GENETIC FACTORS IN THE ORIGIN OF DIVERSITY 395 



chromatids break and recombine at the point where they cross each other, 

 chromatids having the constitution (Ab) and (oB) will be produced. 

 Such exchange of genes between homologous chromosomes is called 

 crossing over. As a result of crossing over in our example a few sperm cells 

 having the constitution (Ab) and (ciB), respectively, will be produced. 



PRIMARY 

 SPERMATOCYTES 



SECONDARY 

 SPERMATOCYTES 



SPERMATIDS 



SPERMS 



FIG. 17.10. Effect upon linkage of (A) meiosis without crossing over, and (B) meiosis 

 with crossing over. 



We see, accordingly, that a group of linked genes tends to behave as 

 a unit in inheritance but that crossing over tends to disrupt this unit. The 

 chromosomes may be thought of as long chains of genes. Crossing over 

 leads to a regrouping of genes in the chain and hence is one of the 

 forces making for genetic diversity. 



