Transcript Chapter 5

Psychology 372
Behavioural Genetics
Behavioural Genetics
• Studies the role of genetics and
environment in animal behaviour
• Intersects with psychology in general
(especially developmental, abnormal,
clinical), human behavioural ecology,
evolutionary psychology, genetics,
population genetics
• Actually, an older field than psychology
Sir Francis Galton
• Hereditary Genius (1869)
– “…a man’s natural abilities are
derived by inheritance, under
exactly the same limitations as
are the form and physical
features of the whole organic
world.”
• Beginning of behavioural
genetics
• Family and twin study designs
• Correlations and regressions
Galton at age 50
<www.galton.org/>
Early Behavioural Genetics
• Traditionally, studied inheritance of
behavioural traits
• Demonstrate genetic influence on behaviour
exist
• Conflict with Behaviorism
Behaviorism
• John B. Watson
• “Hard-line” Behaviorism
– “Give me a dozen healthy infants, well-formed, and my own
specified world to bring them up in and I’ll guarantee to take any
one at random and train him to become any type of specialist I might
select - doctor, lawyer, artist, merchant-chief and, yes, even beggarman and thief, regardless of his talents, penchants, tendencies,
abilities, vocations, and race of his ancestors.”
• Predominant in psychology and social sciences until 1960s
• Environmental control over behaviour with no or minimal
genetic influence
Nature and Nurture
• Back to 17th century philosophy
• Empiricism
– Tabula rasa
• Nativism
Determinism
• Genetic determinism
• Genetic predispositions
Contributing Factors
• Interaction
• Nature
– The genes
• Nurture
– Shared environment
– Unique/non-shared environment
Heritability Estimates
1.0
0.8
Correlation
• Trait 1: low heritability,
high shared
environment
• Trait 2: high
heritability
• Trait 3: low heritability,
low shared
environment, high
unique environment
Correlation of Sibling Traits
in Shared Family Environment
0.6
0.4
0.2
0
Trait 1
Trait 2
Monozygous twins
Biological siblings
Adoptive siblings
Trait 3
More Recent Trends
• Shift from study of inheritance
• Use of quantitative methods
– Estimates degree to which differences in individuals are
due to genetic and environmental differences; doesn’t
specify gene or environmental factors
• Molecular genetics
– Identification of specific genes for behaviours
• Study of quantitative trait loci (QTLs)
Genetic Components
• We’ll get to the behaviours
• Need a basic familiarity with genetic
terminology and elements
Terms
• Gene
– Smallest discrete inherited unit
• Allele
– Different forms of specific gene
• Two alleles of each gene
– Homozygous or heterozygous
• Alleles can be dominant or recessive
• Genotype and phenotype
Punnett Squares
• Standard is to use capitals for dominant,
lower-case for recessive
• Will produce all the possible genotypic
outcomes
• Assumes genes are independent of each
other
• Punnett Square Calculator
Chromosomes
• 23 pairs of chromosomes in humans
– 22 autosomal, 1 sex
• Loci (locus, singular) of gene(s)
<adapted from: www.accessexcellence.org/
RC/VL/GG/human.php>
<encarta.msn.com/media_461543483/Human_Male_Karyotype.html>
Meiosis and Mitosis
• Mitosis
– Non-gamete cell division
– Mitosis animation
• Meiosis
– Production of gametes
– Meiosis animation
Gregor Mendel
• Augustinian priest
– Well trained in mathematics,
physics, biology
• From 1856-1863 cultivated and
tested 29,000 pea plants
• Sought to understand variation
• Work published in 1866, but
largely ignored until rediscovered
in 1900
• Two laws of heredity
<en.wikipedia.org/wiki/Image:Mendel.png>
Mendel’s First Law of Heredity
• Law of Segregation
• Genes “segregate” during gamete formation
– Offspring receive one gene from each parent
• Dominant and recessive forms
Mendel’s Second Law of Heredity
• Law of Independent Assortment
– Inheritance of one gene is not affected by the
inheritance of another gene
• Does get violated in various situations
– Linkage based on proximity of loci on
chromosome
– Recombination (chromosomal crossovers)
– Recombination between linked genes animation
Hardy-Weinberg Equilibrium
• Frequencies of alleles and genotypes don’t
change across generations unless forces
(e.g., natural selection, migration, etc.) alter
them
• For a population, can calculate allele and
genotype frequencies, assuming random
mating
Frequencies
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•
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Consider a single locus with two alleles
Dominant A and recessive a
Frequency(A) = p
Frequency(a) = q
Expected genotype frequencies are the
product of the mother’s (p + q) and father’s
alleles (p + q)
• Thus, (p + q)2 = p2 + 2pq + q2
Punnett Square and HardyWeinberg
Eggs
A
Sperm
A
a
a
AA
Aa
(p2)
0.6x0.6
= 0.36
(pq)
0.6x0.4
= 0.24
Aa
aa
(pq)
0.6x0.4
= 0.24
(q2)
0.4x0.4
= 0.16
p = 0.6
q = 0.4
(p + q)2 = 1
p = 0.6
q = 0.4
p2 + 2pq + q2 = 1
Example
• 1 in 1700 US Caucasian newborns have
cystic fibrosis
– C for normal is dominant over c for cystic
fibrosis
• What percentage of the population have
cystic fibrosis?
