Final answer:
The genotypes of the crossed individuals producing offspring with a 1:1 phenotypic ratio of phenotype "A" to "a" are Aa and aa. This follows Mendelian inheritance principles, where the Aa individual produces gametes with A and a alleles, which when combined with aa gametes from the other individual, results in a 1:1 ratio of Aa to aa offspring. The correct answer is option 1) AA and aa
Step-by-step explanation:
The student's question concerning a test-cross where half of the offspring have phenotype "A" and half have phenotype "a" relates to genetics and Mendelian inheritance. Based on the information provided, when Individual #1 with phenotype "A" is test-crossed, and half of the offspring display phenotype "A" while the other half display phenotype "a", the genotypes of the crossed individuals are Aa and aa. This is because an individual with phenotype "A" and genotype "Aa" would produce gametes with both "A" and "a" alleles. When crossed with an individual with genotype "aa", the possible combinations for the offspring are equally likely to be "Aa" (phenotype A) or "aa" (phenotype a), resulting in a 1:1 phenotypic ratio.
When considering a cross of two Aa individuals, according to Mendelian genetics, the next generation's genotype ratio would be expected to be 1:2:1, which represents AA, Aa, and aa, respectively. However, since AA and Aa both exhibit the dominant phenotype, they can be combined to reflect Mendel's third-generation phenotype ratio of 3:1 for the dominant to recessive trait expression.
Understanding these patterns of inheritance is critical for predicting the probability of genotypes and phenotypes in offspring. A cross between AA and aa individuals would result in all offspring being Aa, with a probability of 100% expressing the dominant phenotype. This demonstrates the fundamental principles of Mendelian inheritance and showcases the importance of the genotype of the parents in determining the potential genotypes and phenotypes of their offspring.