RECOMBINATION IN SEXUAL ORGANISMS 87 



these heterozygotes were to mate at random with individuals of identical 

 genotypes, the result would be the same as if they had inbred among 

 themselves. There would be, on the average, 75 per cent brown-eyed 

 progeny {B/B, B/b, and b/B); the remaining 25 per cent would be blue- 

 eyed individuals {b/b) in whom the recessive b genes, hidden in the Fj 

 heterozygotes, were able to express themselves because of the absence of 

 the dominant B gene. 



When the F^ heterozygotes mate with individuals of B/B genotype, 

 it would be as if they had backcrossed with the B/B parent; only brown- 

 eyed {B/B and B/b) offspring would result. Backcrosses of B/b x b/b 

 can give only B/b and b/b offspring in equal proportions. The 3: 1 ratio 

 of phenotypes in the Fg generation and the results of the backcrosses are 

 a demonstration of Mendel's first law because they are a necessary con- 

 sequence of the 1 : 1 segregation of genes. 



Matings between truly blue-eyed individuals can give rise only to blue- 

 eyed children. The qualification refers to the fact that eye colors are 

 sometimes misclassified. This is because there are more eye colors than 

 blue and brown, and other genes act as modifiers. The incautious in- 

 ference of genotype from phenotype must not be the cause of accusations 

 of inconstancy of affection. 



Mendel's second law of random assortment also predicts specific ratios 

 of genotypes and phenotypes in diploids. Individuals heterozygous for 

 two unlinked genes produce four different kinds of sperm or eggs in 

 equal proportions. From these, sixteen (4^) different combinations of 



TABLE 3.4 

 Inheritance of M and N Blood Groups in Man 



(From Wiener and Wexler, 1958, Heredity of the Blood Groups. 

 Grune and Stratton, New York, p. 43) 



Blood Groups of Parents Number of Families Blood Groups of Children 



M MN N_ 



M X M 



NxN 

 MxN 

 MN X MN 

 M xMN 



NxMN 



° There are six evident exceptions among the 3379 cases shown in the table. 

 They could have been mistakes in blood tvping, instances of mutation, or cases 

 of illegitimacy. Considerations of these possibilities may lead to an intelligent 

 solution of questions of disputed parentage. 



