588 CONTINUITY OF LIFE 



its genetic constitution. The appearance of likelihood that he will be blue-eyed than 



an individual with respect to a specific trait there was for any of the other children. It 



is known as its phenotype, and its genetic must be remembered that at every union of 



constitution as its genotype. The Fi hybrid the egg and sperm the chances are three to 



in the crosses mentioned above are pheno- one for brown eyes no matter what previous 



typically black but genotypically Wio. Pigs unions have produced. 



that are Wiv or WW are phenotypically With this information, let us study vari- 



alike, that is, they are both black, but geno- ous crosses in order to understand how they 



typically different because their genes are work out in actual practice. 



not the same. Likewise, brown-eyed human ... . . . ■ 



, . 1 i_ • 11 11 u 1.U A simple monohybrid cross 



bemgs are phenotypically ahke because the 



gene for brown eyes ( B ) is dominant over In tracing through the simple Mendelian 

 the gene for blue eyes (b), but they may or ratios we shall use the same organism that 

 may not be genotypically alike. They may Morgan first employed because much more 

 possess both genes for brown eyes (BB) or is known about the genetics of Drosophila 

 they may have one of each (Bb); blue-eyed melanogaster than any other animal. This 

 people, of course, have only the one kind of tiny fly possesses four different chromo- 

 gene (bb). somes, all of which have been partially 

 In observing the ratios in actual practice mapped. By the use of x-rays hundreds of 

 one might think that they are not very gene changes (mutations) have been pro- 

 precise. For example, in one litter of guinea duced, so that there are a great number of 

 pigs as a result of a mating between the Fi alleles, many of which have been investi- 

 hybrids in the above experiment there may gated. Through the study of hundreds of 

 be nearly all combinations of the expected thousands of generations of this tiny animal 

 3:1 ratio. If only four pigs were born they has come most of our knowledge of animal 

 may all be black or they may all be white, genetics. 



altiiough the latter possibility is very re- In Drosophila cultures, occasionally a fly 

 mote. It is more probable that there would will appear with shrunken wings, called 

 be some combination of black and white, vestigial wings (Fig. 24-6). If this indi- 

 two of each color or even the expected 3:1 vidual is crossed with a normal fly what 

 ratio. If, on the other hand, the litters of might we expect in the offspring? First let 

 many such matings were used, say 1000 us examine the nature of the germ cells of 

 offspring, the 3:1 ratio would be approxi- these two flies, assuming that the female is 

 mated very closely. This simply means normal-winged and the male has vestigial 

 that all genetic ratios follow the laws of wings. In this cross it makes no difference 

 probability and in small samples these laws, which sex has vestigial or normal wings, 

 while operative, are not very reliable. In During meiosis in the male each sperm will 

 the above cross there are always three receive a single gene, v, for vestigial wings 

 chances out of every four that the pigs because it is homozygous; likewise, each 

 will be black and only one chance that they egg produced by the female will contain a 

 will be white. Likewise, in the matings of gene, V, for normal wings. The flies result- 

 heterozygous brown-eyed people, each ing from the union of these eggs and 

 chfld has three chances of being brown- sperms will be heterozygous (Vi;) and 

 eyed and only one of being blue-eyed, and normal-winged (Fig. 24-6), because normal 

 this holds true no matter how many chil- wing is dominant. Without making this 

 dren there are in the family. If there are cross we have no way of knowing which of 

 three brown-eyed children in a family and these two traits is dominant. If these Fi 

 a fourth one is expected, there is no more hybrids are crossed with one another (in- 



