Weight is a quantitative trait and mice with defective leptin tend to have problems controlling weight since this gene is associated with it.
A quantitative trait is a phenotypic feature controlled by many genes and the interaction with the environmental factors.
Weight, height, behavioral features are all examples of quantitative traits because these types of phenotypic features show continuous variation.
Quantitative traits are modulated by gene interactions and therefore exhibit both epistatic and pleiotropic effects.
The leptin gene is a gene in mammal involved in neuronal control to metabolic signals and thereby it is associated with weight.
In conclusion, weight is a quantitative trait and mice with a defective form of the leptin gene tend to have problems controlling weight because it is associated with this trait.
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