Final answer:
Mendel's Law of Segregation supports a 3:1 testcross ratio based on the segregation of alleles and the resulting genotypic and phenotypic patterns in offspring from monohybrid crosses.
Step-by-step explanation:
Mendel's Law of Segregation is supported by a 3:1 testcross ratio, which is the correct answer to the question posed. This law is based on the premise that each parent has two copies of each gene (alleles), and these alleles segregate (separate) into different gametes during reproduction. When considering monohybrid crosses, the offspring can have one of three genotypes: homozygous dominant, heterozygous, or homozygous recessive. Due to the dominant allele's ability to mask the effect of a recessive allele, both the homozygous dominant and heterozygous offsprings will display the dominant phenotype, leading to a phenotypic ratio of 3 (dominant):1 (recessive).
Understanding the Punnett square further illuminates how we reach the 3:1 ratio. The square allows us to illustrate the possible genetic combinations from parents' gametes. In a cross between two heterozygous individuals (Aa x Aa), the offspring have a 25 percent chance of being homozygous dominant (AA), a 50 percent chance of being heterozygous (Aa), and a 25 percent chance of being homozygous recessive (aa). This ultimately results in a genotypic ratio of 1:2:1. However, the phenotypic ratio considering dominant and recessive traits will be 3:1.