HEREDITY 



449 



that have been used 

 in experiments, and is 

 sometimes called the 

 Lazv of Unit Char- 

 acters (see Fig. 241). 

 This principle will 

 help us to understand 

 how there can be 

 such great diversity 

 among the individ- 

 uals of any given 

 species of plants or 

 animals, or even 

 among the brothers 

 and sisters of any 

 family. The greater 

 the number of char- 

 acters, the greater is 

 the possible number 

 of combinations; and 

 the smaller is the 

 chance of any given 

 combination occur- 



rmg agam. 



Fig. 241. The law of unit characters illustrated 

 by guinea pigs 



Pigmentation in these animals is dominant over albi- 

 nism. Short hair is dominant over long hair. Rough 

 coat is dominant over smooth coat. When two pure 

 individuals like those shown are mated, the offspring 

 will be short-haired, dark, and rough-coated. On mating 

 the hybrids together in sufficient numbers, the segrega- 

 tion will result in producing every combination of these 

 three sets of characters : dark-short-rough ; dark-short- 

 smooth ; dark-long-rough ; dark-long-smooth ; white- 

 short-rough ; white-short-smooth ; white-long-rough ; 

 white-long-smooth. The proportions will be such that 

 for each pair of contrasted characters there will be one 

 recessive to every three dominants. (From photographs 

 lent by Professor W. E. Castle) 



These three laws 

 of heredity — domi- 

 nance, segregation, 

 and nnit character — 

 are known as the 

 Mendelian laws, or 

 principles, because 

 they were first dis- 

 covered by Gregor Mendel. They have been found sufficiently 

 reliable to serve as a basis for practical work of great impor- 

 tance in connection with the breeding of plants and animals. 



