Genetics and Prenatal Development. D. Messinger, Ph.D. Quantitative and molecular genetics. Quantitative genetics: Quantifies the strength of genetic and non-genetic factors Genetics believed to play role in all traits non-genetic influences also important
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Such traits may also be susceptible to environmental influence
How are genes a blueprint?
“The DNA sequence (e.g., ATTCCGGA) . . . spells out the exact instructions required to create a particular organism with its own unique traits.”
A metaphor which describes cases in which there is a specific correspondence between genotype and phenotype
Blueprint-likemodes of genetic transmission
Single gene or Mendellian
Specific genetic defects can be deadly or disabling
sickle cell, phenylkitenuria (but see Knox & Messinger, 1958), etc.
Sex-linked (23rd chromosome)
Traits are transmitted as separate units
Autosomes - 22 pairs
One pair from each parent
When 2 competing traits are inherited
Only 1 trait is expressed
Traits are transmitted as separate units
25% risk of inheriting a “double-dose” of r genes
which may cause a serious birth defect
25% chance of inheriting two N’s
thus being unaffected
50% chance of being a carrier as both parents are
23rd chromosomal pair
Male = XY (Missing an arm)
one Y branch not matched
so allele on corresponding X branch is expressed
Female = XX
each branch is matched
Male’s “x” inherited from mother
Women are carriers
Males represented disproportionately in sex linked disorders
The influence of genetic and environmental factors be distinguished and the influence of each can be quanitified using behavioral genetic methods (Plomin)
Monozygotic vs Dizygotic: human studies of
genetic versus environment
Identical (MZ) twins share 100% of their genes
Fraternal (DZ) twins share 50% of their genes
Both types of twins have similar environments . . .
Greater behavioral similarity of identical twins indexes greater genetic influence
Modeling differences between correlations
A (additive genetics) C (common environment) and E (unique environment); ACE Model.
The correlation we observe between MZ twins provides an estimate of A + C .
Dizygous (DZ) twins have a common shared environment, and share on average 50% of their genes: so the correlation between DZ twins is a direct estimate of ½A + C .
rmz = A + Crdz = ½A + C Where rmz and rdz are simply the correlations of the trait in MZ and DZ twins respectively.
Twice difference between MZ and DZ twins gives us A: the additive genetic effect
C is simply the MZ correlation minus our estimate of A. The random (unique) factor E is estimated directly by how much the MZ twin correlation deviates from 1.
difference between the MZ and DZ correlations is due to a halving of the genetic similarity…
So additive genetic effect 'A' is simply twice the difference between the MZ and DZ correlations:
A = 2 (rmz – rdz) As the MZ correlation reflects the full effect of A and C, E can be estimated by subtracting this correlation from 1
E = 1 – rmz Finally, C can be derived:
C = rmz – A = 2 rdz – rmz
Sources of Variance in Behavior
Gene x environment interaction
No genetic influence
“Most, if not all, reliably measured psychological traits, normal and abnormal, are substantively influenced by genetic factors.” Bouchard, T. J. (2004). "Genetic Influence on Human Psychological Traits: A Survey." Current Directions in Psychological Science13(4): 148-151.
Estimates of genetic and environmental influence
Proportional in samples
Greater environmental variation
Will minimize genetic variation
Greater genetic variation
Will minimize environmental variation
E.g. Downs Syndrome
Gene * Environment interactions
Genetic effects on alcohol use are greater in non-religious than religious households
Genetic effects on seeking specific environments –
Identical twins find similar friends
Identical twins treated more similarly (or differently) than fraternal twins?
Why might adoption studies maximize estimates of genetic influence?
Can genetic effects increase with time?
Transactional perspective on gene*environment interface
“It is not nature vs. nurture, but the interaction of nature and nurture that drives development.” Urie Bronfrenbrenner
Gene * environment interactions
“What will it take to make behavioral genetics truly developmental?”
An analysis of the bi-directional relations from gene action to the external environment over the life course, including the prenatal period. Gottlieb, G. (2003). Human Development46(6): 337-355.
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