AP Psychology 1.1-1.2: Interaction of Traits, Environment, and Evolution

Natural selection

Natural selection is the process by which individuals with heritable traits that improve survival or reproduction in a particular environment tend to leave more offspring. Over generations, those advantageous traits become more common in the population.

Why can a disease-associated allele remain common in a population?

An allele may provide an advantage in a particular environment even if having two copies causes disease. For example, individuals heterozygous for the sickle-cell allele have increased resistance to malaria, which can maintain the allele in regions where malaria is prevalent.

Sickle-cell heterozygote advantage

People with one sickle-cell allele generally do not have full sickle-cell anemia and may have protection against malaria. This advantage is environment-dependent and is strongest in areas where malaria is common.

How does sickle-cell anemia affect red blood cells?

The cells become crescent-shaped rather than normally round. They can obstruct blood vessels, reducing blood flow and causing pain, fever, swelling, and tissue damage.

Genetic variation

Genetic variation is the difference in genetic makeup among individuals of a species. It provides the raw material for adaptation because environmental changes may favor individuals with particular heritable traits.

Chromosome

A chromosome is a long structure composed of DNA that contains many genes. Human gametes typically contain 23 chromosomes, while most body cells contain 46 chromosomes arranged in 23 pairs.

DNA

DNA, or deoxyribonucleic acid, is a molecule composed of nucleotide base pairs and shaped as a double helix. It stores genetic information in the sequences that make up genes.

Gene

A gene is a segment of DNA that influences a particular characteristic or biological function. Some traits are strongly controlled by one gene, whereas others depend on many genes.

Allele

An allele is a specific version of a gene. Different alleles can contribute to different forms of the same trait, such as different hair-color variants.

How does fertilization restore the diploid chromosome number?

Each sperm and egg contributes 23 chromosomes. Their fusion produces a zygote with 46 chromosomes, or 23 pairs, so each genetic contributor supplies half of the offspring's nuclear genetic information.

Genotype

An individual's genotype is the combination of alleles and other genetic information inherited from genetic contributors. It represents genetic makeup rather than directly observable characteristics.

Phenotype

A phenotype is an individual's observable characteristic or outcome, produced by the interaction of genotype with environmental influences. Examples include height, skin color, and body build.

Why does phenotype not always reveal genotype?

Different genotypes can produce the same phenotype, especially when a dominant allele masks a recessive allele. In addition, environmental conditions can alter how a genotype is expressed.

Homozygous and heterozygous

An individual is homozygous when the two alleles for a gene are identical, such as $BB$ or $bb$. An individual is heterozygous when the alleles differ, such as $Bb$.

Dominant allele

A dominant allele contributes to the dominant phenotype when at least one copy is present. In the cleft-chin example, both $BB$ and $Bb$ produce a cleft-chin phenotype.

Recessive allele

A recessive allele produces its associated phenotype only when an individual has two copies and is homozygous recessive. In the cleft-chin example, the smooth-chin phenotype requires $bb$.

What offspring genotypes and phenotypes are expected from a $BB \times bb$ cross?

Every offspring is expected to be $Bb$. If $B$ is the dominant cleft-chin allele, all offspring are expected to have cleft chins.

What offspring outcomes are expected from a $Bb \times bb$ cross?

The expected genotypes are 50% $Bb$ and 50% $bb$. With complete dominance, this corresponds to a 50% dominant phenotype and a 50% recessive phenotype.

Punnett square

A Punnett square is a diagram used to combine parental gametes and predict possible offspring genotypes. It provides expected probabilities for inheritance outcomes, not guarantees for every set of offspring.

Why can two carrier parents produce a child with a recessive disorder?

Each heterozygous carrier has one normal allele and one recessive disease allele. A child who inherits the recessive allele from both parents becomes homozygous recessive and expresses the disorder.

What is the probability that two PKU carriers, each with genotype $Np$, will have an affected child?

For an $Np \times Np$ cross, the expected genotypes are 25% $NN$, 50% $Np$, and 25% $pp$. Thus, the probability of a child expressing recessive PKU is $25\%$.

Phenylketonuria (PKU)

PKU is a recessive genetic condition in which an individual lacks an enzyme needed to convert certain harmful amino acids into harmless products. If untreated, it can cause cognitive impairment, seizures, and increased risk of some psychiatric disorders.

Mutation

A mutation is a permanent change in a gene's DNA sequence. Mutations may be harmful, neutral, or beneficial depending on their effects and the environment in which they occur.

Polygenic trait

A polygenic trait is influenced by multiple genes. Human height, skin color, and body weight are examples, and their variation is often also affected by environmental conditions.

Why is genetic diversity important for evolution by natural selection?

