Final answer:
Mendel's single-factor crosses led to the concept of dominance and the law of segregation, with the former outlining how one allele can mask another's expression and the latter stating that alleles segregate into gametes independently. The correct option is A.
Step-by-step explanation:
Mendel's single-factor crosses chiefly led to two fundamental rules of inheritance: the concept of dominance and the law of segregation. The concept of dominance postulates that when two opposing alleles are present, one may mask the expression of the other, being the dominant allele, while the other is regarded as recessive.
The law of segregation describes how pairs of gene variants (alleles) are separated into different gametes during meiosis, ensuring that offspring have an equal chance of receiving either allele from each parent.
Through his pea plant experiments, Mendel observed that the F2 generation exhibited a 3:1 ratio of dominant to recessive traits, a cornerstone of the law of segregation.
The law of independent assortment was derived from Mendel's dihybrid crosses, demonstrating that different traits are inherited independently when the genes reside on different chromosomes.
This was evident in the phenotypic 9:3:3:1 ratio witnessed in the offspring of a dihybrid cross. However, the correct answer to the provided question regarding single-factor crosses is the concept of dominance and the law of segregation. Option A. is the correct one.