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Quantitative trait and phenotype

1,108 views · Published 25 September 2014 · 5:05 · Indexed 8 November 2026

Channel: Nikolay's Genetics Lessons · 2014 · Education

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What are quantitative traits?

Quantitative, or complex, traits are traits for which phenotypic variation is continuously distributed in natural populations, with population variation often approximating a statistical normal distribution on an appropriate scale. Quantitative traits include aspects of morphology (height, weight); physiology (blood pressure); behavior (aggression); as well as molecular phenotypes (gene expression levels, high- and low-density cholesterol levels).

What causes the continuous distribution of phenotypes for quantitative traits?

The continuous variation for complex traits is due to genetic complexity and environmental sensitivity. Genetic complexity arises from segregating alleles at multiple loci. The effect of each of these alleles on the trait phenotype is often relatively small, and their expression is sensitive to the environment. Allelic effects can also depend on genetic background and sex. Because of this complexity, many genotypes can give rise to the same phenotype, and the same genotype can have different phenotypic effects in different environments. Thus, there is no clear relationship between genotype and phenotype.

Does this mean you can't see Mendelian ratios for quantitative traits?

Yes, because of the small magnitude of the allelic effects on the phenotype. Mendelian variants have large effects on the phenotype so there is a clear correspondence between genotype at a locus and trait phenotype. For any trait there is a continuum of allelic effects from small to large: the large effects segregate as Mendelian variants, while the small effects segregate as quantitative genetic variation. For example, human height is a classic quantitative trait, but achondroplasia (dwarfism) is caused by a Mendelian autosomal dominant mutation in the fibroblast growth factor receptor 3 gene.

Why are quantitative traits important?

Quantitative genetic variation is the substrate for phenotypic evolution in natural populations and for selective breeding of domestic crop and animal species. Quantitative genetic variation also underlies susceptibility to common complex diseases and behavioral disorders in humans, as well as responses to pharmacological therapies. Knowledge of the genetic basis of variation for quantitative traits is thus critical for addressing unresolved evolutionary questions about the maintenance of genetic variation for quantitative traits within populations and the mechanisms of divergence of quantitative traits between populations and species; for increasing the rate of selective improvement of agriculturally important species; and for developing novel and more personalized therapeutic interventions to improve human health.

How can you identify genes affecting quantitative traits?

This is usually done in stages. In the first stage, we map quantitative trait loci (QTLs) affecting the trait. QTLs are genomic regions in which one or more alleles affecting the trait segregate. In the second stage, we focus in on each QTL region to further narrow the genomic intervals containing the gene or genes affecting variation in the trait. The final and third stage is most challenging: pinpointing the causal genes.

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