Inheritance and probability
🎯 Try it
In Pp × Pp, probability of Pp?
✏️ Start with a simplified model
Use an imaginary diploid plant with one gene: allele P produces purple flowers when at least one P is present; pp produces white. This single-gene complete-dominance model does not describe every real trait.
Cross Pp × Pp. Each parent contributes P or p with probability 1/2.
Genotypes: 1/4 PP, 1/2 Pp, 1/4 pp. Phenotypes: 3/4 purple, 1/4 white. Dominant does not mean more common, stronger, or better.
📝 Predict another cross
For Pp × pp, draw the same grid. The pp parent can contribute only p. Half the possible offspring are Pp and half pp: 1/2 purple, 1/2 white.
📝 Probability is not a quota
Four offspring from Pp × Pp do not have to include exactly one white flower. Each independent offspring has probability 1/4 of pp. The chance that two independent offspring are both pp is 1/4 × 1/4 = 1/16.
✅ Check your understanding
In PP × pp, what genotype results?
Does one purple offspring change the next independent probability?
✏️ Test the model without a grid to copy
Predict genotype and flower-colour probabilities for PP × Pp. Then find the probability of two independent white offspring from Pp × pp. State the model assumptions.
🔑 Worked answer
PP × Pp gives half PP and half Pp, so all purple in the stated complete-dominance model. Pp × pp gives white probability 1/2; two independent white offspring have probability 1/4. This assumes the stated single-gene model and independent allele transmission.