PowerPoint slides

Psychology 372

Behavioural Genetics

Behavioural Genetics

• Studies the role of genetics and environment in animal behaviour

• Intersects with psychology in general (especially developmental, abnormal), 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.”

• Rather overstated things, but was the beginning of behavioural genetics

• Family and twin study designs• Correlations and regressions• Both shared genes and shared

environment importantGalton 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 beggar-man 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

• Now seen more as nature via nurture

• Nature– The genes

• Nurture– Shared environment– Unique/non-shared environment

Heritability Estimates

• Trait 1: low heritability, high shared environment

• Trait 2: high heritability

• Trait 3: low heritability, low shared environment, high unique environment

1.0

0.8

0.6

0.4

0.2

0Trait 1 Trait 2 Trait 3

Cor

rela

tion

Correlation of Sibling Traitsin Shared Family Environment

Monozygous twinsBiological siblingsAdoptive siblings

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)

<encarta.msn.com/media_461543483/Human_Male_Karyotype.html><adapted from: www.accessexcellence.org/RC/VL/GG/human.php>

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

• 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 Hardy-Weinberg

A

a

A a

p = 0.6 q = 0.4

p = 0.6

q = 0.4

Spe

rm

Eggs

AA (p2)0.6x0.6 = 0.36

Aa (pq)0.6x0.4 = 0.24

aa (q2)0.4x0.4 = 0.16

Aa (pq)0.6x0.4 = 0.24

(p + q)2 = 1

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

p arm q arm

Chromosome 5

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 sex-linked, 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

• 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 11-34 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– 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