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Lecture 3

Psychology 1000 Lecture Notes - Lecture 3: Robert Trivers, Twin Study, Zygote

Course Code
PSYCH 1000
Derek Quinlan

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Chapter 4 Psych 1000 Week 3
Genetic Influences
Chromosomes & Genes
Gregor Mendel:
Monk who researched peas
Heredity involves passing on specific organic factors
o Stated that it’s not just a simple blending of parent characteristics
Genotype: specific genetic makeup of an individual
Present from conception, never change
Phenotype: observable characteristics produced by that genetic endowment
Can be affected by: (1) other genes, and (2) by the environment
e.g., hens’ teeth
Chickens have genetic code for teeth
Code is prevented from being expressed
Thus, hens’ teeth remains an expression for scarcity
Egg + sperm = zygote (beginning of new individual)
Egg + sperm each carry chromosomes with them
Chromosome: tightly coiled molecule of DNA that is partly covered by PRO
DNA portion of chromosome carries the genes, i.e., the hereditary blueprint
Every cell in body the body except one type has 46 chromosomes
The exception = sex cell (egg or sperm), which has 23
Conception: 23 of sperm and 23 of egg fuse to form a new cell (zygote)
Thus, the zygote will contain 46
Genes within each chromosome occur in pairs
Thus, offspring receives one of each gene pair from each parent
Alleles: alternative forms of a gene that produce different characteristics
Genes affect development and function by one mechanism:
1. Genes code for the production of PROs
50% of all genes target brain structure + function
Each individual gene carries code for a specific PRO
When that gene is activated, the cell produces the specified PRO
Genes can be activated or “turned off”
o Activated = PRO produced
o “Turned off” = PRO production decreases
Dominant, Recessive, & Polygenic Effects
Genotype and phenotype are not identical; some genes are dominant or recessive
Dominant: particular characteristic it controls will be displayed
Recessive: characteristic will not show unless the partner gene inherited from the
other parent is also recessive
e.g., brown eyes = dominant over blue eyes
o Thus, child only has blue eyes if both parents contribute the
recessive gene for blue eyes
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Chapter 4 Psych 1000 Week 3
In many instances, a number of genes pairs combine their influences to create a
single phenotypic trait, i.e., polygenic transmission
It complicates the straightforward picture that would occur if all
characteristics were determined by one pair of genes
Also magnifies the number of possible variations in a trait that can occur
The Human Genome
Human Genome Project (1990)
2001: genetic map was published
Human has ~25,000 genes (previous estimate: 100,000)
Humans have ~same number of genes as a fruit fly
~200 human genes may have arisen from genes bacteria inserted into ancestors
Genetic Engineering: The Edge of Creation
Recombinant DNA procedures: use of specific enzymes to cut the long threadlike
molecules of genetic DNA into pieces, combine them with DNA from another
organism, and insert the new strands into a host organism
Inside the host, the new DNA combination continues to divide and
produce many copies of itself
Scientists have used this to produce HGH
Molecular biologist have developed methods for inserting new genetic material
into viruses that can infiltrate neurons and modify their genetic structure
Knockout procedure: alter a specific gene in a way to prevent its normal function
e.g., mice and serotonin reuptake:
o Loss of reuptake mechanism
o Mice showed increased anxiety and exaggerated stress response
Important note: very little behavior is controlled by a single gene
However, knocking out a single gene may disrupt a wide range of functions
Genetic engineering: potential control over processes of heredity and evolution
Behavior Genetics Techniques
Children get 50% of genetic material from each parent, thus:
P. of sharing any gene with one of your parents is 50%
P. of sharing any gene with one of your siblings is 50%
P. of sharing any gene with one of your grandparents is 25%
P. of sharing any gene with one of your half-siblings is 25%
Behavior geneticists:
Interested in how hereditary and environmental factors combine to
influence psychological characteristics
Heritability coefficient (h2): extent to which variation in a particular characteristic
within a group can be attributed to genetic factors
Heredity: passage of characteristics from parents to offspring by genes
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Chapter 4 Psych 1000 Week 3
Heritability: how much of the variation in a characteristic within a
population can be attributed to genetic differences
o Difference in the trait across individuals, NOT the trait itself
o e.g., h2 = 0.60 = 60% of BW is due to genes, and 40% of BW is
due to environment
o Thus, only applies to variance within groups, not between groups
o e.g., psychology class vs. native tribe: differences in BW between
your class and the tribe are most likely attributable to differences
in the environment, such as differences in the availability of high-
sugar and high-fat foods and the amount of physical activity
When studying heritability, why does knowing the group matter?
e.g., intelligence:
o One group studied can be privileged children, i.e., more resources
o The other = wide-range of backgrounds, i.e., variable resources
How can the same characteristic (e.g., intelligence) have two different h2?
Advantaged group: environmental factors that influence intelligence
would be very similar from one individual to the next and so would be
unable to explain individual differences; if the environment does not
account for the variation in intelligence within this group, then the
difference could be due to genetic factors, and the h2 would be high
Wide-range group: more differences can be attributed to differences in the
environment, and hence the h2 estimate would be low
If a characteristic has higher concordance (i.e., co-occurrence) in people who are
more highly related to one another, then this points to a possible genetic
contribution, particularly if the people have lived in different environments
Adoption study
= Person who was adopted early in life is compared on some characteristic
both with the biological parents and with the adoptive parents
If adoptee = more similar to biological parents than to adoptive parents =
genetic influence suggested
If greater similarity is shown with adoptive parents, then environmental
factors are probably more important
Twin studies
Monozygotic = identical; 1/250 births
Dizygotic = fraternal; 1/125 births
Compare concordance rates of identical and fraternal twins reared in same
o If identical are more similar than fraternal = genetic factors
o Of course, it is possible that because identical are more similar in
appearance, they are treated the same, thus, sharing more similar
environment; could account for more behavioral similarity
Sometimes possible to find identical/fraternal twins reared in different
environments early in life; allows analysis of both genes and environment
o Even if reared in different environment… adopted children still
more similar to biological parents than adoptive parents, and
identical twins still more similar to each other than fraternal twins
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