NORMAL INHERITANCE 



41 



"crossing-over", bear Re and rC. The percentage of cases 

 (times in a hundred) in which crossing-over occurs is called 

 the crossover percentage. The nearer together two genes 

 lie in the same chromosome, the fewer chances there are for 

 breaks to occur between them resulting in recombination 

 or crossing-over. Hence the percentage of crossing-over is 

 taken as a measure of the nearness together of genes or of 

 the linkage strength (16) between them. 



In cases where linkage is involved, new combinations 

 (recombinations) of two characters may be difficult to obtain. 



Fig. 7. Diagram of a pair of chromosomes showing the linkage relationship of two 

 pairs of genes before, during, and after a crossing-over. 



Suppose we select a mouse bearing two genes located in the 

 same chromosome, for example rodless and silver (rrss). 

 Should we cross this rodless-silver to a normal-eyed unsil- 

 vered mouse (RSRS) the offspring will be rsRS and will be 

 normal-eyed unsilvered. When these animals form gametes, 

 r and s being in the same physical chromosome will behave 

 ordinarily as units as also will R and S for a like reason. 

 Hence there will be ordinarily two, and only two, kinds of 

 gametes, namely, rs and RS. If two such animals are mated 

 together they will produce ordinarily but two types of 

 offspring: normal-eyed unsilvered (RSRS or RSrs) and rod- 

 less silver (rsrs). But when there is a crossover, two other 

 types of gametes are produced (rS and Rs), making possible 

 the occurrence of occasional rodless unsilvered animals (rSrS 

 or rsrS) and normal-eyed silvers (RsRs and rsRs). We may 

 detect the percentage of crossovers between r and s by 

 crossing an rsRS animal to an rsrs (see Fig. 8). 



