Explain why a purine base (Adenine & Guanine) must be paired with a pyrimidine base (Thymine &
Cytosine)

Because hydrogen bonds are not as strong as covalent bonds, base pairings can easily be separated, allowing for replication and transcription. Because purines always bind with pyrimidines – known as complementary pairing – the ratio of the two will always be constant within a DNA molecule.

Option A

Explanation:

The heterozygous allele can be defined as the condition in which the organism has two different alleles of the gene.

It is represented as Rr, means a capital letter and a small letter. This hows that the organism has one dominant gene and one recessive gene in it.

This dominant and recessive condition decides the genotype and phenotype of organism.