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LT8 Quantitative Traits
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Continuous and Discontinuous Traits, Heritability, Numerical concepts in quantitative traits
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Quantitative Traits Traits with discrete phenotypes show discontinuous variation – however variable
penetrance, epistasis, complementation can obscure genotype-phenotype
relationship
Phenotypic variation and a genetic component and environmental factors
Discontinuous variation in traits due to underlying genetic variation
Shell character of land snail (Cepaea nemoralis)
- Shell has highly polymorphic appearance – background colours and number of
bands (from 0 – 5+)
- Genetic component: colour and banding determined by alleles at several
independent loci, with clear dominance patterns
Background colour – one locus – alleles for darker colours are dominant
Banding – influenced by several loci
- Natural selection – role is known: variation in gene frequencies correlated with
variation in habitat
Mouse coat colour
- At least 5 interacting loci – epistasis at some loci
- A locus – distribution of colour in hair shaft
- B locus: background colour of hair
- C locus: presence or absence of pigment (epistatic to other loci)
- D locus: expression of pigment
- S locus: pigment distribution on the body
Discontinuous variation in traits due to environmental variation
Often seen where individuals receive reliable cues of their future environment and
where particular phenotypes are advantageous – single genotype can give rise to
different phenotypes
Artic hare – summer has dark fur, winter light fur (temperature effects)
Light: butterfly wing development and growth – Morgan, caterpillars kept under red,
green or blue light, others kept in the dark
- Red light – intensely coloured wings
- Blue light and darkness led to paler but larger wings
Chemicals: Stockard (1907) created cyclopean fish embryos by placing fertilised
Fundulus heteroclitus eggs in seawater and 6g magnesium chloride
, Acorn barnacle – sedentary crustacean
- Has free-swimming larval stage – larvae “settle” to complete development
- Settling – larvae attach their heads to solid object in the water – rocks,
lobsters, ship bottoms
- Alternative shell morphs living in the Gulf of California – bent form found in
Northern Gulf (coincides with that of a predator – snail Acanthina angelica
which uses a spine to scoop barnacles – bent form adapted as predator has
reduced feeding success), conic form is more common
- Bent morphs result from developmental switch – triggered by presence of snail
Experiments – after settlement:
If expose cultures of developing barnacles to the snail – bent form develops, if
not, conic form develops
Environmentally driven occurring after settlement, presumably a chemical cue
from snail’s presence
- Not a case of 2 genetic morphs selecting where to settle in response to
predator – flexibility single genotype can give rise to more than one
phenotype
- Why is the bent morph not universally distributed?
In the absence of the predator – conic form is fitter (eg. feeding efficiency,
growth rate, reproductive success)
Adaptive phenotypic plasticity
- Other examples show maladaptive phenotypic plasticity – plant responses to
deteriorating environmental conditions
Many medically and agriculturally important traits show variation with continuous
range of phenotypes
Range of phenotypes measured quantitatively: quantitative inheritance
Phenotypes result from input of many genes – polygenic
Phenotypes arise from gene action + environmental – complex/multifactorial traits
Three types of quantitative traits
1. Continuous/ metric traits: height, weight, milk yield, growth rate – theoretically
there are infinite number of phenotypes, discrimination limited to measurement
instrument
2. Categorical/meristic traits: expressed in discrete, integral classes – can be
counted – number of ears on a stalk of corn, bristles on a fruit fly, number of
eggs of laid
3. Threshold/dichotomous traits: has small number of discrete phenotypic classes
either present or absent in any one individual
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