One possible form of a gene that codes for a particular trait is known as

Allele

Explanation:

A gene is a stretch of DNA that code for a specific protein. A gene can exist as two or more alternative forms. These alternative forms of a gene are called as alleles. The alleles are responsible for presence of variants of a particular genetic trait.

For example: The gene for plant height has two possible forms, i.e. two alleles. These are symbolized as "T" and "t". The allele "T" impart tallness while allele "t" imparts dwarfism to the plants.

layer 3 is younger than layer 6.