Spillman's genetics

An early observation of Mendelian ratios

Agronomist WJ Spillman (1863 - 1931) is an unrecognized player in the early history of genetics, as the first American who discovered experimentally what would later be called Mendelian ratios. In a 1901 report "Quantitative Studies on the Transmission of Parental Characters to Hybrid Offspring," Spillman described the outcome of crosses with various strains of wheat that differed phenotypically. Like Mendel, he described his crosses in detail, but provided only the ratios of the various hybrid classes, rather than the actual numbers as Mendel had done. Further, as summarized by LPV Johnson (1948) J Hered, pp. 247-252. "[H]e recognized that the relative proportions of the phenotypic classes tended to be constant. He did not see the significance of these proportions clearly enough to assign fundamental ratios." [emphasis added]. Johnson concludes, "We must not claim for Spillman the independent discovery of the Mendelian laws of heredity; he fell short of that."

Spillman's crosses provide an interesting but seldom-used teaching example. The Mendelian expectation for a dihybrid cross with two gene loci that each show complete dominance is of course 9:3:3:1. To reconstruct Spillman's experiment, let A be dominant to a at the "A" locus, and B be semi-dominant to b at the "B" locus. Let gene A control color (AA & Aa green, aa yellow) and gene B pattern (BB close-set vertical lines, vs Bb spaced lines, vs bb no lines).

HOMEWORK: Calculate the expected genotypic & phenotypic ratios in the cross shown between AaBb parental plants. Show the calculation. Compare & Contrast with the expected results from a Mendelian cross of two dominant traits.