Final answer:
Mendel's fourth postulate is the law of independent assortment, stating that genes segregate independently into gametes during meiosis; a principle demonstrated by a dihybrid cross and foundational to classical genetics.
Step-by-step explanation:
Mendel's fourth postulate is often known as the law of independent assortment, which states that pairs of unit factors (genes) segregate equally into gametes such that offspring have an equal likelihood of inheriting any combination of factors. This principle can be illustrated by a dihybrid cross, where two true-breeding parents that express different traits for two characteristics are crossed. The principle of independent assortment ensures that each characteristic is inherited independently of other traits, allowing for a variety of genetic combinations. Mendel's experiments with pea plants led him to conclude that the law of segregation ensures that gametes carry one allele for each trait. During fertilization, alleles from the parents combine at random, resulting in the progeny receiving various combinations of alleles.
This postulate is based on the meiotic segregation of chromosomes during the formation of sex cells, where each gamete receives a single allele for each gene. Although Mendel himself was not aware of the underlying genetic mechanisms, his observations on phenotypic ratios in offspring led to the foundational principles of classical genetics, including the law of segregation and independent assortment.