Class Notes - Chapter 3 – Biology and Behavior
I. Nature and Nurture
Both heredity and environment influence individuals’ characteristics.
Individuals differ from one another by only about 1 to1.5% of their genes.
Genetic and Environmental Influences
People are shaped by several interacting factors
See Figure 3.1 in Siegler et al., 2010
Genotype: the genetic material an individual inherits
Phenotype: the observable expression of the genotype, including body characteristics and
Environment: includes every aspect of the individual, and his or her surroundings, other
these 3 elements are involved in 4 relations that are fundamental in the development of
Four Fundamental Relations
1. Parents’ genetic contribution to the child’s genotype conan is a really tall
2. Contributions of the child’s genotype to his or her own phenotype
3. Contribution of the child’s environment to his or her own phenotype
4. Influence of the child’s phenotype on his or her environment
An example of the model: Conan is really tall & hair texture the way the genotype will
be expressed is influence by the environment
Relation 1: Parents’ and Child’s Genotypes
Genetic material is passed on by chromosomes
o Carry all the instructions involved in the formation and functioning of an
o Genes: sections of chromosomes; basic units of heredity for all living things; each
gene is a segment of DNA that is the code for the production of particular proteins
o Genes affect development and behavior only through the manufacture of proteins
o Sex chromosomes determine an individual’s biolrdical sex.
o Females have two X chromosomes in the 23 pair, whereas males have X Y.
o X chromosomes have 3x more genes than Y. more genes which can possibly
that we can override the genes that are sex-linked. Ex. Colour blindness
o It is always the father who determines the sex of the offspring because half of the
male’s sperm contain an X chromosome and half contain a Y. when a Y bearing
sperm fertilize an egg, offspring will be male, etc
o A gene on the Y chromosome encodes the protein that triggers the formation of
the testes, which produce testosterone, which produces male body features.
1 Why are children of the same two parents so genetically diverse?
- Several mechanisms contribute to genetic diversity among people:
Mutations: changes in sections of DNA caused by random or environmental factors;
those mutation that occur in germ cells can be passed on to offspring;
Crossing over: Sections of DNA switch from one chromosome to another during sex cell
production, further increasing genetic variability; as a result, some of the chromosomes
that parents pass on to their offspring are constituted differently from their own.
Random assortment: Chance determines which member of each pair of chromosomes
goes into the new sperm or egg; during germ-cell division, the 23 pair of chromosomes
are shuffled randomly, with chance determining which member of each pair goes into
each new egg or sperm; for each germ cell there are 2^23 or 8.4 million possible
combinations of chromosomes
Relation 2: Child’s Genotype and Phenotype
Although every cell in the body contains copies of all the genes received from parents,
not all are expressed
Only some of those a person’s genes are expressed at any one time in any given cell.
About a third of human genes have two or more different forms, known as alleles.
The alleles of a given gene influence the same trait or characteristic(eye color), but they
contribute to different developmental outcomes(brown, blue, hazel, gray eyes)
The dominant allele is the form of the gene that is expressed if present.
The recessive allele is not expressed if a dominant allele is present.
A person who inherits two of the same alleles for a trait is described as homozygous.
A person who inherits two different alleles for a trait is described as heterozygous.
Mendelian Inheritance Patterns
Sex linked inheritance:
- Suppose a woman inherits a recessive allele on the X chromosome from
her mother, chances are she will have a dominant allele on the X
chromosome from her father to suppress it.
- Suppose a man inherits the same recessive allele on the X chromosome
from his mother, chances are, because of the much smaller size of the Y
chromosome he inherited from his father, he will not have a dominant
allele to override it, so he will develop the trait
Polygenic Inheritance: When traits (phenotypic outcome) are governed by more than one gene
Applies to most traits and behaviours of interest to behavioral scientists
2 Relation 3: Child’s Environment and Phenotype
The environment plays a role in how the phenotype is expressed.
The phenotype is the unique consequence of a particular genotype developing in a
Example : (nutrition + weight ; EP)
The Case of MAOA (Environment + Genotype)
MAOA: sex linked gene, only from x chromosome; known to inhibit brain chemicals associated
Severe maltreatment increases likeliness of antisocial behavior
Stronger for those individuals who had a relatively inactive MAOA gene.
Video: aggressiveness and genetic
Environment and Phenotype: PKU
Children with phenylketonuria (PKU)—a chromosome 12 disorder —cannot process
A phenylalanine-free diet can prevent mental retardation from PKU. If normal meal,
phenylalanine accumulates in the bloodstream and will cause impaired brain development
Challenge: identify the contribution of genotypewhether the person has chromosome
12 disorder, phenotype how do the disorder manifest, and environment diet
Norm of Reaction: All the phenotypes possible from a single genotype across a range of
Genetic Transmission of Diseases and Disorders
Sex-Linked inheritance: male-pattern baldness, red-green color blindness, hemophilia,
Chromosomal anomalies: Down syndrome (trisomy 21), Kleinfelter syndrome (XXY),
Turner syndrome (XO)
Relation 4: Child’s Phenotype and Environment
Children are active creators of their environments in 2 important ways:
o They evoke certain kinds of responses from others.
o They select surroundings and experiences that support their interests, talents, and
Some very young children develop extremely intense interests in
particular kinds of objects or activities that do not stem from parental
Video: biological genetic temperament
Behavioural Geneticists ask:
Are there aspects of behaviour that depend on genes and combinations of genes?
If so, do experiences modify these characteristics?
3 Behavior genetics
- To the extent that genetic factors are important for a given trait or
behavior, individuals who are genotypically similar should be
- To the extent that shared environment factors are important,
individuals who have been reared together should be more similar
than people who have not