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Quantitative traits
Lecture 8. Quantitative Genetics
Polygenic inheritance Heritability
- Traits with a few distinct phenotypes are DISCONTINUOUS TRAITS. There is
QUANTITATIVE TRAITS A quantitative trait is any trait that varies measurably
usually a simple relationship between the genes responsible and formation of the phenotype.
2. Often, penetrance, expressivity, pleiotropy, epistasis and environmental factors are involved in producing a continuous distribution of phenotypes (CONTINUOUS TRAITS). Quantitative genetics is used to characterize continuous traits.
The first experiment demonstrating that continuous variation is related to polygenic inheritance was conducted by Herman NILSSON-EHLE in 1909: the inheritance of red pigment in the hull of wheat Triticum aestivum
white hulls true-breeding plants x red hulls true-breeding plants
F1 : intermediate color
Self-fertilize
F2: great variation in redness (ranging from white, light red, intermediate red, medium red and dark red)
1:4:6:4:1 ratio
Colors of the hulls are due to the existence of two different genes, each one existing in a red or white allelic form.
These two loci contribute additively to the color of the hull.
There are more types of quantitative traits:
MERISTIC TRAITS Some characteristics are not continuous but are nevertheless considered quantitative because they are determined by multiple genetic and environmental factors. Meristic characteristics are measured in whole numbers. An example is litter size: a female mouse may have 4, 5, or 6 pups but not 4.13 pups.
THRESHOLD TRAITS where INDIVIDUALS ARE CLASSIFIED AS EITHER HAVING THE TRAIT OR NOT, although the underlying basis is quantitative., g y g q e.g. diseases.
For all these types the analytical methods for analysing them are the same.
- The basic tenet of quantitative genetics: THE VARIATION SEEN IN QUANTITATIVE TRAITS IS DUE TO A COMBINATION OF MANY GENES EACH CONTRIBUTING A SMALL AMOUNT, PLUS ENVIRONMENTAL FACTORS O h t ti l i t
Questions Studied in Quantitative Genetics
FACTORS. Or: phenotype = genetics plus environment.
- Basic questions (plus answers):
- What is the genetic basis of quantitative traits? (they are caused by normal genes following Mendel’s rules).
- How can we separate the effects of genetics from the effects of the environment? (by inbreeding to eliminate genetic variation).
- How can we predict and control the outcome of a cross? (by artificial selection).
1. Genes are always expressed in an environmental context. The nature vs. nurture question provides an opportunity to Statistical Tools examine the relative contributions of both.
Applying Statistics to the Study of a Polygenic Characteristic
2. How much of a variation in phenotype (VP) is due to genetic variation (VG ) and how much to environmental variation (VE )? This can be expressed: V (^) P = VG + V (^) **E.
- To work this equation, variation must be measured and then partitioned into genetic and environmental components.**
HERITABILITY
In the study of quantitative traits, a primary goal is to determine HOW MUCH
OF THE PHENOTYPIC VARIATION ARISES FROM GENETIC VARIATION and HOW
MUCH COMES FROM ENVIRONMENTAL VARIATION: analysis of variance:
V T = VG + V E
VT is the total variance. It reflects the amount
of variation that is measured at the phenotypic
level
VG is the relative amount of variance due to
genetic variation
VE is the relative amount of variance due to
environmental variation
H^2 = VG/V T
H^2 is the heritability in the broad sense
VG is the variance due to genetics
VT is the total phenotypic variance
As some alleles affect the phenotype in an
BROAD SENSE HERITABILITY takes into account all genetic variation that
may affect the phenotype
additive way, alternatively other alleles
affect the phenotype as a
dominant/recessive way, or in an epistatic
manner. Then:
VG = VA + V D+ V I
For many quantitative traits, the additive effects of alleles often dominate the phenotypic outcome in genetic crosses. In addition, when the alleles behave additively, we can predict the outcomes of crosses based on the quantitative traits of the parents The heritability of a trait due to the
NARROW SENSE HERITABILITY may be
quantitative traits of the parents. The heritability of a trait due to the additive effects of alleles is called the narrow sense heritability:
h^2 = VA /V T an inaccurate measure of the actual
heritability if VD and VI are not small
values.
