Summary book ‘behavioral genetics’
Chapter 2: historical perspective
Lamarck law of use and disuse: argued that the deliberate efforts of an animal could result in
modifications of the body parts involved incorrect view
Charles Darwin beagle led him to observe the remarkable adaptations of species to their
environment finches: differ in beaks each beak was exactly appropriate for the eating habits of
the species = argument of design = adaptation of animals and plants to the circumstances of their
lives as evidence of the Creator’s wisdom.
Darwin’s theory of evolution natural selection
- variation in a population
- heredity
- surviving + reproducing survival of the fittest
There were gaps in his theory: heredity, the gene was not yet understood (Mendel provides this
answer)
Francis Galton hereditary of mental characteristics
- introduced the use of twins to assess the roles of nature and nurture
- accepted Darwin’s theory
- human differ from animals most in mental ability
- nature prevails enormously over nurture heredity underlies behavior variation
Pre-mendelian concepts of heredity and variation
Darwin + Galton didn’t understand the mechanism by which heredity works
- Heredity: provisional hypothesis of pangenesis = Gemmules, miniature replicas of the parent cells,
were presumably thrown off by each cell throughout its course of development. In embryogenesis
and later development, gemmules from the parents, originally thrown off during various
developmental periods, would come into play at the proper times, thus directing the development of
a new organ like that of the parents.
quite reasonable (although it was wrong). It was particularly compelling because it was
compatible with Lamarck’s notion of “use and disuse” as the source of variation in evolution.
- Variation: blending hypothesis = variation would be greatly reducing each generation by blending
(in fact: halved). One parent tall, other short offspring on average
Darwin struggled
Chapter 3: mendel’s law and beyond
Mendel’s law
Family trees = pedigrees
Huntington disease affected parents, one parent has the disease: caused by dominant allele
from one generation to the next (50% change to develop)
PKU do not usually have affected parents, parents are genetically related two recessive alleles,
offspring must have both, is rare
,1. First law = law of segregation = two elements, by reproduction a child receives one of the two
elements from each parent can dominate or recessive (research with plants)
Mendel’s elements are now known as genes
2. Second law = independent assortment: inheritance of one gene is not affected by the inheritance
of another gene
- genes are carried on chromosomes Genes are located on loci
- human: 23 pairs of chromosomes
- inheritance of two traits at same time
Exceptions (not inheritance independent) linkage + crossover during meiosis
- occurs when two genes are closely linked on the same chromosome
- linkage = inherited together and reside on the same chromosome
- crossover occurs by closeness
Chromosome: A threadlike structure that contains DnA and resides in the nucleus of cells. Humans
have 23 pairs.
Locus (plural, loci): The site of a specific gene on a chromosome. Latin for “place.”
Linkage: Loci that are close together on a chromosome and thus inherited together within families.
Linkage is an exception to Mendel’s second law of independent assortment.
Recombination: A process that occurs during meiosis in which chromosomes exchange parts.
Gene: Basic unit of heredity. A sequence of DNA bases that codes for a particular product.
Allele: Alternative form of a gene.
Genotype: An individual’s combination of alleles at a particular locus.
Phenotype: observed or measured traits.
Dominant allele: An allele that produces the same phenotype in an individual regardless of whether
one or two copies are present.
Recessive allele: An allele that produces its phenotype only when two copies are present
,Beyond Mendel’s law
Complex traits (quantitative traits) more genes are involved. Most psychological traits show
patterns of inheritance that are much more complex than those of Huntington disease or PKU:
- Schizophrenia
A special incidence figure used in genetic studies is called a morbidity risk estimate (also called the
lifetime expectancy), which is the chance of being affected during an entire lifetime. If you have a
second-degree relative (grandparent or aunt or uncle) who is schizophrenic, your risk for
schizophrenia is about 4 percent, four times greater than the risk in the general population. If a first-
degree relative (parent or sibling) is schizophrenic, your risk is about 9 percent.
Clearly, the risk of developing schizophrenia increases systematically as a function of the degree of
genetic similarity that an individual must another who is affected.
- Intelligence = general cognitive ability
parents with high intelligence test scores tend to have children with higher-than-average scores. As
with schizophrenia, transmission of general cognitive ability does not seem to follow simple
Mendelian rules of heredity.
correlation
Multiple-gene inheritance
- Huntington disease + PKU = single gene
- more than one gen (complex disorders) such as schizophrenia and continuous dimensions as
intelligence polygenic trait
Quantitative genetics multiple gene
Theoretically, there should be a continuum of genetic risk, from people having none of the alleles
that increase risk for schizophrenia to those having most of the alleles that increase risk. liability-
threshold model
, Color blindness is a complex trait
Summary:
HD = dominant & PKU = recessive. They follow the basic rules of heredity described by Mendel. A
gene may exist in two or more different forms (alleles). The two alleles, one from each parent,
separate during gamete formation. This rule is Mendel’s first law, the law of segregation.
Mendel’s second law is the law of independent assortment: The inheritance of one gene is not
affected by the inheritance of another gene. However, genes that are closely linked on the same
chromosome can co-assort, thus violating (= overtreden) Mendel’s law of independent assortment.
Such violations make it possible to map genes to chromosomes by using linkage analysis. For
Huntington disease and PKU, linkage has been established and the genes responsible for the
disorders have been identified.
Mendel’s laws of heredity do not explain all genetic phenomena. Genes on the X chromosome, such
as the gene for color blindness, require an extension (=verlenging) of Mendel’s laws. Further, most
psychological dimensions and disorders show more complex patterns of inheritance than do single-
gene disorders such as Huntington disease, PKU, or X-linked conditions such as color blindness.
Complex disorders such as schizophrenia and continuous dimensions such as cognitive ability are
likely to be influenced by multiple genes as well as by multiple environmental factors. Quantitative
genetic theory extends Mendel’s single-gene rules to multiple-gene systems. The essence of the
theory is that complex traits can be influenced by many genes, but each gene is inherited according
to Mendel’s laws. Quantitative genetic methods, especially adoption and twin studies, can detect
genetic influence for complex traits.
Chapter 6: nature, nurture, and human behavior
Most behavioral traits are more complex than single-gene disorders
- mostly multiple genes and environmental
- describe genetic effects on complex behavioral traits by genes and environment
- quantitative genetics = estimates the extent to which observed differences among individuals are
due to genetic differences of any sort and to environmental differences of any sort without specifying
what the specific genes or environmental factors are.
Investigating the genetics of human behavior (which role plays genes in complex traits)
