Practice question

Standard dihybrid 9:3:3:1 assumes each locus acts independently with complete dominance. Chicken comb cross theoretically yields 9 walnut R-P-, 3 rose R-pp, 3 pea rrP-, 1 single rrpp reflecting interaction producing novel walnut phenotype. Observed ratio 8:3:4:1 deviates slightly due to viability differences, incomplete penetrance, sampling variance, or modifiers, yet still retains four phenotypic classes summing to sixteen, indicating two genes controlling one morphological trait with interdependent expression. Recognizing modified ratios prevents misclassification as linkage and emphasizes need for large sample and chi-square evaluation.