Messinger

Genetics and Prenatal Development

PSY344W

D. Messinger, Ph.D.

Psychology of Infancy

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What are the advantages (name some forms of genetic transmission) and disadvantages of thinking of genes as blueprints?

How do environmental and genetic influences interact during prenatal development (provide examples)?

What is the difference between transactional and a behavioral genetics approach to gene * environment interactions?

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Who believes in

Nature – genetics– Genes as blueprint

Nurture – environment– Infinite malleability

Genes and environment working interdependently and interacting?

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Some basics

Genes– Bits of DNA, protein, in each cell– contain information on cell functioning, production,

and reproduction

Chromosomes– Larger groupings of DNA– All non-gamete cells in the body have 23 pairs of

chromosomes– Half of each pair came from each parent

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Chromosomes

Reality Ordered by karotyping

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Human genome project

identify all the approximately 30,000 genes in human DNA,

determine the sequences of the 3 billion chemical base pairs that make up human DNA,

99.9% (of nucleotide bases) are the same in all people

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Chromosome 19

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Genomes to Life Project

Identify the protein machines that carry out critical life functions and the gene regulatory networks that control these machines

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Terms

Phenotype– Observable trait

Genotype– Genetic pattern associated with the phenotype

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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 some aspects of phenomenon– Specific correspondences between genotype

and phenotype

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Blueprint-likemodes of genetic transmission Dominant-recessive

– Single gene or Mendellian Specific genetic defects can be deadly or disabling http://www.uaf.edu/psych/psyc240/exam1/index.html, Jim Allen, Ph.D

– Phenylkitenuria, sickle cell, etc.

Sex-linked (23rd chromosome)

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Dominant-Recessive Inheritance

Traits are transmitted as separate units Autosomes - 22 pairs

– Non-sex chromosomes– One pair from each parent

When 2 competing traits are inherited– Only 1 trait is expressed

Dominant trait Recessive trait

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Dominant-Recessive Inheritance

Norm al(NN )

Carrier(Nr)

Carrier(Nr)

Affected(rr)

Carrier Father (Nr)+

Carrier M other (Nr)

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

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Sex-linked inheritance

23rd chromosomal pair Female = XX

– each branch is matched Male = XY (Missing an

arm)– one Y branch not

matched– so allele on

corresponding X branch is expressed

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Sex-linked inheritance

Male’s “x” inherited from mother– Women are carriers– Males represented disproportionately in sex linked

disorders baldness color-blindness, hemophilia

– Baldness: maternal gf > f

Even sex-linked characteristics are dependent on environmental influences– Expression of baldness depends on circulating

testosterone levels

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Polygenic inheritance – not blue-print like - is the rule Multiple genes influence most traits Sign of polygenic inheritance is range in

phenotype rather than either or– skin/eye/hair color, height, baldness, personality– Reaction Range – Potential variability in expression of a trait

Such traits may also be susceptible to environmental influence

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Disadvantages of the genes-as-blueprint metaphor Genes are bits of protein in a primarily

liquid nucleus in a primarily liquid cell surrounded by other cells in a primarily liquid uterine environment– Without an “environment,” genes are bits of

protein From a lump of jelly to an organism

– How do genes actually work?

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Prenatal development is usually divided into three main periods. Zygote -

– covers the first two weeks after conception– ends when the zygote implants into the wall of the mother's

uterus.

Embryo - – from two to eight weeks following conception– the major organs and bodily systems form

Fetus– from eight weeks after conception until birth – grows tremendously in size and weight.

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Zygotic cell differentiation

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From zygote to embryo

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The question

The zygote grows through cell division– Mitosis - One for one copying of all 23

chromosomes All cells contain the same genetic

information in their nuclei But qualitatively different types of cells

develop in different parts of the body How does this occur?

