Chapter 3 Biology and Behavior

Heredity

the passing on of traits from parents to their offspring

Genes

sections of chromosomes that code for traits

Allele

two or more different versions of a gene (e.g., brown vs. blue iris)

Genome:

the complete set of genes of any organism

Chromosome

molecules of DNA that transmit genetic information

DNA

molecules that carry all the biochemical instructions involved in the formation and functioning of an organism

Genotype

 the genetic material an individual inherits

Phenotype

the observable expression of the genotype, including both body characteristics and behavior

Environment

every experience to which an individual is exposed in their surroundings; everything other than genes

DNA , Chromsome and gene

DNA: long string of molecules containing the “instructions” for building and maintaining an organism 

Chromosome: molecules of DNA that transmit genetic information; in humans, divided into 23 pairs (46 total)

Genes are sections of chromosomes, each containing instructions for making proteins giving rise to specific traits

• Physical traits (eye colour, height)

• Psychological traits (aggression, anxiety

Relation 1: Parents’ Genotype–Child’s Genotype

• involves the transmission of genetic material from parent to offspring.

• The nucleus of every cell in the body contains chromosomes, long threadlike molecules made up of two twisted strands of DNA (deoxyribonucleic acid). DNA carries all the biochemical instructions involved in the formation and functioning of an organism.

• These instructions are “packaged” in genes, the basic unit of heredity in all living things. Each gene is a segment of DNA that is the code for the production of particular proteins. Some proteins are the building blocks of the body’s cells; others regulate the cells’ functioning. Genes affect development and behavior only through the manufacture of proteins.

Variability: Random assortment

• the random assortment of chromosomes in the formation of egg and sperm.

• During gamete division, the 23 pairs of chromosomes are shuffled randomly, with chance determining which member of each pair goes into each new egg or sperm.

• When a sperm and an egg unite, the odds are essentially zero that any two individuals—even members of the same family—will have the same genotype (except, of course, identical twins).

Variability: crossing over

when gametes divide, the two members of a pair of chromosomes sometimes swap sections of DNA.

Variability: mutation

A second mechanism that enhances variability is mutation, a change in a section of DNA. Some mutations are random, spontaneous errors; others are caused by environmental factors. Those that occur in gametes (germ cells) can be passed on to offspring.

Relation 2: Child’s Genotype–Child’s Phenotype

• phenotypes include both physical characteristics, such as height and eye color, and behavioral characteristics, such as temperament and intelligence.

• the simplest pattern of gene expression is the dominant–recessive pattern.

• Some genes have only two alleles, one of which is dominant and the other recessive.

• In this pattern, there are two possibilities: (1) a person can inherit two of the same allele—two dominant or two recessive—and thus be homozygous for the trait in question; or

• (2) the person can inherit two different alleles—one dominant and the other recessive—and thus be heterozygous for the trait.

• When an individual is homozygous, the corresponding trait will be expressed. When an individual is heterozygous for a trait, the instructions of the dominant allele will be expressed

• polygenic inheritance: inheritance pattern in which traits are governed by more than one gene

Relation 3: Child’s Environment–Child’s Phenotype

• eg; of a phenotype produced by a genotype–environment interaction is phenylketonuria (PKU), a disorder related to a defective recessive gene on chromosome 12.

• Individuals who inherit this gene from both parents cannot metabolize phenylalanine, an amino acid present in many foods (especially red meats) and in aspartame, an artificial sweetener. If they eat a normal diet, phenylalanine accumulates in the bloodstream, causing impaired brain development and intellectual disabilities.

Genetic testing

• carrier genetic testing: genetic testing used to determine whether prospective parents are carriers of specific disorders

• prenatal testing: genetic testing used to assess the fetus’s risk for genetic disorders

• newborn screening: tests used to screen newborn infants for a range of genetic and non-genetic disorders

MAOA

• The researchers wanted to determine why some severely maltreated children become violent and antisocial as adults, whereas others who are exposed to the same abuse do not.

• To do so, they focused on the MAOA gene, an X-linked gene that inhibits brain chemicals associated with aggression. The result revealed the importance of a combination of environmental and genetic factors leading to antisocial outcomes. Young men who had a relatively inactive version of the MAOA gene, and who had experienced severe maltreatment, grew up to be more antisocial than other men who had also experienced severe maltreatment but who possessed a more active version of the MAOA gene.

Relation 4: Child’s Phenotype–Child’s Environment

• restates the active child theme—children as a source of their own development. Each child evokes certain kinds of responses from others.

• Children also create their own environments by actively selections surroundings and experiences that match their interests and personalities.

Relation 5: Child’s Environment–Child’s Genotype

• epigenetics: the study of stable changes in gene expression that are mediated by the environment

• The epigenetic mechanism is studied in humans is methylation, which silences gene expression.

• Methyl molecules block transcription in the promotor region of the gene, turning off gene activity.

• DNA methylation typically operates like a dimmer switch, regulating the amount of protein produced by a given gene.

• Epigenetic processes involving methylation are of particular interest to developmental scientists because they provide evidence for long-term epigenetic effects of early adverse experiences on gene expression, impacting later health and well-being

Behavior genetics

the science concerned with how variation in behavior and development results from the combination of genetic and environmental factors

Investigating heredity: Family study

attempts to determine whether phenotypic traits are correlated with the degree to which people are genetically related— parents and children, identical and fraternal twins, non-twin siblings, or adoptive family members.

