Final answer:
Genetic characters are controlled by alleles existing in pairs in individual organisms, with one allele inherited from each parent. These pairs determine traits and segregate during meiosis, combining again upon fertilization.
Step-by-step explanation:
Mendelian genetics posits that genetic characters are controlled by alleles existing in pairs in individual organisms. These alleles are variations of a gene and are found at the same locus on a pair of homologous chromosomes. One allele is inherited from each parent, and this determines the traits of the offspring, a fundamental concept known as the law of segregation.
For example, in the case of pea plants' flower color, there may be a purple-flower allele and a white-flower allele. The combination of these alleles determines the color of the flowers produced by the plant. It's also important to note that while Mendel's experiments suggested that a single gene could control a single trait, we now understand that more complex traits in humans can be influenced by multiple genes with multiple alleles, a phenomenon known as epistasis.
During the process of meiosis, gametes (sex cells) are produced, each carrying only one allele for each trait. When fertilization occurs, the alleles from each parent combine, restoring the two-allele state for the traits in the offspring.