Final answer:
When mating two double heterozygotes for a gene that has a homozygous lethal condition (S) and suppresses another gene (B), the expected phenotypic ratio in surviving offspring deviates from classic Mendelian ratios due to the lethality of SS and the suppression effect of S on B. The predicted phenotypic distribution is complex and requires specific calculations considering the suppressive interactions.
Step-by-step explanation:
Expected Phenotypic Ratio in Double Heterozygotes
When double heterozygotes are mated, and a dominant suppressor gene (S) is present, which is homozygous lethal, but heterozygous suppresses a dominant gene (B) responsible for a black coat color, we see a modification of classical Mendelian ratios. Due to the lethality of the homozygous condition for the S allele, SS individuals will not survive. Consequently, among the surviving progeny, the SS genotype is absent, which distorts the expected Mendelian ratio.
We are considering a cross between two double heterozygotes of genotype SsBb. The lethal nature of the SS genotype means that 1/4 of the offspring that would have been SS at the S locus will not survive. Therefore, we are left with a genotypic ratio of 2:1 among the survivors. If we account for the suppressive effect of the S allele on the B allele, the actual phenotypic ratio will change depending on whether the S allele is present heterozygously or not at all, and the B or b allele combinations.
Mating two SsBb individuals without considering lethality or suppression would normally result in a 9:3:3:1 ratio for a dihybrid cross, with 9 showing the dominant phenotype for both traits, 3 showing the dominant phenotype for one trait and recessive for the other, 3 showing the recessive phenotype for one trait and dominant for the other, and 1 showing the recessive phenotype for both traits. However, with the suppression effect and lethality, one quarter will die (SS genotype), and among the surviving, the B phenotype will only be expressed in the absence of the S allele. Therefore, the phenotypic ratio on surviving offspring considering all these factors would be more complex than standard Mendelian ratios and needs specific calculation based on these suppressive interactions. It would involve an examination of all the possible genotypes that survive and how the S allele affects the expression of the B allele.