Final answer:
Genes are the entities that pass on traits from parents to offspring. These genes influence the observable traits or phenotype of an organism while remaining contained within the genetic makeup or genotype. Mendel's experiments clarified the inheritance of genes and demonstrated that DNA, not proteins, is the primary source of heritable information.
Step-by-step explanation:
Today we know that genes pass on traits. 'Genes' is the correct answer to fill in the blank in the given statement. The observable traits expressed by an organism, known as its phenotype, are the result of the interaction of these genes. The underlying genetic makeup, which includes both physically visible and non-expressed alleles, is referred to as the organism's genotype. Alleles are the different versions of a gene that may be passed on from parents to offspring, and each parent donates one allele for each gene to their offspring.
Mendel's hybridization experiments showed how phenotypes and genotypes can vary even when the observable traits remain the same, demonstrating the inheritance of traits through genes. For instance, when Mendel cross-fertilized true-breeding plants with different pod colors, the F1 hybrid offspring phenotype matched the dominant parent; however, the alleles carrying the recessive trait were still present in their genotype and reappeared in the F2 generation, illustrating the segregation and independent assortment of genes.