Question

A threshold trait

• A. is expressed on a continuous scale
• B. is present in a few discrete classes but is influenced by both genetics and environment
• C. is caused by only a single gene with no environmental influence
• D. is present in very low frequency in population

Hint:

Analysis of threshold traits requires converting the categorical data into a continuous scale of liability using Probit or Logit transformations to accurately estimate breeding values.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
In quantitative genetics, traits can be classified as continuous, meristic, or threshold traits.
A threshold trait exhibits categorical phenotypic expression despite having an underlying polygenic inheritance pattern.

Step 2: Detailed Explanation:
The inheritance of threshold traits is described by the Threshold Model:
The trait is determined by an underlying, unobservable continuous variable known as "liability."
This liability is polygenic, influenced by many genes, each with a small effect, and by environmental factors.
Although liability is continuous, the trait is expressed phenotypically in discrete classes (such as healthy vs. diseased, or pregnant vs. non-pregnant).
The phenotypic expression changes when the underlying liability crosses a specific boundary, known as the threshold.
Common examples in animal breeding include disease resistance (healthy/diseased), fertility (fertile/sterile), and twinning in sheep.
Thus, threshold traits are characterized by a few discrete phenotypic classes but are genetically polygenic and influenced by the environment.

Step 3: Final Answer:
A threshold trait is present in a few discrete classes but is influenced by both genetics and environment.