Question at position 8 Which of the following probabilities is correct (according to Mendel's law of independent assortment) regarding a mating of an Ss RR individual to an individual who is Ss Rr? (A – indicates the second allele is either dominant or recessive.)Homozygous recessive: 10%S– RR: 37.5%ss R– : 15.5%Heterozygous both alleles: 50%Clear my selection

Question

Question at position 8 Which of the following probabilities is correct (according to Mendel's law of independent assortment) regarding a mating of an Ss RR individual to an individual who is Ss Rr? (A – indicates the second allele is either dominant or recessive.)Homozygous recessive: 10%S– RR: 37.5%ss R– : 15.5%Heterozygous both alleles: 50%Clear my selection

BlackTom AI · Accepted Answer

Question at position 8 asks: Which of the following probabilities is correct (according to Mendel's law of independent assortment) regarding a mating of an Ss RR individual to an individual who is Ss Rr? (A – indicates the second allele is either dominant or recessive.)

Option 1: 'Homozygous recessive: 10%'
To evaluate homozygous recessive offspring, we would need both genes to contribute recessive alleles (ss and rr). The S gene cross Ss × Ss yields ss with probability 0.25, but the R gene cross RR × Rr cannot produce rr (it only yields RR or Rr with equal probability 0.5 each). Therefore, ss rr offspring are impossible in this cross, making any percentage for 'homozygous recessive' incorrect. 10% is not achievable here.

Option 2: 'S– RR: 37.5%'
'S–' means at least one S allele (SS or Ss), and 'RR' means the R gene is homozygous dominant. The S gene cross Ss × Ss gives P(S–) = 1 − P(ss) = 1 − 0.25 = 0.75. The R gene cross RR × Rr gives P(RR) = 0.5. Since the two loci assort independently, the joint probability is 0.75 × 0.5 = 0.375 or 37.5%. This matches the option, so it is correct.

Option 3: 'ss R– : 15.5%'
'ss' has probability 0.25 from the S gene cross Ss × Ss. 'R–' (RR or Rr) has probability 0.5 from the R gene cross RR × Rr. Multiplying gives 0.25 × 0.5 = 0.125, i.e., 12.5%, not 15.5%. The stated 15.5% is incorrect due to miscalculation.

Option 4: 'Heterozygous both alleles: 50%'
'Heterozygous both alleles' means Ss for the S gene and Rr for the R gene. The probability of Ss from Ss × Ss is 0.5, and the probability of Rr from RR × Rr is 0.5. The combined probability by independence is 0.5 × 0.5 = 0.25 or 25%, not 50%. Therefore this option overstates the likelihood and is incorrect.

Overall, evaluating each option carefully shows that only Option 2 yields the correct joint probability under Mendel's law of independent assortment. The other choices either miscalculate individual locus probabilities or fail to account for independence between the two loci.

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