Human Development: Genetic Foundations

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Today’s Topics

• Mechanisms of Heredity

– Biology of Heredity

– Genetic Disorders

• Research Methods in Behavioral Genetics

• Gene x Environment Interactions

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The Process of Genetic Transmission

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Chromosome: threadlike

structure that contains the

genetic information DNA: the genetic substance

in chromosomes

Genes: segments of

DNA

The Process of Genetic Transmission

• Genes

–Carry codes for

production of

proteins

• Vital for cell

functions

–Duplicate

themselves

to Behavior

Cracking the Genetic Code

• The Human Genome

Human Development: Genetic Foundations

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The Process of Genetic Transmission

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Chromosomes, Cells, and Sex:

Terminology

Autosomes the 22 pairs of chromosomes that

are not sex chromosomes

Sex

chromosomes

23rd pair of chromosomes

determines sex

XX = female, XY = male

Gametes sex cells: sperm and ova

Zygote sperm and ovum united

The Process of Genetic Transmission

Gametes:

• Ovum

– Largest human cell

– The female germ cell (egg)

– 23 chromosomes (not pairs)

– 23rd is always an X

• Sperm

– Smallest human cell

– The male germ cell

– 23 chromosomes (not pairs)

– 23rd is either an X or a Y 7

Unite to form a single cell,

the Zygote, which has 23

pairs of chromosomes

Mother’s Genes

Father’s Genes

½ of each pair

½ of each pair

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Human Development: Genetic Foundations

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Review:

23 from Father

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23 from Mother

+ =

1 Zygote with 23 pairs

• Phenotype

– An organism’s

actual observable

characteristics

Genes and Behavior

• Genotype

– A particular set of

genes a person inherits

from his or her parents

Alleles

Two forms of the same gene

• Appear at the same place on both

chromosomes in a pair

• One inherited from each parent

• Homozygous – the two alleles are alike

• Heterozygous – the alleles differ

Dominant-Recessive

Inheritance

Human Development: Genetic Foundations

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Complex Heredity • Incomplete dominance

Complex Heredity

• Polygenic inheritance: Multiple genes 1 trait

– Complex behavioral dispositions

– Combination of several genes that ‘work’ together

– Environment plays a major role in gene expression

Genetic Disorders

• Inherited disorders

– Sickle-cell disease

– Huntington’s disease

• Abnormal number of chromosomes

– Down Syndrome

• X-linked Inheritance

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Inherited Disorders

• Sickle-cell disease – Problem is inherited on a recessive allele

– Two recessive alleles = sickle cell disease

– Incomplete dominance of normal “dominant” allele = sickle cell trait

• Huntington’s disease – Problem is inherited on a dominant allele

– Symptoms happen late in life, after most patients have reproduced & passed the trait on to a new generation.

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Human Development: Genetic Foundations

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Abnormal Number of Chromosomes

• Down Syndrome (Trisomy 21)

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Genotype Phenotype

X-linked Inheritance

• What happens if the gene is on a sex chromosome?

– Females: If there’s a “bad” recessive gene on one X, there’s probably a dominant gene on the other X to suppress it

– Males have only 1 X

• e.g., hemophilia

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X-Linked Inheritance Today’s Topics

Mechanisms of Heredity

Biology of Heredity

Genetic Disorders

• Research Methods in Behavioral Genetics

• Gene x Environment Interactions

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Human Development: Genetic Foundations

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Research Methods in Behavioral Genetics

• What is Behavioral Genetics?

– Branch of genetics that deals with inheritance of behavioral and psychological traits (p. 49)

• Family Resemblance Studies

• Twin Studies

– Monozygotic Twins

– Dizygotic Twins

• Adopted vs. Biological Children 21

Heritability

• Proportion of a trait’s variation in a population due to genetic factors

Heritability

= a statistical estimate of how much of the variability in

people‘s physical characteristics is explained by their genetic

makeup

Family Resemblance Studies

3 Factors:

• Genes

• Shared Environment

• Non-shared Environment

Human Development: Genetic Foundations

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Heritability

nonshared environment

shared environment

heritability

IQ adult

IQ child

Reading disability

Personality

Schizophrenia

Major Depression

Autism

Hyperactivity

• Monozygotic (MZ) vs. Dizygotic (DZ) Twins “Identical” “Fraternal”

Share 100% genes Share ~50% genes

Twin Studies

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Twin Studies

• The logic behind twin studies:

If identical twins (same genes) resemble each

other more than fraternal twins (different

genes) do

trait is influenced more by genes

If identical twins (same genes) resemble each

other the same as fraternal twins (different

genes) do

trait is influenced more by the

environment 28

Human Development: Genetic Foundations

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Twins reared together vs. apart

This keeps genes constant & changes the environment.

Separated Identical Twins

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A number of studies compared identical twins reared separately from birth, or shortly thereafter, and found numerous similarities.

Personality, intelligence, abilities, attitudes, interests, fears, brain waves, heart rates, blood pressure

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Biological Versus Adoptive Relatives

• Logic behind adoption studies

resemblance to biological parents

due to genes

resemblance to adoptive parents

due to environment

Biological Versus Adoptive Relatives

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Adoption studies suggest that adoptees (who are biologically unrelated to their families) tend to be

different from their adoptive parents and siblings

more similar to biological parents on a number of characteristics (e.g., personality & intelligence)

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Sibling Studies

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However, even biologically related children turn out to be very different from one another, even when raised in the same family.

So investigators ask:

Do siblings have different experiences within the same family? (nurture)

Even though biological siblings have 50% of the same genes, what effect does the other 50% of non-shared genes have? (nature)

Shared & Nonshared Environments

Ultimate question: What is the effect of parenting?? (nurture)

Parenting does have an effect on both biologically related and unrelated children.

Influences of the Environment

Parenting has greatest influence on children’s

Attitudes, Values

Manners, Beliefs

Faith, Politics

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Small Group Discussion: Which trait is most influenced by heritability?

Shared environment? Non-shared environment?

Twin & Adoption Studies Problems

• Shared environment overestimated

• Same genes, different expression

• Complex genetic interactions ignored

• Based on faulty assumptions

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–Range of reaction (graph)

–What’s not explained by heritability/genes is

usually attributed to the environment

Influences of Genes vs. the

Environment

–Range of reaction

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Influences of Genes vs. the Environment

Gene X Environment Interactions

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Genes can influence traits which affect responses, and environment can affect gene activity.

Nature via Nurture

• genetic predispositions can

– influence how others respond to us.

– drive us to select and create particular environments.

Human Development: Genetic Foundations

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The Epigenetic Framework