Nature and Nurture

• In the 1950s, a deeper understanding of how genetic influences operate came with the identification of DNA which is the basic component of hereditary transmission.

• In recent decades, researchers have mapped the entire genome (the complete set of DNA of any organism, including all of its genes).

• Scientists have announced plans to synthesize an entire human genome, which was downscaled to creating cells that are immune to viral infection. However, researchers have raised concerns that the specific gene variant manipulated in the procedure could be related to reduced life expectancy, perhaps due to increased vulnerability to other viruses.

• Comparisons of genomes of various species has revealed that:

  • 1. humans have roughtly 20-21k protein-coding genes

    2. genes are possessed by all living things

    3. genes make up only about 1% of the human genome

  • Much of the rest of our genome is made up of non-coding DNA which is vital in regulating the activity of protein-coding genes.

Genetic and Environmental Forces

Genotype - genetic material an invididual inherits.

Phenotype - the observable expression of the genotype, including both body characteristics and behaviour.

Environment - every aspect of individuals and their surroundings other than genes.

• Five relations are fundamental in the develoment of every child

1. parents genotype-child’s genotype

2. child’s genotype-child’s phenotype

3. child’s environment-child’s phenotype

4. child’s phenotype-child’s environment

5. child’s environment-child’s genotype

1. Parent’s Genotype-Child’s Genotype

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

• The nucleus of every cell contains chromosomes which are made up of DNA. In DNA is all of the 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.

• Humans have 46 chromosomes, divided into 23 pairs. Each parent passes along one chromosome to their offspring, meaning each individal has one chromosomes from the male, and one from the female.

  Genetic Diversity and Individual Differences

  • Genes guarantee humans will be similar to each other at the species level and at the individual level.

  • They also guarantee differences which contribute to genetic diversity.

  • Random assortment of chromosomes promotes variability (23 chromosomes are shuffled randomly, with only chance deciding which pair goes into each new egg or sperm).

  • Further variation is introduced by the process of crossing over (DNA switches from one chromosome to the other, resulting in some of the chromosomes of the parents different than their own).

  • Mutations (changes in a section of DNA) also encourage variabilities.

    • These mutations are either random errors caused by environmental factors or from inherited disorders.

    • Most are harmful, but some increase disease resistance and allow individuals to adapt to crucial aspects of the environment.

Sex Determination

• Sex chromosomes (the chromosomes (X and Y) are chromosomes that determine an individual’s designated sex at birth.

• Females have two X chromosomes, and males have 1 X chromosome and 1 Y chromosome.

• Because females only have an X chromosome, all her eggs will have an X chromosome as well.

• However, because a male has an X chromosome and a Y chromosome, the male parent determines the genetic sex of an offspring (resulting in either XX or XY zygotes depending on if an X sperm fertilized an egg or if a Y sperm fertilized an egg).

2. Child’s Genotype-Child’s Phenotype

1. Genes influence unobservable, intermediate aspects of the phenotype that impact behaviour, our brain and nervous systems which are known as endophenotypes.

Gene-expression: Developmental changes

• Genes are turned on in certain cells to specialize for arm, hand, and fingerprint formation. Other genes are involved in basic functioning and are almost always on.

• Genes turn on and off when needed, and are regulated by regulator genes.

  • When one gene turns on, another turns off, and the fact that regulator genes can control this in different patterns means one gene can function multiple times in multiple places during development. This results in large diversity in genetic expression.

  • External factors also affect the switching on and off of genes. For example, early visual experience is necessary for the normal development of the visual system because to see, some genes are switched on, which results in other genes in the visual cortex being turned on.

Gene expression: Dominance patterns

• Many of an indivudual’s genes are never expressed and some are only partially expressed.

• 1/3 of human genes have 2 or more different forms which are known as alleles.

  • Alleles influence the same trait or charactistic (ex. eye colour), but they contribute to different outcomes (brown, blue, hazel, gray…).

• Dominant-recessive patterns

  • Some genes only have two alleles - 1 dominant and 1 recessive.

  • 2 possibilities: (1) a person can inherit two of the same (2 dominant or 2 recessive) - homozygous - (2) the person can inherit two different alleles (1 dominant and 1 recessive) - heterozygous

  • When an individual is homozygous, whatever the trait is will be expressed (for example, both parents have brown hair so the child will have brown hair or both parents have blond hair so the child will have blond hair).

  • However, when an individual is heterozygous, the dominant allele will be expressed (for example, one parent has brown hair and the other parent has blond hair. the allele for brown hair is dominant so the child will have brown hair).

• Sex chromosomes make this more complicated.

  • The X chromosome carries roughly 1500 genes and the smaller Y chromosomes only carries about 200.

