562 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 light 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. 



604. 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. 



605. 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 many cases by assuming 

 that interactions occur between genes. It has also been found that genes 

 may change, causing the characters to vary. A heritable change appear- 

 ing 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. It 

 has recently been demonstrated that radiations, particularly X-rays, are 

 capable of bringing about gene mutations (Sec. 590). 



In many cases both traits develop resulting in incomplete dominance 

 of either trait. In such cases the Fi generation differs from both parents 

 and is more or less intermediate with respect to a certain pair of characters, 

 but in the F2 generation there are individuals like both grandparents as 

 well as intermediates. This occurs in the case of plants called four- 

 o'clocks; plants with red flowers crossed with those with white flowers 

 produce hybrids with pink flowers, and when these pink hybrids are 

 crossed the result is a ratio of one red dominant, two pink hybrids, and 



