Massachusetts Comprehensive Assessment System (MCAS) Biology Practice Test 2025 – All-in-One Study Guide for Exam Success!

The best evidence indicating that the resistant trait in soybean plants has a dominant inheritance pattern is when a resistant plant is crossed with a plant that is not resistant, and all of the offspring are resistant. This outcome illustrates that the resistant trait is dominant because when a dominant allele is present in a genotype, it masks the effect of the recessive allele.

In this scenario, the offspring display the dominant trait (resistance to aphids) even though one parent does not possess that trait. This further supports the idea that the resistant trait is controlled by a dominant allele, as the presence of just one resistant allele in the offspring genotype is sufficient to produce the resistant phenotype.

Other situations, such as a mixture of resistance and non-resistance traits, only producing some resistant offspring, or having no offspring exhibit resistance, suggest different inheritance patterns. Specifically, a mixture of traits would indicate that both dominant and recessive alleles are expressed, which does not point towards a clear dominant inheritance. The absence of any resistant traits in the offspring further rules out the possibility of the trait being dominant, as it would imply that the conditions necessary for resistance are not met in the genotype of the offspring.