394 



INTRODUCTION TO EVOLUTION 



only a few of them has been analyzed. The difference in pigmentation 

 between Negroes and "whites" seems to depend upon several pairs of 

 genes with cumulative effects, inheritance following a pattern like that of 

 our hypothetical tall and dwarf plant varieties. 



Linkage and Crossing Over 



So far we have been discussing pairs of genes which are independent in 

 inheritance even though they may not be independent in their effects upon 

 the phenotype. The reason the genes considered were independent in 

 inheritance was because each pair was contained in a separate pair of 

 chromosomes (Fig. 17.5). But since the number of gene pairs far exceeds 

 the number of chromosomes, it is obvious that many genes must be 

 contained in each chromosome. The genes in one chromosome do not 

 exhibit the independent assortment discussed above; they are said to be 

 linked to each other. 



A hypothetical example of such linkage is represented in Fig. 17.9, show- 

 ing a pair of chromosomes in a cell. The maternal (white) member of the 



FIG. 17.9. Chromosomal basis of linkage. Genes may be linked in various ways, 

 close together or far apart, on the chromosomes, as shown. 



pair is shown as containing the dominant genes A and B, the paternal 

 (shaded) member as containing the corresponding recessives, a and b. 

 Fig. 17.9 may be taken as representing the constitution of a spermatogo- 

 nium. What will be the result of meiosis of such a cell? Fig. 17.10A shows 

 what will usually happen (starting with the primary spermatocyte, having 

 each chromosome represented by a pair of chromatids). As a result of 

 meiosis half the sperm cells contain A and B, half contain a and b. 



In our discussion of meiosis, however, we mentioned (p. 382) the 

 fact that during synapsis part of one chromatid may be exchanged with 

 part of another one. As the chromatids separate following synapsis cross- 

 shaped configurations called chiasmata can frequently be observed. Such 

 a chiasma is shown in the primary spermatocyte of Fig. 17.1 OB. If the 



