20 Chromosomes and Genes 



Heterozygous plants and animals show that all individuals 

 that look alike are not necessarily genetically alike. Both the 

 homozygous wild-type fly and the heterozygote have perfectly 

 normal wings and are absolutely indistinguishable in appear- 

 ance. It can be shown that they are different genetically, how- 

 ever, when they are used to produce subsequent generations. The 

 vestigial-winged fly, on the other hand, must have two genes 

 for vestigial wings because if it had even one of the dominant 

 genes for normal wings it would have normal wings. It is pos- 

 sible, therefore, to identify flies that are homozygous recessives 

 by examining their external appearance, or phenotype, and thus 

 to know their genetic constitution, or genotype. All individuals 

 that are homozygous for a recessive gene are alike both pheno- 

 typically and genotypically (with respect to that locus). All 

 individuals that have a dominant gene are alike phenotypically 

 but may be different genotypically for they may be either homo- 

 zygous for the dominant allele or heterozygous. 



Gene Symbols 



It is rather cumbersome to write "the gene for vestigial wings" 

 and ''the gene for normal wings." The geneticist, like the mathe- 

 matician and the chemist, substitutes symbols for such expres- 

 sions, and with a little practice such symbols are readily grasped. 

 The gene for vestigial wings has the symbol vg, and its allele 

 for normal wings has the symbol Vg. The small v indicates that 

 the gene is recessive, the large V indicates a dominant gene. It 

 happens that the allele for the vestigial gene is the one present 

 in flies gathered in from nature; and that this gene is the one 

 found in such wild-type flies is indicated by research workers 

 on Drosophila by modifying the symbol to read vg+ or +^^ or 

 frequently by using merely a plus sign. In this book, however, 

 the symbol Vg is generally used as it is less confusing to begin- 

 ning students. 



In choosing symbols to represent genes, it is helpful although 

 not essential for the symbol to indicate the name and chief effect 

 of the gene. The symbol vg cannot be mistaken for any gene 

 other than vestigial, and the symbol Vg indicates clearly that 

 this gene is the dominant allele of vestigial. However, when 

 plants and animals were first studied genetically, symbols were 

 assigned to genes arbitrarily. In Mendel's original paper, for 