– Genotype cc is q2, so:
– q2 = 1/1700 = 0.00059 = 0.059%
Allele Frequencies
• Then the frequency of the c allele is
– c = q = square root of 0.00059 = 0.024, or 2.4%
• Now, to find the frequency of C,
– C = p = (1 - q) = 1 - 0.024 = 0.976, or 97.6%
Genotype Frequencies
• Frequency of homozygous dominants (CC)
– p2 = 0.9762 = 0.953, or 95.3%
• Frequency of heterozygous condition (Cc)
– 2pq = 2(0.976 x 0.024) = 0.0468, or 4.68%
• Thus, out of 1700 people, 1620 are homozygous
dominant (CC), 79 are heterozygous carriers (Cc),
and 1 is homozygous recessive (cc)
Autosomal Chromosomes
• In humans, 22 autosomal chromosomes
• All chromosomes have a short (p) and long (q) arm
• When stained, distinct “bands” appear on the
chromosome
• Locations of genes identified by the chromosome
number, the arm, the region, and then the band
– E.g., 5p14 is chromosome 5, arm p, region 1, band 4
Chromosome 5
p arm
q arm
Sex Chromosomes
• Two chromosomes that differ for
males and females
• XX and XY
• Sex-determining region Y (SRY)
– Gene located on short arm of Y
chromosome
– Master switch triggering events
converting the embryo into a male;
without the gene, embryo is female
– Evidence: aneuploid humans with
karyotypes XXY, XXXY, even
XXXXY are all functionally male
– SRY transgenic XX karyotype mice
<http://users.rcn.com/jkimball.ma.ultranet
/BiologyPages/S/SexChromosomes.html>
Sex Linked Genes
• X chromosome carries hundreds of genes
• Few have anything to do directly with sex
• Special rules of inheritance because
– Males have only single X chromosome
– Almost all genes on X have no counterpart on Y, thus
– Any gene on X, even if recessive in females, will be
expressed in males
• Genes inherited in this fashion are called sexlinked, or X-linked
Hemophilia Example
• Blood clotting disorder
– Mutant gene encoding clotting factor VIII on X
chromosome
• With only one X chromosome, males who inherit
defective gene from their mother are unable to
produce factor VIII and are hemophiliacs
• For heterozygous female carriers, the normal copy
of the gene provides the needed factor VIII
More Exceptions to Mendel’s
Laws
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•
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Mutations
Chromosomal errors
Repeat sequences
Genomic imprinting
Mutations
• Many genetic diseases involve spontaneous
mutations
• Majority of mutations do nothing
• Of those that do something, practically all
are “bad” (i.e., create a disfunction from
“normal”)
• Only very, very occasionally does a
mutation confer a benefit to the individual
• We’ll come back to mutations in chapter 4
Chromosomal Errors
• Nondisjunction
– Failure to apportion chromosomes equally during
meiosis
– Generally leads to spontaneous abortion in first few
weeks post conception
• Down syndrome
– Three copies (trisomy) of chromosome 21 (one of the
smallest chromosomes)
• Monosomy (one copy) of a chromosome seems to
always be fatal (would be missing essential genes)
Repeat Sequences of DNA
• 1 to 4 nucleotide bases repeat up to a few
dozen times
– Don’t really know why these repeats occur
– Common and normal; perhaps up to 50,000
places in human genome
• Problem when number of repeats at a
particular loci increase beyond normal
range
Example: Huntington’s Disease
• Repeat of three bases (triplet repeat) on
chromosome 4
• Normal (non-Huntington’s) people have 1134 copies of triplet repeat
• Huntington’s allele has 36 or more
– Produces too much glutamine amino acid,
changing protein configuration
Genetic Anticipation
• Where symptoms appear earlier and with
greater severity across generations
• Repeats can expand over generations
• One explanation for genetic anticipation
Genomic Imprinting
• One gene from mother and one from father
• Imprinted genes
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–
–
–
Parental contributor matters
Gene will or won’t be active
Maternally imprinted = from father
Paternally imprinted = from mother
Example: Igf2
• Maternally imprinted (from father)
• Influences embryo growth
– Insulin-like growth factor --> bigger embryo
• Mother
– “Wants” large embryo, but not too large
• Father
– “Wants” largest embryo possible
– Cost to mother doesn’t affect father’s future
reproductive output
Example: Chromosome 15
Deletions
• If deletion inherited from mother, causes
Angleman syndrome
– Severe mental retardation, awkward gait,
inappropriate laughter
• If deletion inherited from father, causes
Prader-Willi syndrome
– Overeating, temper outbursts, depression,
obesity, short height
Multiple-Gene Inheritance
• Polygenetic traits
• Multiple genes interact to produce trait
• Each individual gene inherited according to
Mendelian laws
• But interactive effect of genes (and
environment)
Quantitative Dimensions
• These are continuously distributed traits
• Often approach a bell-curve
• Applied to many psychological and
biomedical traits
• Correlational statistics (0.0 to 1.0) used to
indicate resemblance between individuals