  • So, when a female inherits a recessive allele on the X chromosome from the female, she is likely to have a dominant allele on the chromosome from the male to suppress it, so she will not express the trait.

  • However, when a male inherits the same recessive allele on the X chromosome from the female, he likely will not have a dominant allele from his father to suppress it, so he will express the trait.

  • Because of this, genetic males are more likely than females to suffer from sex-linked inherited disorders caused by recessive alleles on their X chromosome.

• Dominant-recessive pattern of inheritance only pertains to few human traits like hair colour, blood type, abundance of body hair and such, along with topics related to genetic disorders.

• Inheritance patterns are more complicated as they concern traits and behaviours such as physical characteristics to psychological constructs.

  • Example: variability in human intelligence which follows the polygenic inheritance patterns (traits are governed by more than one gene).

3. Child’s Environment-Child’s Phenotype

• Genotypes may develop differently in different environments

• Example: phenylketonuria (PKU) - a disorder related to a defective recessive gene on chromosomes that prevents metabolism of the amino acid phenylalanine.

  • Individuals who inhereit this gene from both parents can’t metabolize this acid, which can lead to impaired brain development and intellectual disabilities if they keep eating food with the acid in it.

  • However, if infants with the PKU gene are identified after birth and kept on a diet without the acid, intellectual impairment can be avoided.

    • Therefore, environment plays an important role in whether a situation results in intellectual disability or relatively normal intelligence.

Parental contributions to the child’s environment

• Parent’s behaviour towards children is genetically influenced along with the preferences, activities, and resources to which they expose their children.

  • Ex. Parents with dyslexia are less likely to provide a reading-oriented environment for their children than parents who are skilled readers.

• A study showed that the child’s educational outcomes were predicted in part by parental alleles that the child did not inherit.

  • By the parent’s phenotypes influencing the family environments, the parent’s genes affected the child’s educational success.

4. Child’s Phenotype-Child’s Environment

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

• For example, children who enjoy books read more, and as they read more, they choose more challenging books, and this gives them a more advanced vocabulary, resulting in greater success in school.

  • Children’s ability to shape their own environments has profound effects on intellectual development.

5. Child’s Environment-Child’s Genotype

• Scientists previously believed that the genotype was “fixed “ at conception, but it is shown that while the genetic code stays the same, genetic expression is altered in life.

• Epigenetic mechanisms are mediated by the environment, and can alter the functioning of genes and create stable changes in their expression. These changes can be passed onto children.

  • In humans, methylation silences gene expression.

    • Methylation works like a dimmer switch with methyl molecules blocking transcription in the promotor region of the gene in order to turn off gene activity.

  • Epigenetic processes involving methylation provide evidence for long-term epigenetic effects of early adverse experiences on gene expression impacting later health and well-being.

  • An experiment with rat pups showed that stable alternations of DNA methylation as a function of maternal behaviour were due to the pups’ postanatal environments, rather than prenatal or genomic differences.

  • A recent study revealed epigenetic changes related to childhood abuse in sperm cells, which can be passed from parent to child.

    • As mentioned before, because of the genetic reshuffling that occurs at conception, a lot of the methylation in germ cells is erased, but this a case for the epigenetic route for the cross-generational transmission of stress.

Behaviour Genetics

• Behaviour genetics - the science concerned wtih how variation in behaviour and development results from the combination of genetic and environmental factors.

• Researchers in the field of behaviour genetics assume all behavioural traits are heritable (genetically transmitted) to an extent.

• To differentiate genetic and environmental contributions, two premises are used:

  • (1) If genetic factors are important for a given trait/behaviour, people who are genotypically similar should be phenotypically similar.

  • (2) If shared environemntal factors are important, people who were raised together should be more similar than people who were raised apart.

Quantitative Genetics Research Designs

• Quantitative behaviour geneticists use stats to study naturally occcuring genetic and environmental variation in a population by studying the phenotypes of people who differ in the amount of genomic DNA they share.

• Family study design

  • Quantitative behaviour genetics research is used to determine whether phenotypic traits are correlated with the degree to which people are genetically related.

  • The resulting correlations are compared to see if they are higher for more closely related people or less closely related people & if they are higher for people who share the same environment verseus those who did not.

• Twin-study design

  • This design is used to compare correlations for identical twins with those for same-sex fraternal twins.

  • Equal environments assumption

    • Both types of twins shared the same prenatal and postnatal environment (in the terms of family and community), so if the correlation between identical twins on a given trait/behaviour is significantly higher than the correlation between fraternal twins, genetic factors are mainly responsible.

    • However, there are notable differences in the degree of placental sharing for identical twins, and identical twins are often treated more similarly than fraternal twins, so this assumption is overly simplistic.

• Adoption study design

  • Researchers examine whether adopted children’s scores are correlated more highly with those of their biological relatives or with those of their adoptive relatives.

