Key Points
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Geneticists are poised to move into a dissection of complex phenotypic traits.
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The complexity of phenotypes probably arises from the segregation of alleles at many, interacting loci (quantitative trait loci, or QTL), the effects of which are sensitive to the environment.
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QTL mapping will allow biologists to make the first steps towards understanding the genetics and evolution of complex phenotypic traits.
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At its most basic level, QTL mapping simply involves finding an association between a genetic marker and a phenotype that can be measured.
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Generally, QTL mapping requires two parental strains with different alleles that affect variation in a single trait and a polymorphic genetic map that allows the two strains to be distinguished genetically.
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QTL analysis of adaptive traits has revealed the number of genes that underlie those traits, their mode of action, and their dependence on the starting material and the environment.
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QTL analysis has provided vital insights into the processes of speciation and plant domestication, as well as having enormous practical potential for agriculture.
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The fw2.2 QTL for fruit size in tomato has been characterized at the molecular level.
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Comparative QTL mapping has revealed insights into plant domestication and has also shown how the merger of genomes of divergent evolutionary histories can produce 'unique avenues' for selection.
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There are numerous caveats for the application of QTL mapping techniques, including the expense of map construction, the bias in location and effect of individual QTL, and the difficulty in moving from QTL to an actual genetic locus.
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QTL analysis is as applicable to humans and animals as it is to plants, although working in a plant system has many advantages, including the ability to generate large numbers of progeny from inbred parental lines and to work in ecologically relevant environments.
Abstract
Gregor Mendel was either clever or lucky enough to study traits of simple inheritance in his pea plants; however, many plant characters of interest to modern geneticists are decidedly complex. Understanding the genetic basis of such complex, or quantitative, traits requires a combination of modern molecular genetic techniques and powerful statistical methods. These approaches have begun to give us insight into understanding the evolution of complex traits both in crops and in wild plants.
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Acknowledgements
I thank M. Arnold, R. Baucom, A. Bouck, C. Goodwillie, A. Johnson, A. Paterson, L. Rieseberg and J. Willis for unpublished material, helpful discussions and constructive comments on the manuscript.
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Arabidopsis Biological Resource Centre (ABRC)
Cereon Genomics Arabidopsis SNP collection
Nottingham Arabidopsis Stock Centre: Columbia × Landsberg RI lines
Rockefeller University's collection of genetic analysis software
The Arabidopsis Information Resource (TAIR)
The Institute for Genomic Research
ENCYCLOPEDIA OF LIFE SCIENCES
Glossary
- MULTIVARIATE NORMAL DISTRIBUTION
-
The central limit theorem assures a normal (bell-shaped) distribution for a variable that is the summation of many independent, random inputs. This applies to single or multiple variables.
- COROLLA
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Collectively, the petals of a flower.
- NECTAR GUIDES
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Markings on the petals of flowers, often in contrasting colours or visible only in ultraviolet wavelengths, thought to act as directional beacons for pollinators, especially bees.
- ANTHER
-
The pollen-bearing part of the male floral structure (stamen).
- STIGMA
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The region, usually the apex, of the gynoecium that receives pollen grains and on which the pollen germinates. The gynoecium is the seed-bearing organ of flowering plants, consisting of the stigma, style and ovary.
- STYLAR TUBE
-
In the genus Iris, the styles (tubular columns of tissue arising from the top of the ovary) look like flower petals and are tightly appressed to the top of the actual petals, forming this tube between the petal and the style.
- PHENOLOGY
-
The timing of periodic biological phenomena that are usually correlated with climactic conditions.
- HYBRID ZONE
-
A region of reproduction between individuals of different species, usually occurring where the ranges of the species come together.
- LIKELIHOOD-RATIO TEST STATISTIC
-
A maximum-likelihood method of hypothesis testing. The likelihood-ratio test statistic is twice the natural logarithm of the ratio of the maximum likelihood that the data fit the alternative hypothesis to the maximum likelihood that the data fit the null hypothesis.
- VARIANCE
-
A statistic that quantifies the dispersion of data about the mean. In quantitative genetics, the phenotypic variance (Vp) is the observed variation of a trait in a population. Vp can be partitioned into components, owing to genetic variance (Vg), environmental variance (Ve) and gene-by-environment correlations and interactions.
- SYMPATRIC
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Occurring in the same area without loss of identity from interbreeding.
- MACROEVOLUTION
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Evolution at or above the level of species.
- INTROGRESSIVE HYBRIDIZATION
-
Incorporation of genes from one species into the gene pool of another species.
- DEHISCENCE
-
The splitting open of a fruit.
- LOD SCORE
-
(Base 10 'logarithm of the odds', or 'log-odds'.) A method of hypothesis testing. The logarithm of the ratio between likelihoods under the null and alternative hypotheses.
- MONTE CARLO SIMULATION
-
The use of randomly generated or sampled data and computer simulations to obtain approximate solutions to complex mathematical and statistical problems.
- PERMUTATION TEST
-
A method of hypothesis testing. In these tests, an empirical distribution of a test statistic is obtained by permuting the original sample many times. Each permuted sample is considered to be a sample of the population under the null hypothesis.
- BAYESIAN APPROACH
-
An alternative statistical method that allows the use of prior information to evaluate the posterior probabilities of different hypotheses.
- SIMPLEX SEGREGATION
-
Segregation in polyploids. Segregation with no crossovers of the simplex genotype Aaaa would result in a gametic ratio of 1/2 Aa and 1/2 aa.
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Mauricio, R. Mapping quantitative trait loci in plants: uses and caveats for evolutionary biology. Nat Rev Genet 2, 370–381 (2001). https://doi.org/10.1038/35072085
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DOI: https://doi.org/10.1038/35072085
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