The Limitations of Heritability
Heritability does not indicate the degree to which a characteristic is genetically determined
An individual does not have heritability There is no universal heritability for a characteristic
Even when heritability is high, environmental factors may influence a characteristic
HHeritabilities it biliti iindicate di t nothingthi aboutb t the nature of population differences in a characteristic
Heritability describes the amount of phenotypic variation due to genetic variation for a particular population raised in a particular environment.
Locating Genes That Affect Quantitative Characteristics Mapping QTLs
The locations on chromosomes that harbor genes that affect the outcome of quantitative traits are called quantitative trait loci (QTLs).
Chromosome regions containing genes that interact with environmental factors and influence a quantitative trait are termed quantitative trait loci, or QTLs.
Quantitative genetics coupled with modern molecular techniques to identify and exploit genetic variation that influences economically important characteristics
Selection by promoting the reproduction of organisms with traits perceived as desirable: artificial selection domestic plants and animals that make modern agriculture possible.
WhenWhen aa quantitativequantitative characteristiccharacteristic isis subjectedsubjected to natural or artificial selection, it will frequently change with the passage of time, provided that there is genetic variation for that characteristic in the population.
RESPONSE TO SELECTION
The response to selection depends on the phenotypic difference of the individuals that are selected as parents; this phenotypic difference is measured by the selection differential, defined as the difference between the mean phenotype of the selected parents and the mean phenotype of the original population.
Suppose that a dairy farmer breeds only those cows in his herd that have the highest milk production. If there is genetic variation in milk production, the mean milk production in the offspring of the selected cows should be higher than the mean milk production of the original herd. This increased production is due to the fact that the selected cows possess more genes for high milk production than does the average cow, and these genes are passed on to the offspringgenes are passed on to the offspring. The offspring of the selected cows possess a higher proportion of genes for greater milk yield and therefore produce more milk than the average cow in the initial herd.
Suppose that the average cow in a dairy herd produces 80 liters of milk per week. A farmer selects for increased milk production by breeding the highest milk producersbreeding the highest milk producers, and the and the progeny of these selected cows produce 100 liters of milk per week on average. The response to selection is calculated by subtracting the mean phenotype of the original population (80 liters) from the mean phenotype of the offspring (100 liters), obtaining a response to selection of 100 80 20 liters per week.
The response to selection is determined primarily by two factors:
First, it is affected by the narrow-sense heritability, which largely determines the degree of resemblance between parents and offspring. When the narrow-sense heritability is high, offspring will tend to resemble their parents; conversely when the narrow-sense heritability is low there
The second factor that determines the response to selection is how much selection there is. If the farmer is very stringent in the choice of parents and breeds only the highest milk producers in the herd (say, the top 2 cows), then all
their parents; conversely, when the narrow sense heritability is low, there will be little resemblance between parents and offspring.
the offspring will receive genes for high-quality milk production. If the farmer is less selective and breeds the top 20 milk producers in the herd, then the offspring will not carry as many superior genes for high milk production, and they will not, on average, produce as much milk as the offspring of the top 2 producers.
The response to selection depends on the phenotypic difference of the individuals that are selected as parents; this phenotypic difference is measured by the SELECTION DIFFERENTIAL, defined as the difference between the mean phenotype of the selected parents and the mean phenotype of the original population.
If the average milk production of the original herd is 80 liters and the farmer breeds cows with an average milk production of 120 liters, then the selection differential is 120 - 80 = 40 liters.
The response to selection (The response to selection ( RR ) depends on the narrow sense heritability () depends on the narrow sense heritability ( hh 2)2) and the selection differential ( S ):
