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CONTINUITY OF LIFE 



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Fig. 24-16. Homologous genes have a tendency to ex- 

 change places during synapsis in some such manner 

 as shown here. 



of certain animals and not in the other 

 is controlled by such autosomal genes. 



Linkage and crossing-over 



During the early studies of Mendelian 

 inheritance it was soon discovered that 

 there were more traits than chromosomes 

 and this fact led workers to conclude that 

 more than one gene existed on each chro- 

 mosome. If tliis were true, then what hap- 

 pens during meiosis? As long as the genes 

 are on separate chromosomes the ideal ra- 

 tios of Mendel come out beautifully, but 

 if they are located on the same chromo- 

 some, the ratios are different, though quite 

 exact. Morgan and his students showed that 

 traits entering a cross often stayed together 

 in subsequent generations, which proved 

 that tliey were linked together on the same 

 chromosome. This condition is called auto- 

 somal linkage to distinguish it from sex 

 linkage. 



There should be as many linkage groups 

 as there are chromosomes, which proves to 

 be the case in those plants and animals 

 where sufficient information is available. 

 Drosophila, for example, has four pairs of 

 chromosomes and four linkage groups. 

 Moreover, the linkage groups even approxi- 

 mate the chromosomes in size. There is a 

 small pair of chromosomes and a small link- 

 age group, a middle-sized pair and a middle- 

 sized linkage group, and there are two pairs 

 of large chromosomes and likewise two 



large linkage groups. These facts, together 

 with information that is to follow, certainly 

 identify genes with chromosomes. 



Linked genes respond in a manner simi- 

 lar to single gene inheritance. For example, 

 if genes A and B are linked in one chromo- 

 some (Fig. 24-15) and their homologous 

 recessives, a and h, are linked on the homol- 

 ogous chromosome, they segregate out in 

 the formation of germ cells as if they were 

 single genes and the subsequent typical 

 one-gene ratio will result. Whereas if the 

 genes were located on separate chromo- 

 somes, the typical 9:3:3:1 ratio would fol- 

 low. Such linkages obviously decrease the 

 possible number of combinations and thus 

 definitely limit variation. 



Strangely enough, genes once linked do 

 not always remain so. During synapsis in 

 meiosis, you will recall, the homologous 

 chromosomes come together and wind 

 about one another just before the tetrads 

 are formed. You will also recall that the 

 homologous genes lie opposite each other, 

 that is, those for the same trait are drawn 

 together by some mutual attraction. When 

 the chromosomes separate during the sub- 

 sequent meiotic divisions they are pulled 

 apart with such force that the chromosomes 

 often fragment where they are entwined 

 about each other. In this way, gene groups 

 from one chromosome become a part of 

 the other chromosome in some such man- 

 ner as indicated in Fig. 24-16. The result- 

 ing chromosomes, containing a mixture of 

 genes, seem to heal perfectly because for 

 the most part they contain their full com- 

 plement of genes. This process of cross- 

 ing-over, as it is called, operates purely 

 fortuitously, so that in it we see another op- 

 portimity for a further juggling of the genes, 

 compensating in part for the limitations 

 placed upon variation through linkage. 



The troublesome ratios that resulted from 

 crossing-over were a blessing in disguise 

 because they made possible the ultimate 

 construction of chromosome maps. Sturte- 

 vant, in 1913, reasoned that crossing-over 