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General processes yield specific outcomes Cells clump together as a sphere This changes the extra-cellular environment

of cells on the inside and outside of the sphere

Differences in environment impact cell’s genetic make-up to activate different proteins

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Gene x Environment interaction

Environmental factors influence development from the start– Cells are environments– The uterus is an environment

The fetus participates in actively constructing its own development– it is not passively constructed

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Creation of a tube

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Embryology

Cells groups in which specific molecular processes occur with boundaries with other groups

Regulator genes activate and de-activate other genes within these groups

Cells impact each other such that a nerve cell transplanted to the liver region becomes a liver cell after several replications– Stem cell debate

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Brain development

General pattern of brain development genetically specified – By 20 weeks, most neurons present– 3rd - 16th prenatal week most crucial– At 8 weeks, head is half of fetus

But specific connections depend on generic growth processes and sensory-motor stimulation– Trillions of connections still forming– Trimming of these connections is developmental task

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The fetus as actively constructing its own development Fetal behavior impacts physical

development– In chicks prevented from moving, cartilage

turns to bone Fetal sensory experience impacts sensory

development– Mice whose tongues were anesthetized had

malformed cleft palates

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Prenatal sensory experience impacts sensory development Hearing typically develops before sight Rats, ducklings, and quail chicks exposed to

visual stimulation prenatally– before they normally would

lose hearing ability at birth Normal sensory development contingent on

extra-fetal environment– being enclosed

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Prenatal behavioral development

9 weeks - movement 16 weeks - frowning, grimacing 25 weeks - moves to drumbeat 26 weeks - remembers sounds 32 weeks - all brain areas functioning 34 weeks - can habituate

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2 perspectives on gene*environment interface Transactional

– “It is not nature vs. nurture, but the interaction of nature and nurture that drives development.” Urie Bronfrenbrenner (what we just heard)

Quantitative– The influence of genetic and environmental

factors be distinguished and the influence of each can be quanitified using behavioral genetic methods (Plomin)

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Behavioral genetics

Measuring genetic and environmental influences on behavior

Finding genes for behaviors?

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Sources of Variance in Behavior

Genetic (heritability) Environmental Gene x environment interaction Error

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Estimates of genetic and environmental influence Proportional in samples

– Greater environmental variation Will minimize genetic variation

– E.g. Poverty

– Greater genetic variation Will minimize environmental variation

– E.g. Downs Syndrome

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Trivia

Why might adoption studies maximize estimates of genetic influence?

Can genetic effects increase with time?– How?

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Environmental effects

Previously modeled but not measured Now parental monitoring, neighborhood

deprivation account for small (2-5%) of environmental variation– What else should we be measuring?

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Twin Studies

Monozygotic vs Dizygotic: human studies ofgenetic versus environment

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Twin studies

Identical (MZ) twins share 100% of their genes– genetic duplicates.

Fraternal (DZ) twins share 50% of their genes– on average

Both types of twins have similar environments . . .

Greater behavioral similarity of identical twins indexes greater genetic influence

• http://www.psych.umn.edu/psylabs/mtfs/special.htm

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Gene * Environment interactions

Development always involves this interaction Specific statistical effects

– Genetic effects on alcohol use are great 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?

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No genetic influence

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50% Dizygotic 100% - Monozygotic

Genetic Relatedness

Twin 1Twin 2

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Complete genetic influence

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Beh

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r S

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50% Dizygotic 100% - Monozygotic

Genetic Relatedness

Twin 1Twin 2

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Additional readings

Plomin, et al. The genetic basis of complex human behaviors. Plomin, R., & Rutter, M. (1998). Child development, molecular

genetics, and what to do with genes once they are found. Child Development, 69(4), 1223-1242.

Rutter. M. (in press [2002]. Nature, nurture, and development: From evangelism through science towards policy and practice. Child Development.

Collins, W. A., Maccoby, E. E., Steinberg, L., Hetherington, E. M., & Bornstein, M. H. (2000). Contemporary research on parenting: The case for nature and nurture. American Psychologist, 55(2), 218-232.

Sleigh, M. J., Columbus, R. F., & Lickliter, R. (1998). Intersensory experience and early perceptual development: Postnatal experience with multimodal maternal cues affects intersensory responsiveness in Bobwhite Quail Chicks. Developmental Psychology, 34(2), 215-223.

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