Genes which code for a pair of contrasting traits are

Alleles

The correct answer is Option (4) → Alleles

Alleles are different forms of the same gene that determine variations in a specific trait. They come in pairs—one inherited from each parent.

Alleles can be dominant or recessive, and they code for contrasting traits. For example, the gene responsible for flower color in pea plants might have two alleles: one for purple flowers and one for white flowers.

The combination of alleles determines the physical expression of the trait (the phenotype). For example, if the allele for purple flowers is dominant over white flowers, a plant with a genotype of either "PP" or "Pp" will have purple flowers.