Chapter 2: Genetics

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Chapter 2 Genetics

• Genetic Foundations
• Heredity Environment

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Genetic Foundations

• Chromosomes limits
• Each of us carries a genetic code that we
  inherited from our parents. Because a fertilized
  egg carries this human code, a fertilized human
  egg cannot grow into an egret, eagle, or
  elephant.

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Genetics and Behavior
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Genes Our Biological Blueprint

• Human Genome Project
• Initial phase completed about the year 2000
• Humans have 20,000 25, 000 genes (21,667)
• There are far more proteins than genes 10-20
  million
• Genes (DNA) are dependent- collaborate with other
  sources of information
• Gene expression/activity is affected by context
  or environment
• Context is affected by hormones, light,
  nutrition, etc.

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The Sex Cells

• Sex cells are formed by meiosis rather than
  mitosis.
• Gametes (sperm and ova) have only 23 chromosomes
  total.
• At conception, these two unite resulting in a
  full complement of 46 chromosomes (23 pairs).
• A fertilized egg is called a zygote.

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Sources of Genetic Variation

• Alleles are normal variations of a gene, found at
  the same location.
• A child who inherits the same allele (type of
  gene) from both parents is homozygous for that
  trait.
• A child who inherits different alleles from each
  parent is heterozygous for that trait.

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Sources of Variation

• Genetic Expression
• Influenced by the environment
• hormones
• light
• nutrition
• behavior
• stress (cortisol may cause a fivefold increase
  in DNA damage)

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Sources of Genetic Variation

• Patterns of Genetic Inheritance
• Dominant-recessive the dominant gene (allele)
  will determine the characteristic

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Patterns of Genetic InheritanceDominant-recessive
inheritance

• Examples of dominant genes
• Dark hair, curly hair, dimples, types A B blood
  (vs. type O), traits for normality in vision,
  hearing, pigmentation, etc.
• Huntingtons Disease

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Patterns of Genetic InheritanceDominant-recessive
inheritance

• Examples of recessive genes
• Cystic fibrosis, PKU, Tay-sachs disease.
  Sickle-cell anemia

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Patterns of Genetic InheritanceCo-dominance and
Additive

• Co-dominance both alleles contribute to the
  phenotype, although not to the same degree.
• Additive They contribute about equally
  (50-50).
• Example of Co-dominance
• Sickle-cell anemia

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X-linked (sex-linked) inheritance

• Female children receive an X chromosome from the
  father which matches locations on the mothers X.
• Male children receive a Y from the father, which
  does not have all the gene locations of an X.
• The defective gene on the mothers X is offset by
  the gene on the normal X in females, but not in
  males.
• So, males will show evidence of the defective
  gene (e.g., hemophilia, RG colorblindness).
• Females will be normal, but carriers of the
  defective gene.

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Polygenic Inheritance

• Many genes interact to influence the
  characteristic
• Most psychological characteristics are polygenic
• (Where environmental factors are included, traits
  are said to be multi-factorial.)

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Chromosomal Abnormalities

• Usually happen during meiosis
• Involve breakage and failure to separate
• Usually result in miscarriage
• Those most commonly survived are
• Down syndrome (trisomy 21)
• Sex-linked abnormalities

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Sex Chromosome Abnormalities

• XXY (Klinefelter) may have verbal difficulties.
  Tall, underdeveloped testes, possible breasts.
  1/800 live male births.
• XO (Turner) have trouble with math and spatial
  skills. Short and have webbed neck may be
  infertile. 1/2500 live female births
• XYY (Are they more aggressive, antisocial?)

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Gene-linked Abnormalities

• Over 7000 known (most rare), including
• Cystic fibrosis
• Diabetes
• Hemophilia
• Huntington
• PKU (phenylketonuria)
• Sickle-cell anemia
• Spina bifida
• Tay-sachs disease

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Genetic Counseling for whom?

• Family history of disease, mental retardation,
  physical defects
• History of miscarriages
• Mother over age 35 (rate of abnormality begins to
  rise sharply)

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Prenatal Diagnostic Methods

• May cause miscarriage (except ultrasound,
  maternal blood samples)
• Is the problem correctible?
• Genetic engineering is still in the future.
• Often the only decision is whether or not to
  abort the fetus.

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Prenatal Diagnostic Methods

• Chorionic villi sampling (6-8 weeks)
• detects genetic defects risk of miscarriage,
  limb deformity
• Amniocentesis (11 weeks, best after 15 weeks)
  detects genetic defects smaller risk of
  miscarriage

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Infertility

• 1 in 6 couples in U.S.
• Waiting too late
• Sexually transmitted diseases
• Fertility technology (IVF, donors)
• Adoption
• Babies culturally unavailable

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Environmental Influence
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Environmental Influence

• Rats reared in an environment enriched with
  playthings show increased development of the
  cerebral cortex

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Twins

• Identical Twins
• develop from a single fertilized egg that splits
  in two, creating two genetically identical
  organisms
• Fraternal Twins
• develop from separate eggs
• genetically no closer than brothers and sisters,
  but they share a fetal environment

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Multiple Births fraternal twins

• Dizygotic (two zygotes)
• Share approximately 50 of their genetic heritage
  like any two siblings.
• Major causes are maternal age and fertility
  drugs.
• Twinning dramatically on the increase since the
  1970s.

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Multiple Births identical twins

• Monozygotic one zygote (same fertilized egg)
• Share 100 of genetic heritage
• Occurs about 3 per 1000 live births worldwide
• Factors may include temperature and oxygen levels
  and late fertilization

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Genetics Research

• Behavior Genetics
• study of the power and limits of genetic and
  environmental influences on behavior
• Molecular Genetics
• subfield of biology that studies the molecular
  structure and function of genes

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Nature-nurture Research

• Molecular genetics
• Human Genome Project
• Behavioral genetics
• Twin studies
• Equal environment assumptions
• Adoption studies
• Concordance rates

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Epigenesis ongoing nature/nurture exchanges
(bi-directional)

• Reaction range
• Canalization
• Genetic-environmental correlation
• Passive
• Evocative
• Active (niche-picking)