  • If correlations for twins raised apart are similar to those for twins raised together, it suggests environmental factors have little effect, but if it is the other way around, it is suggested that environment is influential.

Heritability

• Heritability - a statistical estimate of how much of the measured variance on a phenotypic trait among individuals in a given population is attributable to genetic diferences among those individuals.

• Some heritable traits are temperament, aspects of personality, cognition, and psychopathology.

• Example: intelligence

  • A pattern of results based on more than 100 family studies shows that IQ has both genetic and environmental influences.

  • Identical twins have closer IQs than same-sex fraternal twins, but they are not identical, so environmental factors play a part.

• Heritability estimates necessarily reflect the environments of the populations of individuals from which they are derived due to opportunities and cultures in populations.

Molecular Genetics Research Designs

• Molecular behaviour geneticists examine specific DNA sequences to identify mechanisms that link genes and behaviour.

• DNA-based methods let researchers analyze genetic influences in large samples of unrelated individuals which is helpful in understanding genetically based developmental disabilities and patterns of genes in general across large groups of people.

• Genome-wide association studies (GWAS) are used to link multiple DNA segments with particular traits.

• Genome-wide complex trait analysis (GCTA) uses genetic resemblance across large groups of individuals to make it possible to differentiate aspects of genes and environment that are confounded within families.

  • It also helps determine whether the same genes are implicated in measures of a particular trait across development (ex. intelligence over years).

Environmental Effects

• Estimating heritability automatically estimates the proportion of variance not attributed to genes.

• ex. Shared family

  • Significant shared-environment influence has been shown to be influencial for positive emotion in infancy because identical and fraternal twins had the same results.

  • However, non-shared environments make it more difficult to detect the impact of environments including parental biases towards specific children.

  • Complex phenotypes are very difficult to assess in large-scale experiments.

Brain Development

The Neuron

• The brain has more than 100 billion neurons - cells that are specialized for sending and receiving messages within the brain, and between the brain and all parts of the body.

• Each neuron has a cell body (which contains basic biological material that keeps the neuron functioning), dendrites (neural fibers that recieve input from other cells and conduct it toward the cell body in the form of electrical impulses) and an axon (neural fibers that conduct electrical signals away from the cell body to connections with other neurons).

• Neurons communicate at synapses, which are junctions between the axon terminal of 1 neuron and the cell body of another neuron.

• Glial cells are cells that form a myelin sheath (a fatty sheath that increases the speed and efficiency of information transmission) around axons, function as neural stem and progenitor cells, and protect the brain and potentially aid in regeneration when injured.

The Cortex

• The cerebral cortex is 80% of the brain and consists of 4 lobes - the frontal lobe, the parietal lob, the temporal lobe, and the occipital lobe.

• Occipital lobe: primarily involved in processing visual information.

• Temporal lobe: associated with speech, language, music, and emotional information.

• Parietal lobe: associated with spatial processing and sensory info integration.

• Frontal lobe: associated with working memory and cognitive control.

• The information from multiple sensory systems is processed and integrated in the association areas which lie between the major sensory and motor areas.

• The cortex is divided into 2 cerebral hemispheres.

  • Sensory input from one side of the body goes to the opposite side of the brain.

  • The hemispheres communicate with each other through corpus callosum, which is basically a dense tract of nerve fibers.

• Cerebral lateralization - the specialization of the hemispheres of the brain for different modes of processing (ex. right hemispheres for faces, left hemispheres for speech).

Developmental Processes

Neurogenesis and Neuron Development

• Neurogenesis - the proliferation of neurons through cell division

  • Adult neurogenesis is affected by environmental factors.

• Neurons are pushed along by new cells or go by themselves, and once they each their destination, they grow an axon and a “bush” of dendrites.

  • Axons elongate as they grow towards their targets, and dendrites form new “trees and branches” along with spines on branches which increase the dendrite’s capacity to form connections with other neurons. The entire expansion of dendrites is called arborization.

• Myelination - the formation of myelin around the axons of neurons that speeds and increases information-processing abilities.

  • This is called white matter and lies below the gray matter (cell bodies).

Synaptogenesis

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

• It begins prenatally and proceeds very rapidly before birth and for some time after.

• Differential timing of synpase generation across areas of the brain likely contributes to the developmental timing of beginning of various abilities and behaviours.

Synpase Elimination

• Synaptic pruning - the normal developmental process through which synpases that are rarely activated are eliminated.

• This happens during first months and years of life, but also during adolescence.

• Atypical patterns of pruning have been implicated in autism specrum disorder and schizophrenia.

The Importance of Experience

• The more often a synpase is activated, the stronger the connection between the neurons involved.

• When a synpase is rarely active, it is likely to disappear.

