Chap. 20 



399 



Fig. 20.8 (continued). B, A microphotograph of normal chromosomes from the 

 nucleus of a cell in a salivary gland of a female fruit fly, Drosophila melanogaster. 

 Such chromosomes, among the largest chromosomes that have been discovered in 

 animal cells, have greatly aided the study of the effect of x-rays on the chromo- 

 some. If fruit flies are irradiated, e.g., males, the chromosomes in the nuclei of cells 

 in the salivary glands of the first generation of offspring show various changes. 

 Such changes may be losses of parts, shifts in position of parts, combinations of 

 parts involving two or more chromosomes. Changes in the form or habit of the 

 animal accompany these changes, sometimes its death. (Courtesy, B. P. Kaufmann, 

 Carnegie Institute of Washington.) 



are ww and WW since each one has the diploid number of chromosomes, the 

 product of two sex cells each containing gene w in one case, and gene W in 

 the other. Thus, in each the genes for coat color are similar or homozygous. 

 The sex cells of the black parent (P) each contain a gene for black {W); 

 those of the white parent (P) contain a gene for white (h'). In Fi only black 

 guinea pigs are produced because each receives one gene for black and one 

 for white, with black dominant. Although each animal is black, it is actually 

 a hybrid for color since half of its sex cells contain a gene for black and half 

 of them a gene for white. Because a trait is dominant in one species it does 

 not follow that this will occur in some other species. Black is dominant over 

 white in rabbits and guinea pigs but white is dominant over black in Leghorn 

 poultry. A different type of gene is involved in the two cases. 



Backcross. A backcross (or test cross) of breeding is a method of testing 

 animals that appear alike in one or more characters (phenotypically) but 



