What is the best source of nuclear dna when working with hair samples?

The answer is a hair root.

Nuclear DNA is commonly extracted from the hair root. The hair root consists of keratinocytes. Keratinocytes are cells found in the epidermis. As all other cells, they contain DNA material. When keratynocites die, they get converted into keratoid material in the process of cornification. As a consequence, dead cells do not contain DNA material. Therefore, the hair root is the best source of nuclear DNA than shed or cut hair when working with hair sample.