Differentiate a genetic trait from a selection index.

• A. A trait is an individual characteristic; a selection index combines multiple traits into a single score for selection
• B. A trait is far more important than a selection index
• C. A selection index measures only one trait
• D. A trait is always quantitative; a selection index is qualitative

Answer: (A)

The main idea is to separate an individual characteristic from a tool used to pick animals. A trait is an individual characteristic of an animal—something you can observe or measure in one animal, like birth weight or horns vs no horns. It can be qualitative (yes/no, category) or quantitative (measured amount).

A selection index, on the other hand, is a single numeric score created by combining multiple traits with chosen weights. This index summarizes several characteristics into one value to rank animals for breeding decisions. The weights reflect how important each trait is to the overall breeding goal, so the index helps breeders optimize across traits rather than focusing on a single characteristic.

So the best answer states that a trait is an individual characteristic, and a selection index blends several traits into one score for selection. The other ideas don’t fit because a trait isn’t inherently more important than an index, an index isn’t limited to one trait, and a trait isn’t restricted to one type (traits can be qualitative or quantitative, and an index is a numeric composite that can incorporate both).