556 GENETICS AND EVOLUTION 



homozygous (BB), but if any of the offspring are brown the black parent 

 is heterozygous (Bb). 



Test crosses are of obvious importance to the commercial breeder 

 of animals or plants who is trying to establish a strain which will "breed 

 true" for a certain trait. Formerly, farmers and commercial breeders 

 could select plants to be used lor seed, or animals to be used as breeding 

 stock, only by their phenotypes. Without some means of differentiating 

 homozygous and heterozygous individuals this method is unsatisfactory, 

 for the heterozygous individuals would bear some offspring with the 

 recessive trait. 



In the more modern method, the breeder tests the genotypes of his 

 breeding stock by observing the qualities of their offspring. If the off- 

 spring have the traits desired, then these same parents are used for 

 further breeding. Two bulls, for example, may look equally healthy and 

 vigorous, yet one may have daughters with qualities of milk production 

 which are distinctly superior to the daughters of the other bull. By this 

 method, called progeny selection, the desirable qualities of a strain of 

 animals can be increased rapidly. One geneticist, for example, by progeny 

 selection over a period of eight years increased the average annual egg 

 production of a flock of hens from 114 to 200. 



273. Incomplete Dominance 



In many different species and for a variety of traits it has been 

 found that one gene is not completely dominant to the other. Heter- 

 ozygous individuals have a phenotype which can be distinguished from 

 that of the homozygous dominant; it may be intermediate between the 

 phenotypes of the two parental strains. The mating of red shorthorn 

 cattle with white ones yields offspring which have an intermediate, 

 roan-colored coat. The mating of two roan-colored cattle yields offspring 

 in the ratio of 1 red : 2 roan : 1 white; thus the genotypic and pheno- 

 typic ratios are the same; each genotype has a recognizably different 

 phenotype. This phenomenon, called incomplete dominance, is found 

 with a number of traits in different animals and with some human 

 characteristics. Studies of a number of human diseases inherited by reces- 

 sive genes— sickle cell anemia, Mediterranean anemia, gout, epilepsy and 

 many others— have shown that the individuals who are heterozygous for 

 the trait have slight but detectable differences from the homozygous 

 normal individual. 



274. A Dihybrid Cross 



The mating of individuals that differ in two traits, called a dihybrid 

 cross, follows the same principles as those of the simpler monohybrid 

 cross, but since there is a greater number of types of gametes, the num- 

 ber of different types of zygotes is correspondingly larger. 



If two pairs of genes are located in different (nonhomologous) 

 chromosomes, each pair is inherited independently of the other; each 

 pair separates during meiosis independently of the other. Another pair 



