Chapter Three: The Biological Bases of Behavior

G. Heredity and Behavior: Is it All in the Genes?

To what extent are people's behaviors molded by their genes? This is the domain of behavioral genetics.

1. Basic Principles of Genetics
2. Investigating Hereditary Influence
3. The Interplay of Heredity and Environment

Contents

Basic Principles of Genetics

Chromosomes and Genes

Definitions:

• Chromosomes
• Zygotes
• Genes

Variability of characteristics among family members is due to:

• There are an infinite number of chromosome combinations (o.k., it's only 70 trillion).

• Mutations (changes in genetic material) contribute.

• Crossing over (interchange of material between chromosomes) adds zest.

Genes operate in pairs

• Homozygous condition
• Heteroygous condition
• The dominant gene overrides the recessive gene .
• Example figure 3.32

Genetic Relatedness

The likelihood of sharing the same genes -- members of a family share the most genes, then more distant relatives, and lastly non-relatives.

Genotype Versus Phenotype

• Different genotypes can produce the same phenotype;
• Genotype is fixed;
• Phenotypes can vary over time.

Polygenic Inheritance

Most of our behavior traits are polygenic traits

Investigating Hereditary Influence

Researchers have designed special types of studies to assess the impact of heredity.


Experimental techniques:

Selective breeding - manipulating genetic variability by assigning subjects to mate with each other. Works well with animals, but it is not ethical for human studies (besides we couldn't get anyone to volunteer to let us choose their future mate).

Correlational Studies: Family Studies

• Example -Family risk for schizophrenic disorders (figure 3.33)

• Results are confounded because environmental similarity increases as genetic similarity increases (Your parents can drive you crazy).

Correlational Studies: Twin Studies

There are two kinds of twins:

• Identical (monozygotic) twins have 100% genetic relatedness.

• Fraternal (dizygotic) twins have 50% genetic relatedness.

Logic of twin studies:

In both types of twins, the children grow up in similar environment, but the identical twins have greater genetic similarity.

Therefore
If identical twins exhibit more similarity on a trait than fraternal twins, we would suspect the greater trait similarity is due to heredity rather than the environment.

Identical twins do show greater similarity on intelligence and some personality traits (figure 3.34)

Correlational Studies: Adoption Studies

• If adopted children more closely resemble their biological parents on a trait, genetic factors are at play.

• If adopted children more closely resemble their adoptive parents, environmental factors are more likely.

• Your author presents adoption study research which indicates that both heredity and environment influence intelligence.

The Cutting Edge: Genetic Mapping

Molecular geneticists unravel the genetic code.


Genetic mapping is producing a genetic atlas.

Primer on molecular genetics.
Stanford Human Genome Center

However:

• Gene maps do not reveal which gene governs which traits.

• Most behavioral traits are polygenic so that constellations of genes will have to be identified.

The Interplay of Heredity and Environment

As you may have guessed by now, heredity and experience jointly influence most aspects of behavior.

		next section icon
Quiz Me	Terms	Next	Contents	Go Top