A geneticist from an alien planet that prohibits genetic research brought with him to Earth two pure-breeding lines of frogs. One line croaks by uttering 'rib-it rib-it' and has purple eyes. The other line croaks more softly by muttering 'knee-deep knee-deep' and has green eyes. With a newfound freedom of inquiry, the geneticist mated the two types of frogs, producing F₁ frogs that were all utterers and had blue eyes. A large F₂ generation then yielded the following ratios:
27/64 blue-eyed, 'rib-it' utterer
12/64 green-eyed, 'rib-it' utterer
9/64 blue-eyed, 'knee-deep' mutterer
9/64 purple-eyed, 'rib-it' utterer
4/64 green-eyed, 'knee-deep' mutterer
3/64 purple-eyed, 'knee-deep' mutterer
One set of crosses with his true-breeding lines initially caused the geneticist some confusion. When he crossed true-breeding purple-eyed, 'knee-deep' mutterers with true-breeding green-eyed, 'knee-deep' mutterers, he often got different results. In some matings, all offspring were blue-eyed, 'knee-deep' mutterers, but in other matings all offspring were purple-eyed, 'knee-deep' mutterers. In still a third mating, 1/2 blue-eyed, 'knee-deep' mutterers and 1/2 purple-eyed, 'knee-deep' mutterers were observed. Explain why the results differed.

Verified step by step guidance

1. Step 1: Begin by analyzing the inheritance patterns of the traits involved. The traits include eye color (purple, green, blue) and vocalization type ('rib-it' utterer vs. 'knee-deep' mutterer). The F₁ generation suggests that both traits are controlled by separate genes, and the F₁ phenotype indicates dominance relationships for each gene.
2. Step 2: Examine the F₂ generation ratios. The ratios (27/64, 12/64, etc.) suggest a dihybrid cross involving two genes, each with two alleles. Use the phenotypic ratios to infer the genotypes and dominance relationships. For example, blue eyes may result from a combination of alleles from both genes, while purple and green eyes may be determined by specific alleles of one gene.
3. Step 3: Investigate the results of the crosses between true-breeding purple-eyed, 'knee-deep' mutterers and true-breeding green-eyed, 'knee-deep' mutterers. The differing outcomes suggest that one of the genes involved in eye color is subject to epistasis, where one gene's expression masks or modifies the effect of another gene.
4. Step 4: Consider the genetic mechanisms behind the differing results. In some matings, all offspring are blue-eyed, 'knee-deep' mutterers, indicating that the blue-eye phenotype is dominant and results from a specific combination of alleles. In other matings, all offspring are purple-eyed, 'knee-deep' mutterers, suggesting that the purple-eye phenotype is dominant under certain genetic conditions. The third mating, with a 1:1 ratio, indicates heterozygosity for one of the genes controlling eye color.
5. Step 5: Conclude that the differing results are due to variations in the parental genotypes for the eye color gene(s). The geneticist's true-breeding lines may carry different combinations of alleles that interact in unique ways, leading to epistasis or dominance effects that produce the observed phenotypic variations in the offspring.

Key Concepts

Mendelian Genetics

Mendelian genetics is the study of how traits are inherited through generations based on the principles established by Gregor Mendel. It involves understanding dominant and recessive alleles, where dominant traits mask the expression of recessive ones. In this scenario, the traits of eye color and croaking sound are likely governed by Mendelian inheritance, leading to the observed phenotypic ratios in the F₂ generation.

Genotype and Phenotype

The genotype refers to the genetic makeup of an organism, while the phenotype is the observable expression of that genotype, influenced by environmental factors. In the case of the frogs, the different combinations of alleles (genotypes) result in various phenotypes, such as eye color and croaking sounds. Understanding the relationship between genotype and phenotype is crucial for interpreting the results of the geneticist's crosses.

Incomplete Dominance and Codominance

Incomplete dominance and codominance are genetic phenomena where neither allele is completely dominant over the other. In incomplete dominance, the heterozygous phenotype is a blend of the two homozygous phenotypes, while in codominance, both alleles are fully expressed. The varying results from the geneticist's crosses suggest that the traits for eye color and croaking may not follow simple dominance patterns, leading to different offspring phenotypes in different matings.