtures of inherited characteristics from their parents and why 

 there is variation among the children of the same parents. We might 

 compare the segregation of the chromosomes in meiosis to the shuffling 

 and dealing of a deck of cards. Each gamete represents a new deal and 

 will not be exactly like other gametes in its gene content. 



Linked Genes and Crossing Over 



If you have been studying the principles of independent segregation 

 of genes thoughtfully, you have already realized that there must be 

 some cases where such free assortment of the genes does not occur. 

 The number of genes within a cell is always much greater than the 

 number of chromosomes, so each chromosome must bear many genes. 

 Chromosomes are assorted independently during meiosis and genes on 

 different chromosomes will likewise have such independent assortment ; 

 but what about two genes that lie on the same chromosome? In this 

 case we would expect the genes to remain together during meiosis and 

 retain the parental combinations in the future offspring. Dihybrid 

 crosses involving genes lying on the same chromosome indeed show that 

 such linkage does exist — the genes will tend to stay together in the 

 same combinations in which they existed in the parents of the first 

 generation. This linkage, however, is usually not complete — there will 

 very likely be a few new combinations in the offspring of the second 

 generation. 



Let us take a cross in Drosophila to illustrate linkage. Suppose we 

 select a male with a gray body and red eyes and cross it with a female 

 with a black body and purple eyes. The first generation flies will all 

 have gray body and red eyes, which shows that these two genes are 

 dominant. Now let us chose some of the females of this group and 

 cross them with males having black body and purple eyes as a test cross. 

 In the flies obtained from this second cross, let us assume that we get 

 results as follows : 92 gray red, 96 black purple, 5 gray purple, and 7 

 black red. Since the predominating types are like the first parents it 

 is obvious that these two pairs of genes are linked, but where did we 

 get the recombinations? 



Cytological studies help us learn the answer. When chromosomes 

 pair during the early stages of meiosis it can be seen that portions of 



