Dappled-1 expresses itself only when transmitted through the female of heterozygous plants (homozygous Dappled lines have not yet been isolated). However, the frequency of phenotypically Dappled kernels observed on segregating ears is somewhat less than the expected 50% (average percent Dappled observed = 31.25%, see MNL 61:7). This frequency varies from the base to the tip of the ear, with a higher frequency of Dappled kernels appearing at the base.
To determine whether incomplete penetrance of Dappled accounts for the deficiency of phenotypically Dappled kernels, we selected 25 fully colored purple kernels from the base, and 25 from the tip of 6 different segregating ears of Dappled heterozygotes that had been pollinated by purple aleurone lines (these ears were included in our 1986 report), and grew them in our 1987 selfing block. The resulting ears were scored for the presence of Dappled.
We found that a small percentage of purple kernels did indeed give rise to heterozygous Dappled plants, and that this percentage was lower for kernels from the base (4.20% Dappled) than for kernels from the tip (15.75%, Table 1).
However, these values were not great enough to account for the deficiency of Dappled kernels observed on our 1985 ears (Table 2). When adjustments are made for the full colored purple kernels that are really heterozygous Dappled, the average proportion of Dappled kernels on these ears is 37.82%, still significantly less than 50%. Thus, even accounting for reduced penetrance, we can still conclude that Dappled is transmitted at a reduced frequency through the female. We estimate that the average penetrance of Dappled is 75.7%, although this seems to vary from ear to ear, and may vary in different genetic backgrounds.
When conducting detailed linkage studies, the incomplete penetrance of Dappled necessitates growing the phenotypically wildtype kernels of backcross ears in an open pollination block, and scoring the ears of these plants for Dappled. However, even if this is not done, approximate linkage data can still be obtained by scoring the kernels at the base of the ear, where the penetrance of Dappled is highest.
Table 1. Phenotypes of plants grown from purple nonsectored kernels taken from the base and tip of heterozygous Dappled ears.
Table 2. Observed, and adjusted* percent Dappled kernels on heterozygous Dappled plants crossed as females by purple aleurone stocks.
Philip S. Stinard and Donald S. Robertson
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