The first studies on development were trying to answer the question of which of the following is more influential on the child’s development: the genes or the environment. Since the discovery of the DNA, the basic component of heredity, enormous advances have been made in deciphering the genetic code. Researchers have mapped the entire genome, the complete set of genes of an organism. Later studies on development found both, the genes and the environment, as important influences in the development of a child. The genes and the environment interact continuously with each other, therefore both influence the development of a child.

Which genetic and environmental factors play a role?

There are three elements that are important in the development of a child: genotype, phenotype and the environment. Genotype is the inherited genetic material of an individual. Phenotype is the observable expression of the genotype, namely the body characteristics and behavior. The environment is an all-encompassing aspect of an individual and his / her surrounding aspects, unlike the genes.

These three elements are involved in five relationships that are fundamental to the development of every child: 

How do the genes of the parents influence the child's genotype?

Chromosomes are molecules of the DNA containing genetic information. DNA (deoxyribonucleic acid) is made of molecules containing all biochemical instructions involved in the formation and functioning of an organism. These instructions are packaged in genes, sections of chromosomes that are the basic heredity unit of all living beings. People normally have 46 chromosomes, 23 pairs, in each cell nucleus. Except in the germ cells, these only contain 23 chromosomes.   

A child shows similarities on the general human level (the child has feet and arms) and on an individual level (the child shares similarities with family members). Different mechanisms ensure the genetic diversity between people. One is mutation. Mutation is a change in the components of the DNA. Another mechanism is crossing. Crossing over is a process where parts of DNA swap from one chromosome to another chromosome. Crossing over promotes the variability between individuals. 

Every person has one pair of sex chromosomes. Sex chromosomes transmit the genetic information. A man has an X chromosome and a Y chromosome. A woman possesses two X chromosomes. For this reason, the father always determines the sex of a child. When the man transmits a Y chromosome to the child, it becomes a boy (XY) and when the man passes an X chromosome to the child, it becomes a girl (XX).     

What is the genetic contribution of the child to its own phenotype?

Endophenotypes are intermediate phenotypes influencing physical and behavioral characteristics, including unobservable aspects as the brain and the central nervous system. Regulatory genes regulate the switching on and off of genes. A gene never functions alone. There is a so-called network in which the expression of a gene in turn provides for the expression of another gene. In other words: the expression of a gene is controlled by regulatory genes. This concept could become clearer using a metaphor. The alphabet consists of 26 letters (= genes). Not every letter (= gene) is used in every word (= expression). But there are millions of words, and also millions of types of gene expressions.

Some genes never express themselves because 1/3 of a person's genes have two or more different forms. These are called alleles. The pattern of a gene expression (found by Mendel) is called the dominant-recessive pattern. Some genes contain two alleles: a dominant allele and a recessive allele. A dominant allele is always expressed when present (and is indicated with a capital letter, for example B). A recessive allele is not expressed when there is a dominant allele (and is indicated with a lowercase letter, for example b). When a human being has two identical alleles (two dominant alleles -BB- or two recessive alleles -bb-), the person is homozygous for a certain trait. When a human being has two different alleles (a dominant allele and a recessive allele -Bb-), the person is heterozygous for a specific character trait. Concluding in the following: if someone is homozygous for a certain character trait, the corresponding character trait will express itself, but if someone is heterozygous for a specific trait, the dominant gene for the trait in question will express itself.

It is called polygenetic inheritance when certain traits are regulated by multiple genes.

What is the contribution of the environment of the child to its own phenotype?

Because there is constant interaction between genotype and environment, the genotype will develop differently in different environments. In other words: a given genotype can result in different phenotypes, depending on the environment. This is called norm of reaction. Norm of reaction refers to all phenotypes that can arise from the interaction between a certain genotype and all environments in which it can survive and develop.

An example of how genotype environment interactions work is the disease phenylketonuria (PKU). This disorder is related to a defective recessive gene on chromosome 12. It stops the metabolism of the amino acid phenylalanine. If individuals with PKU are put on a diet free of phenylalanine, they can lead a very normal life, otherwise severe intellectual disabilities would arise.  

Often the environment of the parents also influences the child, such as reading or music. The interaction between parent and child thus influences the environment of the child, and also affects the phenotype of the child. 

Genetic testing is used to diagnose diseases and sometimes to determine what kind of treatment is appropriate. Carrier genetic testing is offered to specific target groups to check whether there are carrying genes of a certain disorders. Prenatal testing is done during pregnancy. To see if the child has the condition, when there are certain risk factors for a genetic disorder. The screening of babies is often done to check for numerous genetic disorders.

A recent development is non-invasive prenatal testing (NIPT). This method studies fragments of the fetal DNA that enter the mother's blood circulation. This technique can be used to screen for aneuploidy (missing or extra chromosomes).

What is the contribution of the phenotype of the child to its environment?

It is about the active child. First, because of their nature and behavior, children evoke certain types of reactions from others. Secondly, the child chooses and creates his / her own environment, for example in the form of activities and friends.

