AP Psychology

2.2 Behavioral Genetics

Genetics

Besides the functioning of the endocrine and nervous system, genetics is another biological factor that affects human behavior and thought• Behavioral Genetics – Genetic and

environmental contributions to personality and behavior

• Human traits are usually caused by genes acting together (not usually one gene)

Nature vs. Nurture• Influence on behavior (ex…body shape,

introversion, temper) • Gene-Environment Interaction (sometimes

hard to determine because genes cause someone to chooser their enviroment)– Nature – heredity (genes determine)– Nurture – environment or life history (begins in

prenatal period)

Evolutionary Psychologists • Study how natural selection

favored behaviors that contributed to survival and spread of our ancestors genes

• Look at universal behaviors shared by all people

Behavioral Geneticists• Study the role played by our

genes and our environment in personality characteristics and behavior (mental ability, emotional stability, temperament, personality, interests, etc.)

• Look at the cause of our individual differences

• Gene-environment Interaction – choose environ because of genes

Twin Studies• Help separate the contributions of heredity

and environment• Zygote – fertilized egg• Identical (monozygotic) twins– Two individuals who share all of the same

genes/heredity because they develop from the same zygote

• Fraternal (dizygotic) twins– Siblings that share about half of the same genes

because they develop from two different zygotes

Twin Studies• Heritability – variation among individuals that is due to

genetic causes (genes)• When twins grow up in the same environment, the

extent to which identical twins are behaviorally more similar than fraternal twins reveal the contribution of heredity

• If fraternal twins are separated at birth and raised in different environments (adoption studies)…behavioral differences may reveal the contribution of environment to behavior; similarities reveal the contribution of heredity

• Adoption studies – kids resemble biological (nature), resemble adoptive family (nurture)

Thomas Bouchard study• Found 100 identical twins given up for adoption

and raised in different families• Studied hundreds of traits and determined the

influence on them (nature vs nurture)• Example – IQ• Correlation coefficient of .69 for identical twins

living apart• .88 for identical twins living together• Showed environment has some effect• Living apart was still a moderate-strong correlation

– showed IQ heavily influenced by genetics

Genetic Concepts• Genome - the entirety of an organism's

hereditary information (book)• Chromosome – structure in the nucleus of

cells that contains genes determined by DNA sequences. (chapters of the book)

• Gene – each DNA segment of a chromosome that determines a trait (words/paragraphs)

• DNA (deoxyribonucleic acid) – genetic material that makes up chromosomes (letters)

• ** Traits are usually influenced by genes acting together **

Human Cells• 46 chromosomes in 23 pairs– 23 from the sperm of the father– 23 from the egg of the mother

• Father contributes a Y sex chromosome (23rd pair), the baby is male; otherwise the baby is female– Males have 44 chromosomes plus X and Y– Females have 44 chromosomes plus X and X

• All of the cells of the embryo/baby have the same 23 pairs of chromosomes which carry the genes for the same traits

Chromosomal Abnormalities• Occasionally, chromosomes will combine (or

fail to) in an unusual way (errors in fertilization) called mutations

• Turner’s Syndrome – females with only one X sex chromosome – Shortness, webbed necks, lack ovaries, fail to

develop secondary sex characteristics at puberty– Usually normal intelligence – although cognitive

deficits in arithmetic, spatial organization, visual perception

Chromosomal Abnormalities

• Klinefelter’s Syndrome – males with XXY chromosomes

• At puberty, male secondary sex characteristics fail to develop, but breast tissue does (minimal sexual development)

• Tend to be passive / introversion

Chromosomal Abnormalities

• Down syndrome – three copies of chromosome-21

• Typically mentally retarded• Have a round head, flat nasal bridge,

protruding tongue, small round ears, a fold in the eye lid (far apart), poor muscle tone and coordination, short fingers and toes

Genetic Disorders• Genotype – genetic make-up of an individual• Phenotype – the expression of the genes• Gene-environment interaction - term used to

describe any phenotypic effects that are due to interactions between the environment and genes

Genetic Disorders• Homozygous – Possessing two identical forms of a

particular gene, one inherited from each parent (both) / individuals express that phenotypic characteristic

• Heterozygous – Possessing two different forms of a particular gene, one inherited from each parent (aka hybrid)

• Dominant gene – gene expressed when the genes for a trait are different

• Recessive gene – the gene that is hidden or not expressed when the genes for a trait are different

• Allele (uh-leel) – a dominant or recessive variant of a particular gene (usually through mutation)

• Mutation – random error in gene replication

Recessive trait with ONE carrier

Recessive trait with TWO carriers

Dominant with ONE carrier

Dominant with TWO carriers

Genetic Disorders• Tay-Sachs syndrome – recessive trait that

produces progressive loss of nervous function and death in a baby

Genetic Disorders

• Albinism – recessive trait that produces a lack of pigment and involves abnormal nerve pathways to the brain resulting in quivering eyes and inability to perceive depth (3D) with both eyes

Genetic Disorders

• Phenylketonuria (PKU) – recessive trait that results in severe, irreversible brain damage unless the baby is fed a special diet low in phenylalanine within 30 days of birth

• Individuals must regulate intake of phenylalanine (artificial sweeteners – aspartame – NutraSweet / Equal)

• The infant lacks the enzyme to process this amino acid which can build up and poison the nervous system

• Gene-environment interaction

Genetic Disorders• Huntington’s disease – dominant gene defect

that involves degeneration of the nervous system, characterized by tremors, jerky motions, blindness, and death

Genetic Disorders• Sex-linked traits – recessive genes located on

the X chromosome with no corresponding gene on the Y chromosome, which results in expression of recessive trait, more frequently in males– Color-blindness – individual cannot see certain

colors, most often red and green– Baldness

• 25 percent of men begin balding by age 30; two-thirds begin balding by age 60

• There is a 4 in 7 chance of receiving the baldness gene

– Hemophilia – long time for blood to clot

Genetic Disorders• Alzheimer’s disease – most common form of

dementia (loss of cognitive functioning)• A form has been attributed to a gene on

chromosome 21, but not in all cases!