INBRED AND HYBRID ANIMALS AND THEIR VALUE 329 



rate of loss of heterozygosis in sex-linked genes is 29% (50% every two 

 generations) instead of 19%. This system is, then, slightly superior to 

 brother-sister mating. The more frequent use of the latter has probably 

 been dictated by its practical convenience. 



It is sometimes required to estimate the percentage of homozygosis in 

 a stock that is inbred, but which has not consistently followed any one 



100 



95 



(fi 



55 



50 



8 



Fig. 



10 12 14 16 18 20 22 24 26 28 30 



GENERATION 



131. — The percentage of homozygosis in successive generations under three 

 different systems of inbreeding. 



system of inbreeding. If pedigree records have been kept this can be done 

 by the use of coefficients of inbreeding (12, 15, 23). 



With brother-sister mating, and any system that results in separate 

 lines of descent, a limit is reached, according to the above calculations, only 

 when complete homozygosis has been attained, that is when all individuals 

 in any one line are genetically identical. 



Haldane (5) has discussed various factors that may affect the conclusions 

 reached above. He has shown that linkage may affect the distribution 

 of the heterozygosis left after inbreeding. Thus, the number of organisms 

 in which all the original heterozygosis has been lost may, as a result of 