If all individuals were genetically identical, environmental change would affect them similarly and selection would have little variation to act on. Genetic diversity allows some individuals to survive and reproduce more successfully under new conditions.

Evolutionary psychology versus behavioral genetics

Evolutionary psychology examines broadly shared cognitive and behavioral patterns that may have evolved as adaptations over many generations. Behavioral genetics focuses on how individual differences arise through current interactions between genes and environments.

How do twin and adoption studies help separate genetic and environmental influences?

Twin studies compare similarity between identical and fraternal twins, while adoption studies compare biologically related and adoptive relatives. Greater similarity among genetically related individuals suggests genetic influence, although these designs cannot eliminate all environmental confounding.

Range of reaction

Range of reaction is the idea that genes establish boundaries for a trait's possible expression, while the environment helps determine where within those boundaries the phenotype falls. For example, a stimulating environment may help an individual approach their intellectual potential.

Genetic-environment correlation

Genetic-environment correlation describes the bidirectional relationship in which genetic characteristics influence the environments people experience, while those environments influence gene expression and development. A child's inherited athletic tendencies may lead to greater exposure to sports, which can further develop those abilities.

Epigenetics

Epigenetics examines changes in gene expression that occur without changing the underlying DNA sequence. Environmental experiences can cause the same genotype to be expressed in different ways, producing different phenotypes.

Identical versus fraternal twins

Identical twins arise when one fertilized egg splits and therefore begin with essentially the same genetic information. Fraternal twins arise from two eggs fertilized by two sperm and are genetically similar to ordinary siblings.

How can identical twins have different diseases or behaviors despite sharing a genotype?

Their environments and life experiences can differ, and these factors can influence gene expression over time. Consequently, identical genotypes can produce different phenotypes.

What does adoption research suggest about schizophrenia risk?

Schizophrenia risk is highest when a person has both genetic vulnerability and a stressful or disturbed rearing environment. Genetic risk alone does not determine the disorder.

How do the schizophrenia adoption-study percentages illustrate gene-environment interaction?

The reported risks were 36.8% for high genetic risk with a disturbed environment, 5.8% for high genetic risk with a healthy environment, 5.3% for low genetic risk with a disturbed environment, and 4.8% for low genetic risk with a healthy environment. The large increase in the high-risk, disturbed-environment group indicates that vulnerability and environment interact.

What is the relationship between genes and psychological characteristics?

Genes can influence temperament, personality, sexual orientation, spirituality, and vulnerability to psychological disorders. They do not act as simple, independent causes; environmental conditions help determine how these influences are expressed.

Central nervous system (CNS)

The CNS consists of the brain and spinal cord. It receives, processes, and coordinates information for the body.

Peripheral nervous system (PNS)

The PNS includes nerves outside the brain and spinal cord. It carries sensory information to the CNS and motor commands from the CNS to muscles, organs, and glands.

Nerve

A nerve is a bundle of axons in the peripheral nervous system. It can contain fibers carrying information toward the CNS and fibers carrying commands away from it.

Afferent versus efferent fibers

Afferent fibers carry sensory information toward the CNS, whereas efferent fibers carry motor instructions away from the CNS. A memory aid is “afferent arrives” and “efferent exits.”

Somatic nervous system

The somatic nervous system is the PNS division associated primarily with sensory input and voluntary control of skeletal muscles. It contains sensory and motor neurons.

Autonomic nervous system

The autonomic nervous system regulates internal organs and glands, generally outside conscious voluntary control. Its sympathetic and parasympathetic divisions work together to maintain internal balance.

Sympathetic nervous system

The sympathetic division prepares the body for stress or high-arousal situations. It increases heart rate and blood pressure, dilates pupils, releases glucose, and redirects resources toward rapid action.

Fight-or-flight response

The fight-or-flight response is a coordinated sympathetic reaction to a perceived threat. It increases energy availability and sensory readiness so an organism can confront the threat or escape.

Parasympathetic nervous system

The parasympathetic division promotes relaxation and restoration after a threat has passed. It lowers heart rate and blood pressure, constricts pupils, restores bladder control, and supports glucose storage.

Homeostasis

Homeostasis is the maintenance of relatively stable internal conditions, such as body temperature. The sympathetic and parasympathetic divisions contribute to homeostasis by adjusting bodily functions to changing demands.

Why can persistent psychological stress harm the body?

Repeated sympathetic activation keeps the body in a high-arousal state even when there is no physical danger. Chronic stress is associated with increased susceptibility to heart disease and impaired immune function.

How do the sympathetic and parasympathetic divisions differ during and after a threat?

The sympathetic division mobilizes energy and prepares the body for action during the threat. Afterward, the parasympathetic division reverses many of those changes and restores routine functioning.