In Binomial Nomenclature

Binomial Nomenclature is a formal system of naming species of living things by giving each a name composed of two parts, both of which use Latin grammatical forms, although they can be based on words from other languages.

The system has several critical rules:
1. The first part of the name identifies the genus to which the species belongs. The genus name is always capitalized.
2. The second part identifies the species within the genus. The species epithet is always written in lowercase.
3. Scientific names are traditionally printed in italics (for example, Homo sapiens) to distinguish them from the surrounding text. When handwritten, they should be underlined.

Let us evaluate the options:
- "genus is written after the species": Incorrect. The genus is written first.
- "genus and species both are printed in italics": Correct. Both parts of the scientific name must be italicized when printed.
- "both the initial letters in genus and species is capitalised": Incorrect. Only the genus starts with a capital letter; the species starts with a lowercase letter.
- "genus and species may be the same": Incorrect. While tautonyms (where the genus and species name are the same, e.g., Naja naja) are allowed in zoological nomenclature, it is not a general rule of binomial nomenclature across all domains (such as botany, where tautonyms are strictly forbidden). Thus, printing them in italics is the universally accepted rule.

Therefore, the correct option is "genus and species both are printed in italics".