You have four guinea pigs for a genetic study. One male and one female are from a strain that is pure-breeding for short brown fur. A second male and female are from a strain that is pure-breeding for long white fur. You are asked to perform two different experiments to test the proposal that short fur is dominant to long fur and that brown is dominant to white. You may use any of the four original pure-breeding guinea pigs or any of their offspring in experimental matings. Design two different experiments (crossing different animals and using different combinations of phenotypes) to test the dominance relationships of alleles for fur length and color, and make predictions for each cross based on the proposed relationships. Anticipate that the litter size will be 12 for each mating and that female guinea pigs can produce three litters in their lifetime.

Verified step by step guidance

1. Step 1: Identify the genotypes of the pure-breeding guinea pigs based on the given traits. Since short fur (S) is proposed dominant to long fur (s), and brown fur (B) is proposed dominant to white fur (b), assign genotypes: short brown guinea pigs as SS BB (homozygous dominant) and long white guinea pigs as ss bb (homozygous recessive).
2. Step 2: Design Experiment 1 to test fur length dominance. Cross a pure-breeding short fur guinea pig (SS) with a pure-breeding long fur guinea pig (ss), keeping the color constant by using brown furred parents (BB x bb). The F1 offspring should be heterozygous (Ss) for fur length and heterozygous (Bb) for color. Predict the fur length phenotype of the F1 generation based on dominance.
3. Step 3: Design Experiment 2 to test fur color dominance. Cross a pure-breeding brown fur guinea pig (BB) with a pure-breeding white fur guinea pig (bb), keeping fur length constant by using short fur parents (SS x ss). The F1 offspring will be heterozygous (Bb) for color and heterozygous (Ss) for fur length. Predict the fur color phenotype of the F1 generation based on dominance.
4. Step 4: For each experiment, perform test crosses by mating F1 heterozygous offspring (SsBb) with homozygous recessive individuals (ssbb) to observe segregation of traits in the F2 generation. Use Punnett squares to predict the expected phenotypic ratios for fur length and color in litters of size 12.
5. Step 5: Use the predicted phenotypic ratios from the test crosses to confirm or refute the dominance relationships. For example, if short fur is dominant, expect approximately a 1:1 ratio of short to long fur in the test cross offspring. Similarly, for color, expect a 1:1 ratio of brown to white fur if brown is dominant.

Key Concepts

Mendelian Dominance and Allele Relationships

Dominance refers to the relationship between alleles where one allele masks the expression of another in heterozygotes. In this question, short fur is proposed as dominant over long fur, and brown color as dominant over white. Understanding dominance helps predict offspring phenotypes from specific crosses based on parental genotypes.

Pure-Breeding Strains and Genotype Inference

Pure-breeding strains are homozygous for specific traits, meaning they carry two identical alleles. Using pure-breeding parents allows clear inference of offspring genotypes and phenotypes in crosses. This is essential for designing experiments to test dominance by observing segregation patterns in progeny.

Experimental Cross Design and Phenotypic Predictions

Designing genetic crosses involves selecting parents with known genotypes or phenotypes to test hypotheses about inheritance. Predicting offspring phenotypes based on proposed dominance relationships allows validation of genetic models. Considering litter size and multiple matings helps ensure statistically meaningful results.