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Step-by-Step Explanation
Step 1: Understanding the Context of the Cross
To determine the different types of gametes produced by a pea plant of genotype $AaBb$, geneticists use a test cross. A test cross involves crossing the plant in question with another individual that is completely recessive for the traits of interest. In this case, the recessive genotype is $aabb$.
Step 2: Principle of a Test Cross
The plant with the genotype $AaBb$ can produce four possible gametes: $AB$, $Ab$, $aB$, and $ab$. When these gametes are fertilized by the gametes from the $aabb$ plant (which can only produce the “ab” gamete), the resulting offspring phenotypes will directly reveal the types of gametes produced by the $AaBb$ parent. This is because each type of gamete from $AaBb$ will combine with “ab” to reflect a unique combination in the progeny.
Step 3: Reasoning Behind Choosing $aabb$
The homozygous recessive parent ($aabb$) does not mask any of the alleles contributed by the $AaBb$ parent. Therefore, every offspring’s genotype and phenotype correspond directly to the gamete it received from $AaBb$. This makes $aabb$ the ideal choice for clearly determining the variety of gametes formed by the plant of unknown composition (or in this case, to verify the types of gametes from $AaBb$).
Step 4: Conclusion
Since the purpose is to identify all types of gametes produced by $AaBb$, crossing it with a homozygous recessive $aabb$ plant is the correct strategy. Hence, the answer is aabb.
