Final answer:
The variability in nucleic acids is provided by alternate forms of genes known as alleles, which are responsible for genetic diversity and hereditary traits. The sequence of nucleotides in DNA dictates protein synthesis, affecting the organism's phenotype.
Step-by-step explanation:
Nucleic Acids and Genetic Variability
Nucleic acids offer variability because they contain alternate forms of genes called alleles. Alleles are different versions of a gene that arise through mutations and are found at the same place on a chromosome. Genetic variability among individuals is largely due to the differences in these alleles. The sequence of nucleotides in DNA, which comprises adenine (A), thymine (T), guanine (G), and cytosine (C), forms the genetic code that dictates the synthesis of proteins. The proteins created according to this genetic code determine an organism's phenotype, which includes physical traits and predispositions to certain diseases.
During reproduction, alleles are passed from parents to their offspring, and this transmission is the basis for heredity. The DNA housed in the cell's nucleus becomes the blueprint for the next generation. Despite having only four nucleotide bases, the potential combinations are virtually boundless, which explains the remarkable diversity found in the biological world. Thus, the correct answer to the Introduction to Genetics Pre-Test question is 'A. Alleles'.