42 



LABORATORY MOUSE 



Gametes of rsRS parent 

 RS Rs rS 



Gametes 



of 



rrss 



parent 



Crossover classes 



If the characters silver and rodless were not linked, we 

 should expect equal numbers of all four classes. If they are 

 linked, we expect the second and third classes to be small in 

 comparison with the first and fourth. The second and third 



Fig. 8. Diagram of a back-cross illustrating the linkage between rodless retina (r) 

 and silver pelade {x). Animals with unshaded eyes are rodless. 



classes are produced only from gametes in the genesis of 

 which a crossover has occurred between silver and rodless. 

 A back-cross of the kind described (98) gave 9 unsilvered 

 normals, 1 unsilvered rodless, 1 silver normal, and 5 silver 

 rodless. More extensive breeding tests indicate that a cross- 

 over takes place between silver and rodless at the formation 

 of about 12 per cent of the gametes. 



If three determiners are located at different points in the 

 same chromosome, as are albinism (172), shaker (126), and 

 pink-eye dilution (172), we may determine their relative posi- 

 tions by the percentage of crossovers obtained in breeding ex- 

 periments. If a crossover occurs between albinism and shaker 

 3 times out of a hundred and between shaker and pink-eye 

 dilution 15 times out of a hundred and between albinism and 