Investigating heredity: Twin studies

• a specialized form of the family study used to compare the correlations for identical (monozygotic, or MZ) twins with those for same-sex fraternal (dizygotic, or DZ) twins.

• the equal environments assumption, the claim is that both types of twins shared the same prenatal environment, were born at the same time (so experienced societal changes similarly), grew up in the same family and community, and are always the same age when tested.

Investigating heredity: adoptive studies

• researchers examine whether adopted children’s scores on a given measure are correlated more highly with those of their biological parents and siblings or with those of their adoptive parents and siblings.

• Looks to see how much adopted children resemble their biological vs. adoptive parents. study in which identical twins who grew up together are compared to identical twins who were separated shortly after birth and raised apart.

Brain development: the neuron

cells that are specialized for sending and receiving messages between the brain and all

parts of the body, as well as within the brain itself

The neuron: 3 components

(1) a cell body, which contains the basic biological material that keeps the neuron functioning;

(2) dendrites, fibers that receive input from other cells and conduct it toward the cell body in the form of electrical impulses;

(3) an axon, a fiber (anywhere from a few micrometers to more than a meter in length) that conducts electrical signals away from the cell body to connections with other neurons.

Neurons communicate at synapses, microscopic junctions between the axon terminal of one neuron and the dendritic branches of another. Electrical and chemical messages cross the synapses and cause the receiving neurons either to fire, sending a signal on to other neurons, or to be inhibited from firing.

Glial cells

perform a variety of critical functions, including the formation of a myelin sheath around axons, which increases the speed and efficiency of information transmission. Glial cells also function as neural stem and progenitor cells during prenatal brain development, and some glial cells continue to do so into adulthood. When the brain is injured, some glial cells react by rapidly increasing in numbers, protecting the brain and potentially aiding in regeneration.

Cerebral cortex

Its the the “gray matter” of the brain, consisting of four distinct lobes.

• The occipital lobe is primarily involved in processing visual information.

• The temporal lobe is associated with speech and language, and the processing of emotion and auditory information.

• The parietal lobe engages in spatial processing and is also involved in the integration of information from different sensory modalities.

• The frontal lobe, the brain’s “executive,” is involved in cognitive control, including working memory, planning, decision making, and inhibitory control.

• association areas parts of the brain that lie between the major sensory and motor areas and that process and integrate input from those areas

The cortex

• The cortex is divided into two cerebral hemispheres. For the most part, sensory input from one side of the body goes to the opposite side of the brain, and the motor areas of the cortex control movements of the opposite side of the body.

• The hemispheres communicate with each other primarily by way of the corpus callosum, a dense tract of connective nerve fibers between the two hemispheres.

• cerebral lateralization: the specialization of the hemispheres of the brain for different modes of processing

Developmental process: Neurogenesis

• the proliferation of neurons through cell division, begins 42 days after conception (in humans) and is virtually complete by the midway point of gestation

• After their “birth,” neurons migrate to their ultimate destinations— typically outwards from the center of the brain toward the developing neocortex. Some neurons are pushed along passively by the newer cells formed after them, whereas others actively propel themselves toward their ultimate location.

• After their “birth,” neurons migrate to their ultimate destinations— typically outwards from the center of the brain toward the developing neocortex. Some neurons are pushed along passively by the newer cells formed after them, whereas others actively propel themselves toward their ultimate location.

• myelination: the formation of myelin (a fatty sheath) around the axons of neurons that speeds and increases information-processing abilities

Developmental process: synaptogenesis

• the process by which neurons form synapses with other neurons, resulting in trillions of connections

• synaptic pruning: the normal developmental process through which synapses that are rarely activated are eliminated

Plasticity and its 2 kinds

• the capacity of the brain to be affected by experience

• Experience-expectant plasticity: involves the general experiences that almost all infants have just by virtue of being human.

• Experience-dependent plasticity involves specific, idiosyncratic experiences that children have as a result of their particular life circumstances—such as growing up in New York City or in the Amazon rain forest, experiencing frequent cuddling or abuse, and so on.

experience-expectant plasticity

the process through which the normal wiring of the brain occurs in part as a result of species-typical experiences

experience-dependent plasticity

the process through which neural connections are created and reorganized throughout life as a function of an individual’s experiences

Brain development REVIEW

• Neurons, the basic units of the brain’s informational system, transmit information between the brain and body via electrical signals. The brain’s cortex is composed of several major areas, or lobes, specialized for different functions.

• Brain development involves several processes, beginning with neurogenesis and differentiation of neurons. In synaptogenesis, an enormous profusion of connections among neurons is generated, starting prenatally and continuing for the first few years after birth. Through synaptic pruning, excess connections among neurons are eliminated. Myelination, another important process for neural processing, begins before birth and continue through adulthood.

• Experience plays a crucial role in the strengthening or elimination of synapses and hence in the normal wiring of the brain. The fine-tuning of the brain involves experience- expectant processes, in which existing synapses are preserved as a function of stimulation that virtually every human encounters, and experience-dependent processes, in which new connections are formed as a function of experience and learning.

• Plasticity makes it possible in certain circumstances for the brain to rewire itself in response to damage. It also makes the developing brain vulnerable to the absence of stimulation at sensitive periods in development. The ability of the brain to recover from injury depends on the age of the child.