5 7\ 9, 11 13 15 17 19 21 23 X 



\\ \\ 





4' 6 8' 10' 12 14 16 18 20 22 Y 



I'aililrl' 



CHROMOSOMES IN MAN 



The 24 pairs of chromosomes are shown in order of size. The members of each pair 

 ore indistinguishable, except that the smallest, marked X and Y, differ in size in the 

 male. During reduction division, when sperm cells are being formed, the X goes to 

 one sperm and the Y to another. All eggs, however, have the X chromosome. Struc- 

 tural variations which the microscope may reveal among chromosomes of different 

 individuals cannot be related to personal or racial traits 



Chromosome Maps We have seen that it was through the idea of 

 linkage that Morgan and his fellow workers came to place certain genes to- 

 gether in particular chromosomes — that is, from fcllowing up exceptiotis to 

 Mendel's law of independent assortment. Since a chromosome is generally 

 an elongated structure, it seems reasonable to suppose that the genes are 

 probably arranged along the length of the chromosome. Now the question 

 naturally arose. Is there any way of locating particular genes more exactly 

 along any particular chromosome? 



This problem was solved by studying the exceptions to the idea of linkage. 

 Characters are coupled; but linkage is not 100 per cent consistent. Certain 

 pairs or groups of linked characters become separated in succeeding genera- 

 tions more frequently than others. If we suppose that the genes are arranged 

 in a series, we should expect that those which are close together in a chromo- 

 some would seldom become separated, whereas those at opposite ends of a 

 chromosome might become separated more frequently. But what happens in 

 the chromosomes to produce such a break in the chain (see illustration, 

 p. 494).? 



The observed fact that parts of chromosomes sometimes break off and 

 cross over to the other member of the pair seems to run parallel with the 

 experimental fact that characters which are usually coupled together in suc- 



491 



