Alleles 19 



"vestigial gene" can be considered just a different form of the 

 "normal-wing gene" which is present at that locus and vice 

 versa. Two genes at the same locus but producing somewhat 

 different effects on the developing individual are called allelo- 

 morphs or, more usually, alleles. Therefore, the vestigial gene 

 is an allele of the gene for normal wings which is at the same 

 locus. 



Flies with two normal-wing genes have normal wings and 

 those with two genes for vestigial wings will have vestigial wings. 

 Every normal fly must have two members of chromosome II, but 

 some may have one chromosome with a gene for normal wings 

 and the other with a gene for vestigial wings. The question may 

 well be asked whether the wings of such flies will be normal, 

 vestigial, intermediate, or something else. In this particular 

 case, such an adult fly would have normal wings because it hap- 

 pens that the effect produced by the wild-type gene during the 

 development of the fly completely overcomes the effect produced 

 by the vestigial gene. Whenever one allele alone is expressed 

 to the exclusion of the other, the allele whose effect is expressed 

 is said to be dominant over the one whose effect is not expressed, 

 known as a recessive gene. 



An organism in which the two genes at one locus are identical 

 is homozygous for that gene. Thus the vestigial-winged fly and 

 the fly with the two genes for normal wing are homozygous. The 

 normal-winged fly that has one dominant gene and also the 

 recessive allele for vestigial wings is heterozygous. An indi- 

 vidual in which the two genes at any one locus are different is 

 heterozygous for that pair of alleles. 



The two members of all pairs of alleles do not show this 

 dominant-recessive relationship for some heterozygous indi- 

 viduals are intermediate in nature. Although it is usually true 

 that the wild-type or "'normal" gene is dominant over its allele, 

 sometimes the "normal" gene is recessive to the "abnormal." 

 Thus in Drosophila the gene for bar eyes that produces an eye 

 in which most of the facets are undeveloped is partially domi- 

 nant over the normal, and the gene for hairless body is dominant 

 over its allele that produces the wild-type or normal hairy condi- 

 tion. Such traits as normal wings, vestigial wings, bar eyes, and 

 hairless bodies are frequently referred to as characters. 



