Final answer:
Mendel proposed that discrete entities, now known as genes, are transmitted from parents to offspring in pairs, one from each parent. The different versions of these genes are known as alleles, and Mendel's work demonstrated the patterns of inheritance, including dominant and recessive traits manifesting in a 3:1 ratio in heterozygous crosses.
Step-by-step explanation:
Based on the observation that F1 offspring always resembled one of the true-breeding parents, Mendel rejected the idea of blended inheritance and instead proposed that what passes from parents to offspring are discrete entities. These entities, which account for the different traits in parents and are transmitted to the offspring in pairs, with one coming from each parent, are what Mendel called 'factors'. We now understand these factors as genes, and the different versions of these genes as alleles.
Mendel's groundbreaking work laid the foundation for what we now call Mendelian inheritance. When Mendel crossed peas with contrasting traits and examined their offspring, he observed a consistent pattern of inherited characteristics. This led him to articulate the principles of random segregation and independent assortment, where each parent contributes one allele for every gene, leading to the offspring's genotype. Consequently, Mendel's experiments revealed a predictable 3:1 dominant-recessive ratio for traits when crossing heterozygous individuals, showing that the recessive trait can resurface in a predictable manner.