Final answer:
In studying Biology, the phenotypic frequencies for sex-linked loci A and B in Drosophila that are 20 map units apart would include around 20% recombination in offspring. Male offspring phenotypes mirror their alleles due to only one X chromosome, while females can have typical Mendelian ratios. The proportion of offspring exhibiting dominant phenotypes at all four loci (A, B, C, and D) is 81/256.
Step-by-step explanation:
The subject of the question is Biology, specifically genetics and the study of genetic linkage and recombination in Drosophila, a common fruit fly used as a model organism in genetic research. When loci A and B in Drosophila are sex-linked and 20 map units apart, we can predict phenotypic frequencies using recombination frequencies. A recombinant frequency of 20% suggests that approximately 20% of the offspring will be recombinant types, while 80% will exhibit parental phenotypes.
For male Drosophila, which have only one X chromosome, the phenotype will directly reflect the allele present at each locus, as there is no homologous allele to mask the effect of a recessive one. For females, which have two X chromosomes, typical Mendelian genetic principles apply. If we assume that the distance between these loci result in approximately 20% recombinant phenotypes, then in a large enough population of offspring, we would expect a 1:1:1:1 ratio of the following phenotypes: AB, Ab, aB, ab (where AB represents the parental dominant phenotype).
When considering the question's reference to phenotypic proportions and genotypic proportions, the expected proportion of offspring with the dominant phenotype at all four loci (A, B, C, and D) is calculated by combining the probabilities of obtaining a dominant phenotype at each locus, which results in 81/256 when using the product rule.