Expected inheritance ratios and limited evidence
A hypothetical plant has flower color determined by a single gene with allelesR andr. The exercise specifies complete dominance ofR, equal viability of all genotypes, independent offspring and no environmental change in flower color. An Rr plant is crossed with another Rr plant. In a second experiment a red-flowered plant of unknown genotype is crossed with rr and ten offspring are red. A separate class compares genetically matched seedlings under two light conditions while controlling water and temperature. All inheritance conditions belong to this invented model and should not be generalized to every flower trait. A student treats an expected ratio as a guarantee for four offspring. Another treats ten red offspring as logical proof that their red parent is RR. Distinguish probability, genotype and the strength of the evidence.
1. State the expected genotype and phenotype proportions for Rr×Rr.
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The expected genotype proportions are1/4RR,1/2Rr and1/4rr. Under complete dominance the expected phenotype proportions are3/4red and1/4recessive. A finite family need not match these proportions exactly.
2. Does the second experiment prove the unknown parent is RR?
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No. An Rr parent crossed with rr has red-offspring probability1/2 for each independent offspring. Ten red offspring still have probability(1/2)^10=1/1024 under that genotype. The result favors RR over Rr under the model but does not logically eliminate Rr.
3. What does a difference in matched seedlings under controlled light support?
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With appropriate replication and other conditions controlled, it supports a light-related effect on the measured phenotype. It does not by itself establish that a new inherited allele arose.