Final answer:
The question involves Mendelian genetics and requires knowing the genotypes of parent fruit flies to perform a cross. This involves using Punnett squares to predict offspring ratios for genotypes and phenotypes. Examples include single trait crosses with 3:1 phenotype ratios and test crosses yielding 1:1 ratios.
Step-by-step explanation:
The question refers to a genetic cross involving fruit flies (Drosophila melanogaster), which is a classic example in genetics used to illustrate Mendelian inheritance and dihybrid crosses. The genotype mentioned, a/a . b/b refers to a fruit fly that is homozygous recessive for two traits. The question is incomplete as it doesn't provide the genotype of the second fruit fly for the cross. However, to answer a similar question, you would consider the genotypes of the parents and perform a Punnett square analysis to predict the genotypic and phenotypic ratios of the offspring.
For example, if you cross an Aa individual with another Aa individual for a single trait, the genotype ratio in the next generation will be 1:2:1 (one AA, two Aa, one aa), and the phenotype ratio, assuming complete dominance, would be 3:1, where 3 represents the dominant phenotype and 1 represents the recessive phenotype.
A test cross, which is a cross of a heterozygote with a recessive homozygote, would yield a 1:1 genotypic and phenotypic ratio if a single trait is being examined. The principles illustrated by these crosses are fundamental in Mendelian genetics and serve as the basis for understanding more complex inheritance patterns.