Appendix A: 

 Primer on Molecular 

 Genetics 



gives gene order. Rough information about gene position is sometimes available also, 

 but these data have to be used with caution, because recombination is not equally likely 

 at all places on the chromosome: thus, the genetic map, compared to the physical map, 

 stretches in some places and compresses in others. It is as though the genetic map were 

 drawn on a rubber band. 



How difficult it is to find an actual disease gene of interest depends largely on what is 

 already known about that gene and, especially, on what sort of alterations in the gene 

 have resulted in disease (Fig. 17). If disease results from a single altered DNA base, 

 spotting the disease gene is very difficult; sickle cell anemia is an example of such a 

 case, as are probably the majority of major human inherited diseases. When disease 



Fig. 16. Finding genes. A genetic 

 map will reveal that a given gene of 

 interest (C) lies in a region, perhaps 

 encompassing 10 million base pairs 

 (10 Mbp). The physical map allows 

 one to dissect that region and to 

 locate likely pieces on which the 

 particular gene may reside. 



Genetic 

 map 



cloned 

 DNA 



A 



1 - 10 Mb 



— D 



c 



■O 



B 



restriction 

 fragments 



1 Mb 



Goal; Find the fragment that contains gene C. 



136 



