534 GENERAL CONSIDERATIONS 



If two pairs of characters, one pair for curly hair and straight hair, 

 curly being dominant, and the other pair for dark color and hght color, 

 dark being dominant, are represented in the breeding of hybrid indi- 

 viduals, then nine of the phenotypes will have dark, curly hair; three will 

 have dark, straight hair; three will have light, curly hair; and one will 

 have hair which is both light and straight. 



The result when three pairs of characters are involved is illustrated 

 by the work of Castle on guinea pigs. When a short-haired, dark- 

 colored, and smooth-coated guinea pig is crossed with one which is long- 

 haired, white, and rough-coated, all of the Fi generation will belong to 

 one phenotype and will be short-haired, dark-colored, and rough-coated, 

 since these are the dominant characters. When these individuals are 

 bred together, however, they will produce eight different phenotypes. 

 Twenty-seven will have hair which is short, dark, and rough; nine will 

 have hair which is short, white, and rough; nine, hair which is long, dark, 

 and rough; nine, hair which is short, dark, and smooth; three, hair which 

 is long, white, and rough; three, hair which is short, white, and smooth; 

 three, hair which is long, dark, and smooth; and one, hair which is 

 long, white, and smooth. 



602. Breeding the Test for Characters. — In determining the genetic 

 constitution of an animal the test applied is that of breeding. If indi- 

 viduals bred generation after generation show only one character, then 

 they must be homozygous for that character; if they are hybrid, breeding 

 will betray the fact. The genetic constitution of an animal the genotypic 

 character of which is unknown may be determined by breeding it with 

 other animals the genotypic character of which is known. 



603. Variations in Inheritance. — The result of an enormous amount 

 of experimental work has shown that while there are certain characters 

 in plants and animals that behave exactly according to Mendel's prin- 

 ciples, others do not. This has been explained in some cases by assuming 

 that there are unpaired genes but in more cases by assuming that inter- 

 actions occur between genes. It has also been found that genes may 

 change, causing the characters to vary. A heritable change appearing 

 in a line of descent which cannot be traced to any ancestor is known as a 

 mutation. A mutant is an animal which shows such a change. 



With respect to the absence of a gene it is evident that dwarfness 

 might be due to a gene for dwarfness or to the absence of a gene for 

 tallness. The first alternative could be indicated graphically by the 

 use of the letters T and D, T being dominant ; the second by using T and t, 

 t showing the absence of T. A pure recessive would be either DD 

 or tt. 



An example of the interaction of genes is what is known as blending 

 inheritance, in which case the Fi generation differs from both parents and 

 is intermediate with respect to a certain pair of characters, but in the F2 



