Independent Recombination by Nonalleles 



51 



types 1, 2, 4, 5, another by genotypes 3 and 

 6, another by 7 and 8. and another by geno- 

 type 9, producing the 9:3:3:1 ratio already 

 discussed. Dominance causes a reduction 

 in the number of phenotypic classes. 



What phenotypic ratios are expected when 

 two different, independently active, pairs of 

 genes affect the same trait in the same man- 

 ner or direction? If one or more allelic 

 combinations for one gene pair produce the 

 same phenotype as one or more allelic com- 

 binations for the other gene pair, the num- 

 ber of phenotypes will also be reduced from 

 the maximum (9 when identical dihybrids 

 are crossed). Of course, the number of 

 different phenotypes detected will be further 

 reduced if the absence of dominance in both 

 gene pairs is changed to dominance in one 

 gene pair, and still further reduced if both 

 gene pairs show dominance. Thus, if A 

 and B produce equal amounts of melanin 

 pigment in human skin, the amount of pig- 

 ment being cumulative, and A' and B' pro- 

 duce none, dominance being absent, a cross 

 between identical dihybrids yields the ratio 

 1 "black" (type 1 ) :4 "dark" (types 2, 4) : 

 6 "mulatto" (types 3, 5, 7) :4 "light" (types 

 6, 8):1 "white" (type 9), instead of 9 dif- 

 ferent phenotypes. Moreover, if both A 

 and B show complete dominance, for exam- 

 ple either gene producing full flower color, 

 the phenotypic ratio becomes 15 colored 

 (types 1-8) :1 colorless (type 9). Note 

 that when different pairs of genes act on 

 the same trait in the same direction or way, 

 they have a common phenotypic background 

 on which their effects superpose, and the 

 effect of one gene pair interferes with the 

 detection of the effect of the other pair. 



Sometimes different gene pairs act inde- 

 pendently on the same trait in different — 

 antagonistic or cooperative — ways. For ex- 

 ample, in Drosophila (Figure 4-10), A' is 

 a recessive allele which reduces the wing to 

 a stump, whereas B' is a recessive allele 



which causes the wing to be curled, the 

 dominant allele A making for normal sized 

 wings and the dominant allele B straight 

 wings. A cross between two identical dihy- 

 brids does not produce the customary 9:3: 

 3:1 ratio. In the present case, the ratio 

 becomes 9 flies with long, straight wings: 3 

 with long, curled wings: 4 whose wings are 

 mere stumps (of which one quarter would 

 have had curled wings if the full wing had 

 formed). Here, then, the phenotypic ex- 

 pression of one gene pair can prevent de- 

 tection of the phenotypic expression of an- 

 other gene pair. 



In another case, either of two pairs of 

 genes may prevent a given phenotype from 

 occurring. Suppose the dominant alleles A 

 and B each independently contribute some- 

 thing different but essential for the produc- 

 tion of red pigment, whereas their corre- 

 sponding recessive alleles A' and B' fail to 

 make the respective independent contribu- 

 tions to red pigment production. Then 

 crosses between two identical dihybrids will 

 produce 9 red: 7 nonred (composed of 3 

 homozygotes for A' only, 3 homozygotes for 

 B' only, and 1 homozygote for both A' and 

 B ') . Notice that if the recessive alleles are 

 considered, we have just dealt with examples 

 of unilateral and mutual opposition to pheno- 

 typic expression, respectively, but if the dom- 

 inant alleles are considered these become 

 cases of unilateral and mutual cooperation 

 in phenotypic expression. 



In all cases where two pairs of genes af- 

 fecting the same trait interact phenotypically 

 by superposition, antagonism or cooperation, 

 one pair of genes has had an influence upon 

 distinguishing the effects of the other. The 

 general term epistasis may be used in these 

 cases to describe the interference with — 

 suppression or masking of — the phenotypic 

 expression of one pair of genes by the mem- 

 bers of a different pair. Genes whose de- 

 tection is hampered by nonallelic genes are 



