Genetics and Behavioral Genetics — Page-by-Page Summary

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  • Genotype: genetic makeup of an individual

  • Phenotype: expression of genotype in observable or measurable characteristics

  • Interaction: genes and environments drive phenotype

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  • Mechanisms of Genetic Transmission

  • Genes: basic units of inheritance

  • DNA: deoxyribonucleic acid

  • Chromosomes: located in the cell nucleus

  • Cell types: Somatic/body cells; Gametes/sex cells

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  • Humans have homologous chromosome pairs; for each type, one chromosome from the mother and one from the father

  • Homologous chromosomes carry the same genes in the same order

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  • A homologous pair of chromosomes

  • Chromosome from mother; Chromosome from father

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  • A homologous pair of chromosomes

  • A duplicated chromosome (two sister chromatids)

  • Duplicated chromosomes can come from either parent (as shown: from father/mother)

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  • Gametes: Egg (ovum) and Sperm

  • Each gamete has 2323 chromosomes

  • Fertilisation forms a Zygote with 4646 chromosomes in 2323 pairs

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  • Somatic/body cells: MITOSIS

    • Prophase; Metaphase; Anaphase; Telophase

    • Duplicated chromosome with two sister chromatids

  • Gametes/sex cells: MEIOSIS

    • Meiosis I and II; crossing over at chiasma

    • Reduction division: from diploid to haploid

  • Key concepts: Parent cell duplication; Homologs separate in meiosis I; Sister chromatids separate in meiosis II

  • Example: 2n = 6; after meiosis I/II, n = 3; 2n=6<br>ightarrown=32n = 6 <br>ightarrow n = 3

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  • MEIOSIS I: Prophase I (crossing over at chiasmata), Metaphase I (homologs align), Anaphase I (homologs separate), Telophase I

  • MEIOSIS II: similar to mitosis but with haploid cells; Sister chromatids separate

  • Result: four haploid daughter cells; genetic variation due to crossing over

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  • Crossing over during meiosis occurs at chiasmata

  • Increases genetic variation among gametes

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  • Locus: position of a gene on a chromosome

  • Alleles: different forms of a gene

  • Example: eye color locus with alleles on maternal and paternal chromosomes

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  • Homozygous vs. Heterozygous

  • Example: Hair texture with alleles C (curly) and c (straight)

  • Four possible parental genotype combinations

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  • Homozygous: CC (curly) or cc (straight)

  • Curly from each parent yields CC; straight from each yields cc

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  • Heterozygous: Cc or cC

  • Phenotype depends on dominance; if curly is dominant, Cc shows curly

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  • Three expressions of heterozygous traits: 1) Intermediate, 2) Combined, 3) Dominance

  • Rare fourth: Genomic imprinting

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  • With dominance: Heterozygous (Cc) and homozygous dominant (CC) share the same phenotype (curly)

  • Recessive phenotype appears only with cc (straight)

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  • Dominant traits: curly hair, brown eyes, dark hair, normal color vision, immunity to poison ivy, normal hearing, normal blood clotting, normal metabolism

  • Recessive traits: straight hair, gray/green/blue/hazel eyes, light hair, color blindness, myopia, PKU, sickle-cell anemia

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  • Three main expressions of heterozygous combinations: Intermediate, Combined, Dominance

  • Fourth rare possibility: Genomic imprinting (Prader-Willi Syndrome / Angelman Syndrome)

Page 18

  • PKU: single-gene recessive disorder; autosomal recessive

  • Genetic variants: P (normal) and p (PKU allele)

  • Phenotypic outcome: phenylalanine compounds accumulate when pk allele is present in homozygous form (pp); normal when at least one P allele

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  • Punnett-style cross example for PKU: Pp × PP or Pp × Pp scenarios illustrate carrier and affected outcomes

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  • Huntington's Disease: single-gene dominant disorder

  • Genotypes: Hh or HH affected; hh is normal

  • Cross example: Hh × hh yields 50% affected offspring

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  • Achondroplasia: single-gene dominant disorder; FGFR3 gene

  • About 80% of affected individuals have unaffected parents due to a new mutation

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  • Sex determination and sex-linked characteristics

  • 22 autosomes; 23rd chromosome determines sex

  • Female karyotype: XX; Male karyotype: XY

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  • Offspring sex determined by Dad’s sperm

  • If sperm carries X, offspring is female; if it carries Y, offspring is male

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  • Transmission of sex-linked characteristics

  • X-linked patterns; typically more impact on males; females can be carriers

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  • Case 1: Affected father (XaY) and Normal mother (XX)

  • Daughters: XaX (carriers); Sons: XY (unaffected)

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  • Case 2: Normal father (XY) and Carrier mother (XaX)

  • Daughters: 50% XA-? outcomes: XaX (carrier) or XX (normal)

  • Sons: 50% affected (XaY) or normal (XY)

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  • Case 3: Affected father (XaY) and Carrier mother (XaX)

  • Daughters: 50% affected (XaXa) or carrier (XaX)

  • Sons: 50% affected (XaY) or normal (XY)

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  • Case 4: Normal father (XY) and Affected mother (XaXa)

  • Daughters: all XaXa (affected)

  • Sons: all XaY (affected)

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  • When will daughters show the disorder?

