Final answer:
The expected phenotypic ratio for the offspring of a CcDD x Ccdd cross would be a 3:1 ratio of offspring showing both dominant phenotypes to those showing the dominant phenotype for the C gene but the recessive phenotype for the D gene.
Step-by-step explanation:
The expected phenotypic ratio of the offspring resulting from a CcDD xCcdd cross can be determined through the understanding of Mendelian genetics. For the genes in question, 'C' represents a dominant allele and 'c' represents its recessive counterpart, while 'D' is one dominant allele with 'd' being its recessive counterpart.
When we consider each gene independently, we look at the Cc x Cc cross for the C gene, which produces a phenotypic ratio of 3:1 (where 3 represents the dominant phenotype and 1 the recessive phenotype). For the Dd x dd cross, all offspring will have at least one D allele and thus show the dominant phenotype. Therefore, the phenotypic ratio for this gene will be 4:0 since 'D' is dominant and even a single copy is sufficient for the dominant phenotype to be expressed, irrespective of the 'd' allele.
Since these two genes are independently assorted, we can multiply the ratios to obtain the combined phenotypic ratio. The ratio for the C gene (3:1) multiplied by the ratio for the D gene (4:0) gives us a combined phenotypic ratio of 12:4 or simplifying to 3:1, indicating a 3:1 ratio of offspring showing both dominant phenotypes to those showing the dominant phenotype for C but recessive for D.