576 GENETICS AND EVOLUTION 



I8O1 



Number 



Height in inches of 1083 adult men 



Figure 33.4. An example of a "normal curve," or curve of normal distribution: 

 the heights of 1083 adult white males. The blocks indicate the actual number of men 

 whose heights were within the unit range. For example, there were 163 men between 

 67 and 68 inches in height. The smooth curve is a normal curve based on the mean 

 and standard deviation of the data. (Villee: Biology.) 



same genetic constitution. Human identical twins (p. 646) have identical 

 sets of genes. The individuals of these strains, and human identical 

 twins, are not identical in all of their characters, however, for the varia- 

 tions due to environmental influences remain. One method of estimating 

 the relative importance of genetic and environmental factors on a given 

 character is to compare the variability of that character in a genetically 

 heterogeneous group and in a genetically homogeneous one. 



When a commercial breeder attempts to establish a new strain of 

 hens that will lay more eggs per year, or a strain of turkeys with more 

 breast meat, or a strain of sheep with longer, finer wool, he selects indi- 

 viduals which show the desired trait in greatest amount for further breed- 

 ing. There is a limit, of course, to the effectiveness of selective breeding 

 in increasing some desirable trait or in decreasing some undesirable one. 

 When the strain becomes homozygous for all the genetic factors in- 

 volved, further selective breeding will be ineffective. 



