554 GENETICS AND EVOLUTION 



receives only one b gene. Similarly, during meiosis in the female, the 

 BB genes separate and each egg receives only one B gene. There is only 

 one type of sperm, those containing a b gene, and one type of egg, those 

 with a B gene, and their union leads to a single type of individual, Bb. 

 Thus, all the offspring, the Fj generation, are similar. Since these in- 

 dividuals have one gene for black color and one gene for brown color, 

 you might guess that the offspring would be dark brown, or gray, or 

 perhaps spotted. However, all the Fi individuals are just as black as the 

 mother. The black gene is dominant to the brown one and produces 

 black coat color even in the presence of the other gene. The brown gene 

 is said to be recessive to the black one. By convention, the dominant 

 gene is symbolized by a capital letter and the recessive gene by the cor- 

 responding lower case letter. The phenomenon of dominance supplies 

 part of the explanation as to how it is that an offspring may resemble 

 one of its parents much more than the other, despite the fact that both 

 parents make equal contributions to its genetic constitution. 



An animal or plant with two genes exactly alike, two blacks (BB) or 

 two browns (bb), is said to be homozygous or "pure" for the character. 

 An organism with one dominant and one recessive gene (Bb) is said to 

 be heterozygous or "hybrid." Thus, in the mating under consideration 

 the black and brown parents were homozygous, BB and bb, respectively, 

 and the offspring in the F^ were all heterozygous, Bb. Recessive genes 

 are those which will produce their effect only when homozygous; a 

 dominant gene is one which will produce its effect whether it is ho- 

 mozygous or heterozygous. 



In the process of gamete formation in these heterozygous black Fi 

 guinea pigs, the chromosome containing the B gene undergoes synapsis 

 with, and then separates from, the homologous chromosome containing 

 the b gene, so that each sperm or egg has a B gene or a b gene. No sperm 

 or egg is without one or the other and none has both. Since there are 

 two kinds of eggs and two kinds of sperm, the mating of two of these 

 heterozygous black guinea pigs permits four different combinations of 

 eggs and sperm. To see these possible combinations of eggs and sperm 

 it is conventional to arrange them in a Punnett square (Fig. 32.1), de- 

 vised by the English geneticist, R. C. Punnett. Gametes containing B 

 genes and ones containing b genes are formed in equal numbers. There 

 is no special attraction or repulsion between an egg and a sperm con- 

 taining similar genes; an egg containing a B gene is just as likely to be 

 fertilized by a B sperm as by a b sperm. The four possible combinations 

 occur with equal frequency. 



The possible types of eggs are written across the top of the Punnett 

 square and the possible types of sperm are arranged down its left side, 

 then the squares are filled in with the resulting zygote combinations 

 (Fig. 32.1). Three-fourths of the offspring are either BB or Bb, and con- 

 sequently have a black coat color, and one-fourth are bb, with a brown 

 coat color. This three to one ratio is characteristically obtained in the 

 second generation of a monohybrid cross, i.e., a mating of two individ- 

 uals which differ in a single trait governed by a single pair of genes. The 



