Psychology 372 Behavioural Genetics Behavioural Genetics Studies the

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 1960 s • Environmental control over behaviour with no or minimal genetic influence

Nature and Nurture • Back to 17 th 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 Correlation • Trait 1: low heritability, high shared environment • Trait 2: high heritability • Trait 3: low heritability, low shared environment, high unique environment 1. 0 Correlation of Sibling Traits in Shared Family Environment 0. 8 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 • • • 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 = p 2 + 2 pq + q 2

Punnett Square and Hardy. Weinberg Eggs A Sperm A a a AA Aa (p 2) 0. 6 x 0. 6 = 0. 36 (pq) 0. 6 x 0. 4 = 0. 24 Aa aa (pq) 0. 6 x 0. 4 = 0. 24 (q 2) 0. 4 x 0. 4 = 0. 16 p = 0. 6 q = 0. 4 (p + q)2 = 1 p = 0. 6 q = 0. 4 p 2 + 2 pq + q 2 = 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 q 2, so: – q 2 = 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) – p 2 = 0. 9762 = 0. 953, or 95. 3% • Frequency of heterozygous condition (Cc) – 2 pq = 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. , 5 p 14 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 /Biology. Pages/S/Sex. Chromosomes. 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 • • 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 – Parental contributor matters – Gene will or won’t be active – Maternally imprinted = from father – Paternally imprinted = from mother

Example: Igf 2 • 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