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Back of the leopard frog (Rana pipiens) can be either patterned or non-patterned - VCE - SSCE Biology - Question 1 - 2003 - Paper 1
Question 1
Back of the leopard frog (Rana pipiens) can be either patterned or non-patterned. Several patterned frogs were allowed to breed and they produced 75 patterned offspr... show full transcript
Worked Solution & Example Answer:Back of the leopard frog (Rana pipiens) can be either patterned or non-patterned - VCE - SSCE Biology - Question 1 - 2003 - Paper 1
Step 1
Which of the phenotypes, patterned or non-patterned, is dominant?
Answer
The patterned phenotype is dominant.
This conclusion is based on the offspring ratio observed. Out of 100 offspring, 75 were patterned and 25 were non-patterned, indicating a 3:1 ratio which is indicative of a dominant-recessive inheritance pattern.
Step 2
Explain your answer to i.
Answer
The dominance of the patterned phenotype can be inferred from the observed ratio of the offspring. In a typical Mendelian inheritance pattern, if a dominant trait is present, it will manifest in approximately 75% of the offspring when a homozygous dominant or heterozygous parental genotype is crossed with a homozygous recessive one.
In this case, the presence of 75 patterned offspring indicates that patterned is likely the dominant trait, as it overrides the recessive non-patterned phenotype.
Step 3
Using your own allelic notation, show the genotypes with their respective phenotypes for the parents and offspring of the cross between the patterned frogs described above.
Answer
For the patterned frogs, we can denote the alleles as follows:
- Let 'P' represent the dominant allele for patterned phenotype.
- Let 'p' represent the recessive allele for non-patterned phenotype.
Parents:
- Parent 1: PP (homozygous patterned)
- Parent 2: pp (homozygous non-patterned)
Offspring Genotypes:
- All offspring will be Pp (heterozygous patterned).
- Offspring Phenotypes: all patterned (75%).
Step 4
Explain why the outcome of crosses A and B are different.
Answer
Despite both crosses having patterned parents, their differences in offspring results stem from variations in genotypes. In Cross A, one parent was homozygous non-patterned (pp), ensuring all offspring were patterned (Pp). However, in Cross B, if both parents were heterozygous (Pp), a typical Mendelian ratio would yield patterns of 1/4 PP, 1/2 Pp, and 1/4 pp, which would produce 3 patterned to 1 non-patterned offspring. The genetic composition of the parents thus directly influences the phenotype ratios observed in their offspring.