Final answer:
A sex-limited trait is one that is expressed in only one sex, even though it may be autosomal, thus not linked to the X or Y chromosomes. Therefore, the correct answer is 4) Sex-limited trait.
Step-by-step explanation:
The trait that is expressed in only one sex but may not be X-linked or Y-linked is known as a sex-limited trait. These traits can be autosomal, meaning they are located on non-sex chromosomes, and their expression is limited strictly to one sex due to hormonal differences, sexual development, or other physiological factors.
Sex-limited traits are different from sex-linked traits, which are located on the sex chromosomes and often exhibit different patterns of inheritance between males and females.
An example of a sex-limited trait could be beard growth in human males, which is determined by autosomal genes but usually only expressed in males due to the influence of male sex hormones. Therefore, the correct answer is 4) Sex-limited trait.