Final answer:
The correct answer to the question of which option is an example of a test cross is D. A-B- X Aabb. This is because it involves crossing an organism with a dominant phenotype and unknown genotype with an organism that is heterozygous for the traits being examined.
Step-by-step explanation:
Understanding a Test Cross
A test cross is a genetic cross between a homozygous recessive individual and an organism with a dominant phenotype but unknown genotype. This cross is designed to reveal the genotype of the unknown parent. The essence of a test cross is to determine if the organism expressing the dominant trait carries the recessive allele (is heterozygous) or not (is homozygous).
Looking at the options provided:
A. A-B- X AaBb is not a test cross because both organisms have dominant traits, and the genotype of the first organism expressed with '-' indicates unknown alleles which are not necessarily homozygous recessive.
B. A-bb X aabb is also not a test cross since it does not include an organism with unknown genotype for the dominant trait.
C. A-B- X A-B- is not a test cross, as neither parent is homozygous recessive.
The correct answer is D. A-B- X Aabb, where the presumed homozygous recessive individual is crossed with an organism with dominant phenotype and unknown genotype.
If we delve into an example, using a Punnett square, you can predict the offspring's possible genotypes and phenotypes. For instance, in a Punnett square crossing Bb (heterozygous dominant) with bb (homozygous recessive), you would expect a 50% chance for offspring to be Bb and a 50% chance to be bb, resulting in a 3:1 phenotypic ratio if B is dominant over b.
Therefore, out of the given options, option D represents an example of a test cross: A-B- X Aabb.