GENETICS 



167 



dominant when a white and black animal mate and produce half-breeds 

 which are all black in color. However, as has already been stated, the 

 color is only in the somatoplasm. It is important to remember that the 

 gerrmlasm remains pure; that is, some of the eggs in the mother and 



some of the sperm in the father will 

 be of the black variety and others 

 will be of the white variety. To put 

 this in other words, there will be no 

 half-breed eggs or sperm. 



Whatever unit character shows 

 up in the somatoplasm in animals 

 of different breeds is said to be 

 dominant, while that unit character 

 which is present in the germplasm 

 but is not seen in the somatoplasm 

 is said to be recessive. 



In our example of the mating 

 of a white and black guinea pig the 

 offspring, though black, have white- 

 ness in their germplasm even if it 

 does not show externally in the 

 somatoplasm. Blackness in this 

 case is therefore dominant; white- 

 ness, recessive. 



In their accounts of breeding ex- 

 periments, geneticists use a formula 

 to represent dominant and recessive 

 genes. The capital letter represents the dominant gene and the small 

 letter the recessive. In the example we have been discussing, the capital 

 letter B would represent the gene which carries blackness, the domi- 

 nant color ; and the small letter w will represent the gene carrying 

 the recessive white. The formula in our example of a halfbreed black 

 and white, therefore, is Bw . 



In those cases where pure blooded blacks would meet with pure 

 blooded blacks the formula would be BB, while in the breeding of two 

 pure whites the formula would be WW. It will therefore be noted that 

 we may have the various formulas BB, Bw, WW, Wb, provided, of 

 course, that a recessive black could be found. 



Wherever two genes are alike, so that either has BB, or WW, the 

 resultant zygote is called a homozygote, while the organism resulting 

 from a homozygote is said to be homozygous. If the two genes of the 

 mating pair are different, such as Bw or Wb, the zygote is called hetero- 

 zygote, the resultant animal being called heterozygous. It is, of course, 

 quite common for the same animal to be homozygous for some charac- 

 ters and heterozygous for others. 



Fig. 82. 



Diagram of two chromosomes, each square 



representing a gene. 



An insect, for example, with these two 

 chromosomes would possess a 'normal 

 wing, a minature wing, a rudimentary 

 wing, and forked bristles. All these char- 

 acters could be transmitted to the offspring. 

 The insects' body and eyes, however, 

 would be heterozygous. 



