Inbreeding 369 



of wise and able rulers, but regularly adhered to a custom of 

 brother-and-sister marriages. The marriage of Charles Darwin 

 to his first cousin, Emma Wedgwood, produced offspring that 

 were well above the average. If inbreeding is not always harm- 

 ful, what is its effect? 



The effects of inbreeding were effectively demonstrated and 

 stated by the Danish geneticist, Johannsen, in 1903. He chose 

 a commercial variety of the common garden bean, known as the 

 Princess Bean, for studies of the effect of selection on weight of 

 the seed. Taking a sample from a mixed lot of beans of dif- 

 ferent sizes, he showed that the progenies of the heavier beans 

 in general weigh more than those of the lighter beans, and that 

 not all the seeds of the same size produce offspring of the same 

 average weight. He also compared the average weight and the 

 variability of offspring of individual mother plants and found 

 that the progeny of each particular plant showed much less 

 variability than the whole group with which he started. He then 

 self-fertilized these strains for several generations, being espe- 

 cially careful to prevent one line from becoming crossed with 

 another. 



The results of several successive generations of inbreeding 

 from different original mother plants (of which he happened to 

 use nineteen) showed that each of these different inbred lines 

 had a certain average weight and that this value was essentially 

 the same for each generation of any given line. For example, 

 the line with the smallest seed weight had an average of about 

 35 eg, a value essentially the same for all generations that were 

 produced by self-fertilization within that line. Another line had 

 a seed-weight average of about 64 eg, and this weight was main- 

 tained for all generations that were produced by self-fertiliza- 

 tion of any seed in the line. When we point out that the average 

 weight of the seeds of any generation in one of the lines has a 

 certain value, we do not mean that there was no variation in 

 that group of plants for, of course, there was. The interesting 

 feature of this variation, however, is that it is not inherited. For 

 example, the average seed weight of line 2 of the original nine- 

 teen lines was 55.8 eg, and yet beans were found which weighed 

 as little as 40 eg or as much as 70. To ,test the efficacy of selec- 

 tion within this particular inbred line, progeny were obtained 

 from the self-fertilization of plants of that line which weighed 



