The back of the leopard frog (Rana pipiens) can be either patterned or non-patterned - VCE - SSCE Biology - Question 1 - 2003 - Paper 1

In the offspring produced from the patterned frogs, there are 75 patterned and 25 non-patterned individuals. This indicates that the patterned phenotype appears more frequently than the non-patterned phenotype in the offspring. In genetic terms, the ratio can be analyzed: 75 patterned to 25 non-patterned simplifies to a ratio of 3:1, which is characteristic of a dominant-recessive inheritance pattern. This clearly suggests that the allele for patterned is dominant over the allele for non-patterned.

Let's use 'P' to denote the allele for patterned and 'p' for non-patterned. The probable parental genotypes could be:

• Parents: 1 patterned (Pp or PP), 1 patterned (Pp or PP)
• Offspring:
  • If both parents are Pp: 1/4 PP (patterned), 1/2 Pp (patterned), 1/4 pp (non-patterned).
  • Therefore, the expected phenotypic ratio of offspring would be 75% patterned and 25% non-patterned.

Although the parents in crosses A and B have the same phenotypes (both patterned), the difference in their outcomes can be attributed to the genotypes of the parents. In cross A, the parents may be homozygous (e.g., PP) resulting in all patterned offspring. In contrast, in cross B, if one or both parents are heterozygous (e.g., Pp), then the offspring can exhibit both patterned and non-patterned phenotypes, following the expected Mendelian ratios.