What is the contribution of the child's environment to its own genotype?

Epigenetic factors can help explain why identical twins do not have the same life path: different environments can change gene expression in subtle ways during development. This happens, for example, through methylation. The presence of methyl molecules blocks transcription in the promoter region of the DNA strand. Methylation is an epigenetic process that can take place due to environmental influences. For example, it has been found that young rats that are neglected by their mother show less activity in the glucocorticoid receptor gene as a result of methylation. Research has shown that the same process can take place in neglected children. A recent study has identified a large number of genes in which methylation can occur as a result of early-life stress. Methylation due to child abuse can also be passed on to the next generation.  

What are behavioral genetics and what research designs can be used?

According to behavioral genetics, the interaction between genes and the environment causes variation. Why do people differ from each other? Traits are initially hereditary, but mostly multifactorial. Hereditary refers to every trait or characteristic that can be transmitted through genes. Multifactorial refers to traits and characteristics that are influenced by both environmental factors and genetic factors. 

Family studies focus on measurements of differences in characteristics between individuals (family members and non-family members). These studies examine whether family members are more similar than non-family members. If this is the case, one can conclude that for this trait the genetic contribution is greater than the contribution of the environment.

Twin studies focus on differences in characteristics between identical twins and also between fraternal twins, and then examine whether the genes are responsible for the differences. In fact, identical twins share 100% of the same genes and fraternal twins share 50% of the same genes. When identical twins are more alike than non-identical twins, it can be concluded that the studied trait is more genetic than non-genetic. 

Adoption studies focusses on measurements of differences in characteristics between adopted children and biological children, and then examine whether the adopted children are more like their biological parents than on the adoptive parents.  

Adoption of twin studies focusses on measurements of characteristics between identical twins that have grown up separately and identical twins that grew up together, and then examine whether the differences are due to genetic or environmental influences.

Studies on IQ show surprising results: the genetic influence on intelligence increases with years.     

How can differences in heritability be used in research?

Heredity is a statistical measurement of the proportion of the measured variance of a given trait between individuals in a given population, attributable to the genetic differences between individuals.

Genome-wide association studies show that genetic effects are cumulative, individual chromosomes do not correlate with certain characteristics.  

There are a number of misunderstandings about heredity:

• It is often assumed that heredity says something about individuals. Heredity, however, only says something about populations. 
• It is also often assumed that heredity is generalizable. However, heredity can only say something about a specific population in a specific environment.
• In addition, it is often assumed that heredity depends on genes only and not on the environment. However, estimates of heritability by definition depend on the environment of the population from which this estimate is determined.
• People often assume otherwise, but heredity can change due to developmental factors. For example, the difference in intelligence in twins often increases with time.
• In addition, it is often thought that traits with high heritability are more robust; but high heritability does not necessarily imply high immutability.
• Finally, it is often thought that heritability studies can be generalized to the world population. It is important to note that most studies of heredity consist of WEIRD participants only (i.e., White, Educated, Industrialized, Rich, and Democratic participants).

Genome-wide association studies (GWAS) attempt to link specific DNA segments and genes to specific traits.The genome-wide complex trait analysis (GCTA) uses genetic similarity based on large groups of individuals rather than families. In this way aspects of genes and environment which could be disrupted in families can be unraveled more easily. 

How can differences in environmental factors be used in research?

The amount of variance determined by the environment is calculated automatically through the 100 heredity (%). In studies comparisons are made between shared environmental influences and non-shared environmental influences. Shared environment has to do with having the same experiences that are a result of growing up together in the same family. Non-shared environment mean that children grew up in the same family, but do not share the same experiences with each other. These experiences can take place within the family or outside the family. 

How does the brain develop?

What different brain structures do we distinguish?

The brain is kept active by information exchanges. These information exchanges are regulated by neurons. Neurons are cells that specialize in sending and receiving information between the brain and all other body parts. Sensory neurons exchange information from the sensory receptors of the environment (both external and internal stimuli) or within the body itself. Motor neurons exchange information between the brain and the muscles and limbs. Inter-neurons exchange information between sensory neurons and motor neurons. 

A neuron consists of three main elements: the cell body, the dendrites and the axon. The cell body is the nucleus of the neuron, which keeps the neuron working. The dendrites receive information from other neurons through the synapses and transmit them to the cell body. The axon transmits information from the cell body to other neurons via synapses. A synapse is the space between the dendrites of the one neuron and the axon of the other neuron. 

Glial cells are cells that fulfill all sorts of supportive functions. They ensure the formation of a myelin sheath around axons. The myelin sheath is a greasy sheath that increases the speed and efficiency of information exchange.

The cerebral cortex is seen as the most human part of the brain, the so-called 'gray matter'. The cerebral cortex takes up 80% of the brain. The large areas of the cerebral cortex are called the lobes. The lobes are characterized by the general categories of behavior that each lobe specializes in. The occipital lobe is mainly important in the processing of visual information. The temporal lobe is specialized in memory, visual recognition, auditory information, and the processing of emotions. The parietal lobe is specialized in non-verbal processing, sensory input integration, and information about emotions. The frontal lobe is important for planning ahead and organizing behavior to achieve a certain goal. Information from different sensory systems is processed in the association areas that lie between the large sensory, and motor areas. Areas are mostly not functionally specific, often different areas work together.

