Phenotype purple dark red red light red white
Notice that the purple and white phenotypes are each encoded by a single genotype, but other phenotypes may result from several different genotypes.
From these results, we see that five phenotypes are possible when alleles at two loci influence the phenotype and the effects of the genes are additive. When alleles at more than two loci influence the phenotype, more pheno-types are possible, and this would make the color appear to vary continuously between white and purple. If environmental factors had influenced the characteristic, individuals of the same genotype would vary somewhat in color, making it even more difficult to distinguish between discrete phenotypic classes. Luckily, environment played little role in determining kernel color in Nilsson-Ehle's crosses, and only a few loci encoded color; so Nilsson-Ehle was able to distinguish among the different phenotypic classes. This ability allowed him to see the Mendelian nature of the characteristic.
Let's now see how Mendel's principles explain the ratio obtained by Nilsson-Ehle in his F2 progeny. Remember that Nilsson-Ehle crossed a homozygous purple variety (A+A+B+B+) with the homozygous white variety (A-A-B-B-), producing F1 progeny that were heterozygous at both loci (A+A-B+B-). All the F1 plants possessed two pigment-producing alleles that allowed two doses of color to make red kernels. The types and proportions of progeny expected in the F2 can be found by applying Mendel's principles of segregation and independent assortment.
Let's first examine the effects of each locus separately. At the first locus, two heterozygous F1s are crossed (A+A- X A+A-). As we learned in Chapter 3, when two heterozygotes are crossed, we expect progeny in the proportions
1/4 A+A+, 1/2 A+A-, and 1/4 A-A-. At the second locus, two heterozygotes also are crossed, and again we expect progeny in the proportions 1/4 B+B+, 1/2 B+B-, and 1/4 B-B-.
Break into simple crosses
Number of pigment genes Phenotype
►1/4 b+ b+-► 1/2 X 1/4 =2/16 3 /4 b b a+ a- b+ b+ 3
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