Behavioural Genetics

• Heredity prior to Mendel


  Blending hypothesis, pangenesis theory, germ plasm theory.
• Blending Hypothesis


  Parents' characteristics merge in their offspring, though this would decrease variation over time.
• Weismann'sgerm plasm theory


  Only germ cells in the gonads (ovaries and testes) pass on heritable characteristics, and somatic cells cannot. An experiment where mice whose tails were cut off had offspring with tails, showing that you can't inherit acquired traits from somatic cells.
• Galton's work


  - Studied 1,000 eminent (intelligent) men, founf higher eminence when more closely related to an eminent man.

  - Also studied twins: "nature prevails enormously over nurture".

  - Founded regression to the mean: "extreme outcomes tend to be followed by more moderate ones", e.g., taller parents have more average height offspring.
• Galton's rabbit tranfusion emperiment


  Thought gemmules were transmitted in blood. Transmitted blood between rabbits with different characteristics, expecting to find their own and their offpsring's characteristics to change, which they didn't.
• Lamarck's theory of evolution


  Offspring inherit characteristics that their parents acquired due to their behaviour. e.g., a giraffe's neck lengthening and offspring inheriting long neck.
• Fisher's paper 1918


  Integrated Mendelian and biometrical inheritance models (studying continuous traits) via a new statistical concept - variance.
• Variance


  Mean of the squared deviation, measures the degree of dispersion in scores
• Eugenics


  - Movement encouraging breeding in those with desirable traits, and reducing in those with undesirable traits.

  - Boomed after the large migration after industrial revolution.

  - People searching for answer to solve social problems.

  - Influenced US public policy on sterilisation laws.
• Marie Stopes


  British birth control pioneer.
• Margaret Sanger


  Founded Planned Parenthood in US.
• Buck vs. Bell (1927)


  - Virginia Eugenical Sterilisation Act allowing sterilisation of inmates in institutions.

  - Carrie Buck forced to be sterilised on the grounds of epileptic fits and 'feeble-minded', after becoming pregnant after being raped.

  - Forced sterilisation laws from 1907-1963, with over 64,000 sterilisations.
• The Kallikaks, Henry Goddard (1912)


  - Published story that demonstrated the rationale for eugenics movement. 'Normal' (kallos) line descended from Quaker woman, and 'degenerate' (kakos) line descended from barmaid.

  - Printed in Germany in 1914, then re-printed in 1933.
• 1960s behavioural genetics work


  - Twin/family designs

  - Fuller & Thomson wrote monograph in 1960s.
• 3 areas of behaviour genetics

  - General psychology and social sciences.

  - Clinical psychology, psychiatry, and genetic epidemiology.

  - Evolutionary biology, psychobiology, and animal behaviour.
• Modern controversies


  - Around racial and social class differences

  - Jensen 1969 published article claimed compensatory education for lower income citizens didn't work + racial differences in intelligence.

  - Hernstein & Murray 1994's The Bell Curve, suggested social class differences

  - President of Behavioural Genetics Association 1995, claimed intelligence, aggression ,etc., differences between races are cause by genetic differences.
• WAIS (IQ test) standardisation sample (1981)


  - More variation within groups than between groups

  - Intelligence measured in technical vs unskilled jobs, with roughly same variation as in entire population, despite differences in mean.
• How much DNA varies between individuals?

  0.1%
• Genetic variation found within and between continental groups?


  ~85-90% genetic variation found within continental groups, with only 10-15% more variation found between.

  ~90% genetic variation can be seen in a single continent.
• Mendel's pea plant experiments


  - Method: collect pollen from male's anthers, remove stamens before self-fertilisation, add pollen from stamen to pistil (female)

  - Cross-fertilisation experiments of garden peas with different discrete characteristics

  - 34 varieties of true-bred same species

  0 7 pairs of phenotypic traits (seed/pod colour, shape, stem height)
• Medel's laws of heredity


  1. Principle of segregation2. Principle of independent assortment
• Mendel's monohybrid crosses


  - Single trait crossing of smooth SS and wrinkled ss seeds

  - Cross 2 purebred (SS, ss), then cross first generation (Ss, Ss), and second generation shows 3:1 ratio (SS, Ss, Ss, ss)
• Mendel's dihybrid crosses


  - Two pairs of alternative traits crossed

  - 9:3:3:1 ratio of smooth yellow:smooth green: wrinkled yellow:wrinkled green

  - SSYY x ssyy separated into haploid gametes SY and sy

  - F1 of SsYy and SsYy
• Principle of segregation


  - Recessive traits that are not seen in first generation reappear in second generation.-Alleles separated in the formation of gametes, where each half carries an allele each.
• Principle of independent assortment


  - Genes for different traits can segregate independently during the formation of gametes

  - Alleles of one gene sort into gametes independently of another gene

  - Violated when genes loci are close on same chromosome
• When were cells discovered


  - 17th century

  - 1840s observed nucleus divided first

  - 1870s discovered nuclei of two gametes fuse during fertilisation
• 3 regularities about chromosomes


  1. Nucleus of somatic cells contains fixed number of chromosomes for species

  2. Chromosomes in nuclei of somatic cells usually in pairs/diploid

  3. The germ cells produce gametes with nuclei that containing set of chromosomes/ are haploid
• What does meiosis produce?


  4 haploid daughter cells that are not identical
• What does mitosis produce?

  2 diploid identical cells
• 2 sources of genetic diversity


  1.Crossing over:occurs in prophase 1, where a pair of homologous chromosomes cross over at a chiasma, exchanging genetic material.2.Independent assortment:genes independently separate during meiosis
• When were chromosomes discovered?

  1880s by Fleming, using cell microscopy.
• Chromosome theory of inheritance

  - 1902, Boveri & Sutton saw similarities between chromosome transmission and Medelian inheritance.

  - Supported by Morgan's experiment
• Morgan's experiment

  - Experimental support from sex chromosome linkage in drosophila (fruit flies)

  - Also has XY chromosomes (XX = female, XY = male)

  - White eyes lined to Y chromosome

  - Crossed red-eyed male XWY, and white-eyed female XwXw.

  - White-eyed females only produced white-eyed males

  - When both parents had red eyes, all offspring did too (all had XW from female)
• Autosome


  Non-sex chromosome
• Polygenic trait


  - A trait influenced by 2 or more genes (traits with additive effects)

  - A higher frequency of alleles associated with a gene increases the trait (e.g., height intelligence)
• Central limit theory


  Sum of a large number of independent random quantities (multiple genes and environmental factors) is bound to converge to a normal distribution.
• Infinitesimal model


  The character is determined by an infinite number of unlinked and non-epistatic loci, each with an infinitesimal effect.
• P = G + E


  Phenotypic value = Genotypic value + Environmental deviation
• Contributions to genetic variance

  d = 0 -> additive influence to give heterozygote trait

  d = a -> complete dominance of A over a

  d =/ 0 -> some degree of dominance

  
• Genetic relatedness


  - Correlation coefficients between pairs of relatives allow estimations of the % variance due to heritable factors.

  - Phenotypic resemblance should increase as relatedness increases, if trait is influenced by genes.