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Quantitative trait loci in Drosophila

Nature Reviews Genetics volume 2, pages 11–20 (2001)Cite this article

Key Points

  • Quantitative trait phenotypes are continuously distributed in natural populations, due to segregation of alleles at multiple quantitative trait loci (QTL) and environmental sensitivity of QTL alleles.

  • Determining the genetic and environmental bases of variation for quantitative traits is important for human health, agriculture, and the study of evolution.

  • Complete genetic dissection of quantitative traits is currently feasible only in genetically tractable and well characterized model systems, such as Drosophila melanogaster.

  • Genomic regions containing QTL affecting within-species variation (in sensory bristle number, wing shape, life span and other life history traits) and variation in morphological differences between species have been mapped by linkage to neutral, polymorphic molecular markers. Multiple QTL affect variation in each of the traits studied.

  • Drosophila QTL often have sex- and/or environment-specific effects, and can interact non-additively (that is, exhibit epistasis).

  • Drosophila QTL are mapped with high resolution using a quantitative version of deficiency complementation mapping.

  • Genetic loci that interact with QTL alleles are identified using quantitative complementation tests, provided stocks with mutant alleles exist.

  • Screens for viable P transposable element-induced mutations with quantitative phenotypic effects define novel pleiotropic effects of known loci and functions of predicted genes, and provide mutant stocks that can be used in quantitative complementation tests.

  • Linkage disequilibrium mapping is used to determine which candidate genes correspond to QTL, and which molecular polymorphisms within candidate genes are associated with quantitative variation in phenotypes.

  • Molecular polymorphisms in non-coding regions of candidate genes are associated with quantitative variation in phenotypes; such associations are often sex-specific.

  • Quantitative traits in other organisms are likely to have equally complex genetic architectures.

Abstract

Phenotypic variation for quantitative traits results from the simultaneous segregation of alleles at multiple quantitative trait loci. Understanding the genetic architecture of quantitative traits begins with mapping quantitative trait loci to broad genomic regions and ends with the molecular definition of quantitative trait loci alleles. This has been accomplished for some quantitative trait loci in Drosophila. Drosophila quantitative trait loci have sex-, environment- and genotype-specific effects, and are often associated with molecular polymorphisms in non-coding regions of candidate genes. These observations offer valuable lessons to those seeking to understand quantitative traits in other organisms, including humans.

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Figure 1: Characteristics of quantitative traits.
Figure 2
Figure 3: Quantitative trait locus mapping.
Figure 4: Quantitative trait locus effects.
Figure 5: Linkage disequilibrium mapping.

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Acknowledgements

Work in the author's laboratory is supported by the National Institute of Health and by the W. M. Keck Center for Behavioral Biology of North Carolina State University.

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  1. Department of Genetics, Box 7614

    Trudy F. C. Mackay

  2. North Carolina State University, North Carolina, 27695, USA

    Trudy F. C. Mackay

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FURTHER INFORMATION

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ENCYCLOPEDIA OF LIFE SCIENCES

Quantitative genetics

Glossary

INTROGRESSION

Transfer of genetic material from one strain to another by repeated backcrosses. With marker-assisted introgression, markers that distinguish the parental strains are used to track the desired interval and select against the undesired genotype.

ANTAGONISTIC PLEIOTROPY

Alternative homozygous genotypes (A1A1, A2A2) have opposite phenotypic effects under different conditions.

CONDITIONAL NEUTRALITY

The difference between quantitative trait loci genotypes is only expressed under some conditions.

VARIANCE

A statistic to quantify dispersion about the mean. In quantitative genetics, the phenotypic variance, VP, is the observed variation of the trait in a population. VP is partitioned into components due to variation in the additive (VA) dominance (VD) and epistatic (VI) genetic variance, the variance attributable to the environment (VE), and gene–environment correlations and interactions.

BACKCROSS

The cross of the F1 progeny of two parental strains to either of the two parents.

PROGENY TEST

Crossing an individual at random to a number of unrelated individuals in the population, and determining the mean phenotype of the progeny. The progeny mean phenotype is a more accurate measure of the genetic merit of the tested individual than the individual's own phenotype.

HERITABILITY

The fraction of the phenotypic variance attributable to additive genetic variance (VA/VP).

LIKELIHOOD RATIO

A method for hypothesis testing. The maximum of the likelihood that the data fit a full model of the data is compared with the maximum of the likelihood that the data fit a restricted model and the likelihood ratio (LR) test statistic is computed. If the LR test statistic is significant, the full model provides a better fit to the data than does the restricted model.

PERMUTATION TEST

A statistical test in which the data are randomized many times to determine the statistical significance of the experimental outcome (in this case, the association of a quantitative trait phenotype and a multi-locus marker).

OVERDOMINANT

Heterozygote superiority. The phenotype of the heterozygote is greater than that of either homozygotes. Overdominance for fitness can lead to the maintenance of both alleles in the population.

STABILIZING SELECTION

Intermediate phenotypes have greater fitness than extreme high and low scoring phenotypes.

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Mackay, T. Quantitative trait loci in Drosophila. Nat Rev Genet 2, 11–20 (2001). https://doi.org/10.1038/35047544

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