All polymorphisms were detected by 11 clones randomly selected from an adult cod liver cDNA library.Examples of cDNA-based polymorphisms are shown in Figure 2. Common alleles at all loci are restriction enzyme-dependent, suggesting that the genetic basis of the polymorphism is the gain or loss of restric- tion sites. The mean sizes of restriction fragments de- tected by cDNA clones (weighted by their frequencies) ranged from a low of 1.78 kh at locus GM865 to a high of 12.4 kb at locus GMGl5 and exhibited overall mean of 5.48 kb. The Mendelian nature of common alleles at all loci was confirmed by performing pair crosses and resolving restrictionf ragments in DNA samples ex- tracted from 21day-old larvae (G. H. POGSON, unpub- lished data). With only two exceptions (both involving rare alleles) heterozygous individuals possessed two re- striction fragments of similar intensity (the fainter up- per bands seen in Figure 2, A and D, are due to less efficient hybridization to higher molecular weight frag- ments). This banding pattern is expected if the polv- morphic restriction site falls outside the coding region represented by the cDNA clone. In the hvo exceptional
cases, three-banded heterozygotes were observed with the sizes of the two smallest fragments summing to that of the largest fragment. One of these alleles is shown in Figure 2C. This banding pattern is consistent with the polymorphic site occurring within the boundaries of the coding region hybridizing with the cDNA clone. In these instances it is not possible to determine if the variable site occurs within an exon of the structural gene probed, or within an intron, if present.