Heredity and Environment

What determines how a child develops? In reality, it would be impossible to account for each and every influence that ultimately determines who a child becomes. What we can look at are some of the most apparent influences such as genetics, parenting, experiences, friends, family relationships and school to help us understand the influences that help contribute to a child's growth.

Think of these influences as building blocks. While most people tend to have the same basic building blocks, these components can be put together in an infinite number of ways. Consider your own overall personality. How much of who you are today was shaped by your genetic inheritance, and how much is a result of your lifetime of experiences?

This question has puzzled philosophers, psychologists and educators for hundreds of years and is frequently referred to as the nature versus nurture debate. Are we the result of nature (our genetic background) or nurture (our environment)? Today, most researchers agree that child development involves a complex interaction of both nature and nurture. While some aspects of development may be strongly influenced by biology, environmental influences may also play a role. For example, the timing of when the onset of puberty occurs is largely the results of heredity, but environmental factors such as nutrition can also have an effect.

From the earliest moments of life, the interaction of heredity and the environment works to shape who children are and who they will become. While the genetic instructions a child inherits from his parents may set out a road map for development, the environment can impact how these directions are expressed, shaped or event silenced. The complex interaction of nature and nurture does not just occur at certain moments or at certain periods of time; it is persistent and lifelong.

In this article, we'll take a closer look at how biological influences help shape child development. We'll learn more about how our experiences interact with genetics and learn about some of the genetic disorders that can have an impact on child psychology and development.

Going From One Cell to Trillions

At its very beginning, the development of a child starts when the male reproductive cell, or sperm, penetrates the protective outer membrane of the female reproductive cell, or ovum. The sperm and ovum each contain chromosomes that act as a blueprint for human life. The genes contained in these chromosomes are made up of a chemical structure known as DNA (deoxyribonucleic acid) that contains the genetic code, or instructions, that make up all life. Except for the sperm and ova, all cells in the body contain 46 chromosomes. As you might guess, the sperm and ova each contains only contain 23 chromosomes. This ensures that when the two cells meet, the resulting new organism has the correct 46 chromosomes.

From Genotype to Phenotype

So how exactly do the genetic instructions passed down from both parents influence how a child develops and the traits they will have? In order to fully understand this, it is important to first distinguish between a child's genetic inheritance and the actual expression of those genes. A genotype refers to all of the genes that a person has inherited. The actual express of these traits is the person's phenotype. The phenotype can include physical traits, such as height and color or the eyes, as well as nonphysical traits such as shyness, a high strung temperament or a thirst for adventure.

Remember our building block metaphor from earlier? While our genotype may represent a blueprint for how children grow up, the way that these building blocks are put together determines how these genes will be expressed. Think of genes as something like a blueprint to a house. Two houses can be constructed from the exact same blueprint, but the materials chosen build each house can vary dramatically from one to the next.

Influences on Gene Expression

Whether or not a gene is expressed depends on two different things: the interaction of the gene with other genes and the continual interaction between the genotype and the environment.

Genetic Interactions: Genes can sometimes contain conflicting information, and in most cases, one gene will win the battle for dominance. Some genes act in an additive way. For example, if a child has one tall parent and one short parent, the child may end up splitting the difference by being of average height. In other cases, some genes follow a dominant-recessive pattern. Eye color is one example of dominant-recessive genes at work. The gene for brown eyes is dominant and the gene for blue eyes is recessive. If one parent hands down a dominant brown eye gene while the other parent hands down a recessive blue eye gene, the dominant gene will win out and the child will have brown eyes.

Gene - Environment Interactions: The environment a child is exposed to both in utero and throughout the rest of his or her life can also impact how genes are expressed. For example, exposure to harmful drugs while in utero can have a dramatic impact on later child development. Height is a good example of a genetic trait that can be influenced by environmental factors. While a child's genetic code may provide instructions for tallness, the expression of this height might be suppressed if the child has poor nutrition or a chronic illness.
Genetic Abnormalities

Genetic instructions are not infallible and can go off track at times. Sometimes when a sperm or ovum is formed, the number of chromosomes may divide unevenly, causing the organism to have more or less than the normal 23 chromosomes. When one of these abnormal cells joins with a normal cell, the resulting zygote will have an uneven number of chromosomes. Researchers suggest that as many as half of all zygotes that form have more or less than 23 chromosomes, but most of these are spontaneously aborted and never develop into a full-term baby.

In some cases, about 1 in every 200 births, a baby is born with an abnormal number of chromosomes. In every case, the result is some type of syndrome with a set of distinguishing characteristics.

Down Syndrome

The most common type of chromosomal disorder is known as trisomy 21, or Down syndrome. In this case, the child has three chromosomes at the site of the 21st chromosomes instead of the normal two. Down syndrome is characterized by facial characteristics including a round face, slanted eyes and a thick tongue. Individuals with Down syndrome may also face other physical problems including heart defects and hearing problems. Nearly all individuals with Down syndrome experience some type of intellectual impairment, but the exact severity can vary dramatically. No matter the severity of the syndrome, early intervention can result in much better outcomes, allowing many people with Down syndrome to care for themselves and gain more independence.

Abnormalities of the Sex Chromosomes

The vast majority of newborns, both boys and girls, have at least one X chromosome. In some cases, about 1 in every 500 births, children are born with either a missing X chromosome or an additional sex chromosome. Klinefelter syndrome, Fragile X syndrome and Turner syndrome are all examples of abnormalities involving the sex chromosomes.

Kleinfelter's syndrome is caused by an extra X chromosome and is characterized by a lack of development of the secondary sex characteristics and as well as learning disabilities.

Fragile X syndrome is caused when part of the X chromosome is attached to the other chromosomes by such a thin string of molecules that it seems in danger of breaking off. It can affect both males and females, but the impact can vary. Some with Fragile X show few if any signs, while other develop mild to severe mental retardation.

Turner syndrome occurs when only one sex chromosome (the X chromosome) is present. It affects only females and can result in short stature, a "webbed" neck and a lack of secondary sex characteristics. Psychological impairments associated with Turner syndrome include learning disabilities and difficulty recognizing emotions conveyed through facial expressions.

Final Thoughts

Clearly, genetics have an enormous influence on how a child develops. However, it is important to remember that genetics are just one piece of the intricate puzzle that makes up a child's life. Environmental variables, including parenting, culture, education and social relationships also play a vital role.