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.
• Each person has the same set of genes
• Differences from slight variations (i.e., brown or red hair)
Genetic and Environmental Influences
• 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
Four Fundamental Relations
1. Parents’genetic contribution to the child’s genotype
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
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
• Sex chromosomes determine an individual’s biological sex.
• Females have two X chromosomes in the 23 pair, whereas males have X Y.
• X chromosomes have 3x more genes than Y.
• Agene on the Y chromosome encodes the protein that triggers the formation of the testes,
which produce testosterone, which produces male body features.
Why are children of the same two parents so genetically diverse?
• Mutations: changes in sections of DNAcaused by random or environmental factors
• Crossing over: Sections of DNAswitch from one chromosome to another during sex cell
production, further increasing genetic variability
1 • Random assortment: Chance determines which member of each pair of chromosomes
goes into the new sperm or egg
Relation 2: Child’s Genotype and Phenotype
• Only some of those a person’s genes are expressed at any one time.
• About a third of human genes have two or more different forms, known as alleles.
• 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.
• Aperson who inherits two of the same alleles for a trait is described as homozygous.
• Aperson who inherits two different alleles for a trait is described as heterozygous.
Mendelian Inheritance Patterns
Polygenic Inheritance: When traits are governed by more than one gene
• Applies to most traits and behaviours of interest to behavioral scientists
Relation 3: Child’s Environment and Phenotype
• The environment plays a role in how the phenotype is expressed.
The Case of MAOA(Environment + Genotype)
• Severe maltreatment increases likeliness of antisocial behavior
• Stronger for those individuals who had a relatively inactive MAOAgene.
Environment and Phenotype: PKU
• Children with phenylketonuria (PKU)—a chromosome 12 disorder —can not process
• Aphenylalanine-free diet can prevent mental retardation from PKU.
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
o They evoke certain kinds of responses from others.
2 o They select surroundings and experiences that support their interests, talents, and
Behavioural Geneticists ask:
• Are there aspects of behaviour that depend on genes and combinations of genes?
• If so, do experiences modify these characte