28 



Chromosomes and Genes 



strains of beans, another gene, Gr, is found at the same locus. 

 This gene produces plants with green p6ds and green foliage. 

 At this locus, therefore, three alleles may be found, but any one 

 bean plant can have only two of the three. More than two 

 alleles at one locus are called multiple alleles, and series of mul- 

 tiple alleles are quite common among plants and animals. In 

 beans, Gr is dominant over both G and g, and G is dominant over 

 g. Any plant may be homozygous for any one of the three or 

 heterozygous for any two. Thus these combinations will result: 



It is customary to differentiate most of the members of a 

 series of multiple alleles by the addition of a superscript to the 

 symbol of the recessive although this rule has not always been 

 adhered to. Thus at the white locus in Drosophila melanogaster, 

 in addition to the genes w (white) and w^ or W (red or wild 

 type), are found w'^'^, wine; w'^^, coral; w^^, blood; w^, cherry; 

 u'°, apricot; w^, eosin; w\ ivory; w^, buff; w\ tinged; and w^^y 

 €cru. 



QUESTIONS AND PROBLEMS 



1. In maize, the sugary gene su is recessive to its allele, the starchy 

 gene Su. Would plants of these genotypes be sugary, starchy, neither, 

 or both: Susw, susu; SuSu? 



2. In four-o 'clocks, gene w produces white flowers when homozygous 

 and gene W, its allele, produces red flowers when homozygous. There is 

 no dominance, and the heterozygote is pink. What would be the color 

 of plants with these genotypes : WW, ww, Ww ? 



3. If you had a starchy corn plant, how could you tell whether it w^as 

 homozygous or heterozygous? If the plant was a sugary plant could 

 you tell? 



4. Can you tell the genotypes of these plants by merely looking at 

 the plant: white four-o'clock; sugary maize; red four-o'clock; starchy 

 maize; pink four-o'clock? 



5. Suggest workable symbols for these characters that Mendel 

 studied: 



