Final answer:
To find the proportion of offspring with more than 11 spots from a cross between two heterozygous individuals, a Punnett square is used to calculate the possible combinations of the additive traits, and the product rule is applied for independent events.
Step-by-step explanation:
The question involves calculating the proportion of offspring with a certain phenotype after a cross, specifically relating to the number of spots which is governed by an additive trait. Since the cross is between two individuals who are heterozygous for both alleles (S1/S2; P1/P2), we can use the Punnett square to determine the possible combinations of alleles that the offspring might inherit. The S1 allele adds 2 spots to the phenotype, S2 adds 3 spots, P1 adds 5 spots, and P2 adds 1 spot. To find out the proportion of offspring with more than 11 spots, we need to consider all possible genotypic combinations.
After creating the Punnett square, we'll calculate the spot number for each genotypic combination. Any offspring with a total spot count above 11 would meet the criteria. Remember that the S and P traits assort independently, allowing us to use the product rule for independent events to calculate the probability of each phenotype. The proportion of offspring with more than 11 spots can therefore be determined by summing the probabilities of all the genotypic combinations that result in a phenotype with more than 11 spots.