Final answer:
The condition where normal hair is replaced with fine, colorless hair is called lanugo growth. Lanugo is typically found on fetuses and sheds around the time of birth, but if hair does not replace properly, fine or sparse hair may occur in adults as well.
Step-by-step explanation:
The condition when normal hair is replaced with fine, colorless hair is known as the growth of lanugo. Lanugo is a type of very fine, soft, and usually unpigmented hair that covers the body of a fetus. This hair type grows as the first coat of hair and is usually shed and replaced by more substantial hair growth shortly before or after birth. Within hair growth stages, the anagen phase involves active hair growth, the catagen phase represents the transition, and the telogen phase is a rest period. If the replacement of shed hair does not occur normally, hair can become very fine or sparse due to various reasons such as hormonal changes, dietary issues, or aging.