The cortex is divided into two halves. These two halves are called cerebral hemispheres. Information coming in from the left side of the body is processed in the right hemisphere. Information coming in from the right side of the body is processed in the left hemisphere. The two brain halves communicate with the help of the corpus callosum. Each hemisphere has its own specialization. This division of specialization over the two hemispheres is called cerebral lateralisation. For example, the left hemisphere specializes in verbal aspects such as language, logic, and sequential tasks, and the right hemisphere specializes in non-verbal aspects as visual information.

What are the developmental processes that take place in the brain?

Neurogenesis is the proliferation of neurons by cell division, this process begins 42 days after conception. The second development process is cell migration, which is followed by cell differentiation. First an axon grows, and then a 'bunch' of dendrites are forming. The biggest change in dendrites is 'arborization', an enormous increase in the size and complexity of the dendrites through growth, branching and the formation of so-called spines that increase the capacity to form connections with other neurons. Arborization allows neurons to grow in complexity during the first years of life, expanding the surface of the cortex and making the layers of the cortex thicker.

Myelination is the process whereby myelin sheaths are formed around the axons. Synaptogenesis also takes place, where neurons form synapses with other neurons, resulting in trillions of connections. Synaptic pruning also takes place, through which superfluous synapses (which are rarely activated) are removed. Synaptic pruning is a normal process, but abnormal patterns of synaptic pruning have been linked to conditions such as autism and schizophrenia.

There are different techniques for mapping the functioning of the brain. These techniques have led to a better understanding of the brain and its development. Electroencephalography (EEG) is a technique that measures the electrical activity generated by neurons using electrons on the skull. Event-related potentials (ERPs) are changes in electrical activity that arise from the presentation of a particular stimulus. Magnetoencephalography (MEG) detects magnetic fields generated by electrical currents in the brain. Functional magnetic resonance imaging(fMRI) is a technique in which fluctuations in cerebral blood flow are measured by means of an electromagnet, which is used to measure activity in brain areas. Positron emission tomography (PET) measures brain activity by detecting metabolic processes; radioactive material is injected. Near-infrared spectroscopy (NIRS) is an optical imaging technique in which metabolic changes are observed that lead to different absorption of infrared light.    

What is the importance of experience?

Certain processes in the brain are indeed dependent on experiences. This is made clear by the degree of plasticity. This refers to the capacity of the brain that will be influenced by experiences. There are distinct forms of plasticity: experience-expectancy-plasticity and experience-dependent plasticity. The experience-expectancy-plasticity is a process in which the brain forms as a result of contemporary experiences in a normal environment. Experience-dependent plasticity is a process in which the neural connections are made and reorganized as a result of individual experiences.

Because of the plasticity, the brain can be rewired to a certain extent. Children would therefore have a better chance of recovery after damage because a lot of reorganization can still take place. On the other hand, this depends on how serious the damage is and at what point in the development this takes place. In severe damage, the brain can no longer develop properly.

What processes of growth and development take place in the body?

The variability in physical growth depends on genetic and environmental factors. The genes mainly influence growth and sexual maturation through the production of hormones. The environmental factors mainly influence the secular trends, in which changes in physical development occur over the years, such as growing in length or the menstrual cycle. Failure to thrive is a condition in which babies become malnourished and do not grow or gain weight without a clear medical reason. This has to do with a combination of genetic and environmental factors. 

Nutrition is an important aspect in physical developments. Breastfeeding has many advantages for the baby, it is good for the immune system and for cognitive development. Babies have a strong preference for sweet tastes. Young children often do not want to eat unfamiliar things. The preferences are influenced by associative learning; positive associations with the food lead to positive feelings about the food.

Overeating leads to obesity. This is mainly due to the Western style of eating with a lot of fats and sugar. As children grow older, they retain the problems with their weight, which in turn leads to an unhealthy lifestyle, (mental) health problems, and social problems. On the other hand, malnutrition is prevalent around the world. This is strongly related to poverty and therefore mainly occurs in underdeveloped countries. Poverty is often the cause and the consequence of malnutrition.       

Babies in different socioeconomic groups do not differ in brain development up to the age of 12 months. However, a lower SES leads to reduced brain growth. Children who live in poverty are also more likely to have health problems and benefit less from treatment than children from richer families.

The availability of vaccines is another important factor in healthy physical development. Vaccines consist of a weak variant of a disease that is injected into the body so that the body starts to produce antibodies against this disease. These antibodies protect against subsequent exposure to the disease. Some parents believe that vaccines lead to autism, but the research on which this theory is based was found to be fraudulent. The effectiveness of vaccines is based on herd immunity, which means that a certain percentage of the population must be vaccinated to protect it from the disease.