HEREDITY 



449 



that have been used 

 in experiments, and 

 it is known as the 

 Law 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- 

 ring again. 



These three laws 

 of heredity domi- 

 nance, segregation, 

 and unit 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. 



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) 



