Multiple Alleles and Lethals 57 



SUMMARY AND CONCLUSIONS 



The diflFerent alternative states which a gene may assume in a multiple allelic series may 

 produce different degrees of effect upon a quantitative phenotypic result, or may involve 

 apparently different qualitative effects, or both. Dominance is absent when the alleles in 

 the hybrid produce qualitatively different effects, and may or may not obtain when purely 

 quantitative effects are involved. 



The establishment of isoallelism in any given case is largely a matter of the precision 

 and variety of the testing procedures employed. Different alleles may produce detectable 

 differences upon viability by acting at any stage in the life history of individuals, and may 

 modify the expected phenotypic ratio so that certain classes of offspring are in excess, or 

 in reduced frequency, or are absent. The effect mentioned last is produced by dominant 

 and (homozygous) recessive lethal genes. 



REFERENCES 



Hadorn, E., Developmental Genetics and Lethal Factors, New York, Wiley, 1961. 



Race, R. R., and Sanger, R., Blood Groups in Man, 3rd Ed., Springfield, 111., C. C Thomas 

 1959. 



Wiener, A. S., and Wexler, I. B., Heredity of the Blood Groups, New York, Grune & Stratton, 

 1958. 



QUESTIONS FOR DISCUSSION 



9.1. How would you prove that you were dealing with multiple alleles, rather than multiple 

 pairs of genes? 



9.2. How many different genotypes are possible when there are four different alleles of a 

 single gene? 



9.3. Does the discussion in the text imply that: (a) there is an infinite variety of isoalleles? 

 (b) no two genes are ever identical? Explain. 



9.4. Describe how you would proceed to test whether the genes for white eye in two different 

 populations of Drosophila were alleles, isoalleles, or nonalleles. 



9.5. An agouti rabbit crossed to a chinchilla rabbit produced an agouti offspring. What 

 genotypic and phenotypic results would you expect from crossing the Fi agouti with 

 an albino? 



9.6. For each of the following matings involving Nicotiana give the percentage of aborted 

 pollen tubes and the genotypes of the offspring. 



o^ 9 



a. si s2 X si s3 



b. si s3 X s2 s4 



c. si s4 X si s4 



d. s3 s4 X s2 s3 



9.7. Two curly-winged stubble-bristled Drosophila are mated. Among a large number of 

 adult progeny scored there are 4 curly stubble : 2 curly only : 2 stubble only : 1 neither 

 curly nor stubble (which were therefore normal, wild-type). Explain these results 

 genetically. 



9.8. Could you prove the existence of multiple allelism in an organism that reproduces 

 asexually only? Explain. 



9.9. Discuss the factors which can modify the expected phenotypic ratio. 