  • In X-linked recessive cases, daughters show disorder if they inherit the mutant Xa from both parents (rare); more commonly, they are carriers or unaffected

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  • Chromosomal Abnormalities: Down Syndrome (Trisomy 21)

  • Nondisjunction leading to an extra chromosome on chromosome 21

  • Other examples illustrated (not detailed here)

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  • Klinefelter’s Syndrome: (47, XXY)

  • Features include male with extra X chromosome

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  • Turner Syndrome: (45, X)

  • Affects females; monosomy X

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  • Behavioral Genetics: Genetic and environmental contributions to individual differences

  • Why do people differ? Multifactorial; polygenic

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  • Variance and Heritability concepts

  • Heritability is the proportion of behavioral variability due to genetic factors

  • Notation: h2=racextgeneticvarianceextgeneticvariance+extenvironmentalvarianceh^2 = rac{ ext{genetic variance}}{ ext{genetic variance} + ext{environmental variance}}

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  • Heritability: the proportion of variability in a behavior attributable to genetic factors; depends on population and environment

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  • Definition of Heritability: h2=racextgeneticvarianceextgeneticvariance+extenvironmentalvarianceh^2 = rac{ ext{genetic variance}}{ ext{genetic variance} + ext{environmental variance}}

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  • Behavioral Genetics (BG) Methods

  • Family studies; Twin studies; Adoption studies; Combination twin/adoption studies

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  • BG Methods: Family Studies

  • % Genes in common: 1st degree relatives 50%; 2nd degree 25%; 3rd degree 12.5%; Unrelated 0%

  • Familial influence typically: r1st > r2nd > r3rd > unrelated

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  • BG Methods: Family Studies – Correlation patterns for cognitive abilities show decreasing correlations with lesser genetic relatedness

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  • BG Methods: Twin Studies

  • Identical/MZ twins share 100% of genes; Fraternal/DZ twins share ~50%

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  • Twin Study illustrations: Identical (MZ) vs Fraternal (DZ) twins

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  • BG Methods: Twin Studies – Dizygotic vs Monozygotic twin development and chorionic/amnionic arrangements

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  • BG Methods: Twin Studies

  • If resemblance is due to genes, rMZ > rDZ; if rDZ ≈ 1/2 rMZ, genetics contribute strongly

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  • Example correlations across traits (median):

  • Height: MZ 0.930.93, DZ 0.480.48

  • Weight: MZ 0.910.91, DZ 0.580.58

  • Ridge count: MZ 0.960.96, DZ 0.490.49

  • Siblings: ~0.51; Parent-child: ~0.46

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  • BG Methods: Twin Studies – Genetic influence: rMZ > rDZ

  • Shared environmental influence: rDZ > 1/2 rMZ; Nonshared environmental influence: r_MZ < 1

  • If resemblance is due to genes, rDZ ≈ 1/2 rMZ

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  • Genetic and environmental influences can be assessed in home vs. lab settings; examples show higher correlations in home environments for some traits

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  • Equal Environments Assumption

  • If mislabelled twins (MZ treated as DZ or vice versa) bias results: MZ mislabelled as DZ may appear less alike; DZ mislabelled as MZ may appear more alike

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  • BG Methods: Adoption Studies

  • Genes in common: Biological parent-adopted child ≈ 50%; Adoptive parent-adopted child ≈ 0%

  • Genetic influence: r(biological parent–adopted child) > 0; Shared environment: r(adoptive parent–adopted child) > 0

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  • Adoption Studies designs:

  • 1) Parent-offspring Design: Biological parent share genes; Adoptive parent share environment

  • 2) Adopted/Nonadopted Sibling Design: Nonadopted siblings share genes; Adopted siblings do not; compares genetic vs environmental influence

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  • BG Methods: Combination Twin/Adoption Studies

  • Groups: Identical twins reared together (MZT) = 100% genes; Identical twins reared apart (MZA) = 100% genes; Fraternal twins reared together (DZT) = 50%; Fraternal twins reared apart (DZA) = 50%

  • Genetic influence: rMZ > rDZ or rMZA > 0; Environmental influence: rtwins reared together > r_twins reared apart

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  • Minnesota Study of Twins Reared Apart: Trait correlations show substantial genetic influence across traits such as Finger Ridge Count, Height, Weight, IQ, Personality

  • Which 2 variables show no shared environments influence

    • Finger ridge count and personality


Page 52

  • Genotype-Environment Interactions

  • Genetically based variations in individuals’ responsiveness to environments (range of reaction)

  • Examples: Juan, Tony, Freddie across different environments

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  • Case example: Maltreatment in childhood and depression moderated by 5-htt gene (Caspi et al., 2003)

  • Genotypes s/s, s/l, l/l show different probabilities of major depression depending on maltreatment exposure

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  • Genotype-Environment Correlations

  • Correlation between a person’s genotype and the environments they experience

  • correlation between person genotype and type of environment a person experiences is crucial for understanding how genetics can influence behavior. There are three primary types of genotype-environment correlations: passive, evocative, and active. These correlations illustrate how individuals might inherit specific gene patterns alongside their environmental contexts, shaping their developmental pathways. The passive correlation occurs when parents pass on their genes to their children along with the environments they create, while evocative correlations arise when individuals elicit certain responses from their environment based on their genetic predispositions. Lastly, active correlations involve individuals seeking out environments that match their genetic tendencies, further influencing their behavior and life experiences.

  • Types: Passive (infants young passive gene environment reflects parental traits) , Evocative (reactive), Active

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  • Molecular Genetic Methods

  • Genome-wide association studies (GWAS): identify SNPs associated with traits

  • Single-nucleotide polymorphisms (SNPs)

  • Gene hunting here

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  • Genome-wide Complex Trait Analysis (GCTA) uses snips to estimate the proportion of phenotypic variance that can be attributed to additive genetic variance in complex traits.

  • Purpose: estimate heritability from SNP data

  • Terms: GCA (genetic component/analyzed aggregate)

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  • Epigenesis

  • The environment can influence gene expression via methylation

  • Example: licking/grooming affects NGFI-A and GR expression, altering corticosterone and anxiety

  • High licking: higher GR expression; low corticosterone; lower anxiety; Low licking: lower GR expression; higher corticosterone; higher anxiety