• Plasticity - the capacity of the brain to be affected by experience.

  • Because of plasticity, less info needs to be encoded in genes.

• Children’s brains are more plastic then adult’s brains, so children who suffer from brain damage have a better chance of recovering lost function than adults from the same injury.

Experience-Expectant Processes

• The process where normal wiring happens just because of the individual being a member of the species.

• Ex. Human Evolution

  • Infants experience patterned visual simulation, auditory simulation, and physical simulation so the human brain can expect these experiences will be available to fine-tune its circuitry.

• Because experience helps shape the brain, fewer genes are needed which means less info needs to be installed in the brain.

• However, if the experience does not occur, development may be compromised.

• During sensitive periods, the human brain in especially sensitive to specific external stimuli, and the neural organization that occurs (or does not occur) during these periods is typically irreversible.

Experience-Dependent Processes

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

• Ex. Experiment: animals raised in complex environments full of objects to explore versus those raised in bare laboratory cages.

  • Those who grew up in richer environments had more synpases and supportive tissues and the like, which resulted in more neuronal and synaptic function than the ones who grew up in a bare space.

The Body: Physical Growth and Development

Growth and Maturation

• Body growth is uneven over time and uneven across the body.

• The “cuteness” traits in infants across species exist to increase parent’s motivation to care for offspring.

Variability

• There is great variability across people in all aspects of physical development due to genetic and environmental factors.

• Genes affect growth & sexual maturation by influencing the production of hormones, especially growth hormone and thyroxine.

• The influence of environmental factors is particularly evident in secular trends (marked changes in physical development that have occured over generations).

• ex. In modern industrialized nations, adults are several inches taller than great-grandaparents due to improvements in nutrition and general health.

Nutritional Behaviour

Infant Feeding

• Breast milk is naturally free of bacteria, strengthens the infant’s immune system, and contains important antibodies.

• It also has a positive effect on cognitive development, which leads to higher IQ scores.

• For those in countries with unsafe drinking water and fewer public health resources, breast-feeding is essential in promoting positive health outcomes.

Development of Food Preferences

• Some food preferences are innate.

• Bitter flavours elicit negative responses, sour flavours elicit varied responses, and salty flavours do not elicit much of a reaction until infants can have a preference for it after 4 months of age.

• Taste preferences can be influenced by prenatal environment and by the flavour of breast milk which takes on whatever the mother eats.

• Most young children demonstrate food neophobia: an unwillingness to eat unfamiliar foods.

  • This avoidance is an adaptive response to keep children safe.

• Bribing & pressuring children to try new foods (ex. “if you have 2 bites of brocolli, you can have dessert”) only increases the value of the dessert, and makes them think of the food as punishment, leading children to overindulge in the treat. This can lead to obesity.

• Eating is an inherently social act and children learn about food and are more likely to accept it from familiar people. Children also prefer foods endorsed by those similar to them in social dimensions (gender, native language, ethnicity).

• Foods marketed to young children are branded with popular cartoon characters, and children are more likely to select and like foods with these characters, or food that other children like (Associative learning)

  • This can lead to obesity, but when used with healthy food, it can lead to better health.

Obesity

• Childhood obesity is a global problem, mostly because societies all over the world are adopting a “western diet” of foods and drinks high in fat and suguar and low in fiber. This is also because there is less physical activity in urbanized world.

• Why are some overweight but not others?

  • Genetic and environmental factors play roles in this.

  • Genetic factors are proven in the findings that (1) adopted children’s weight is more correlated with biological parents than adoptive parents (2) and identical twins raised apart have more similar weight than fraternal twins.

  • POMC - a genetic deletion - can lead to human obesity.

  • Childhood impulsivity also leads to obesity.

  • In terms of environmental factors:

    • A lack of physical education programs, recess activities, healthy cafeteria lunches.

    • Children spend less time outside and more with screens.

    • Less sleep because children spend more time on screens.

    • Unhealthy food is cheaper and more readily available than healthy food.

• Obese children are more likely to struggle with friendships and are generally poorly treated by peers.

Undernutrition

• Undernutrition is associated with poverty, limited access to healthcare, warfare, famine, natural disasters.

• Malnutrition with all these facts adversely affects all areas of development (brain development, physical growth, cognition, social development, educational attainment…).

• Improved water sanitation, community-based interventios, and food stamps result in positive adulthood outcomes.

Vaccines

• Vaccines deliver a weak version of the disease to the to promp the body to produce anitbodies to attack it.

• Antibodies provide protection against subsequent encounters with disease.

• Some parents believe vaccines are harmful and refuse to take them or to let their children take them.

  • This creates a risk for children and others in the community.

  • Some vaccines operate on herd immunity, meaning a certain population needs to be vaccinated for it to work, so this behaviour can lead to several deaths.