Human Genetics

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While all humans share the same biological processes and mechanisms, we are unique biological specimens with very unique thoughts and behaviors. Psychologists understand that the genetic code inside each cell of our bodies is a kind of unique software that allows for a wide variety of biological and psychological reactions to the world around us. For example, a particular combination of genes may make us more or less susceptible to a disease like malaria or cystic fibrosis. In fact, it has been found that some very rare diseases are confined to particular ethnic groups or regions or can be tracked back through particular families who have a genetic mutation, such as the mutation that causes Machado-Joseph’s disease, a neurodegenerative disease. Similarly, some psychological disorders like schizophrenia may have a genetic component. Some individuals may be at high genetic risk for getting the disease. Thus, psychological researchers study genetics in order to better understand the biological basis for some behaviors.

Genetic Variation

Genetic variation, the genetic difference between individuals, is what contributes to a species’ adaptation to its environment. In humans, genetic variation begins with an egg from the woman, about 100 million sperm from the man, and fertilization. Fertile women ovulate, or release an egg from the ovary to the uterus, roughly once per month. During the egg's journey from the ovary through the fallopian tubes to the uterus, a sperm may fertilize an egg.

The egg and the sperm each contain 23 chromosomes. Chromosomes are long strings of genetic material known as deoxyribonucleic acid (DNA). DNA is a helix-shaped molecule made up of nucleotide base pairs. In each chromosome, sequences of DNA make up genes that control or partially control a number of visible characteristics, known as traits, such as eye color, hair color, and so on. A single gene may have multiple possible variations, or alleles. An allele is a specific version of a gene. So, a given gene may code for the trait of hair color, and the different alleles of that gene affect which hair color an individual has.

When a sperm and egg fuse, their 23 chromosomes pair up and create a zygote with 23 pairs of chromosomes. Therefore, each parent contributes half the genetic information carried by the offspring. We can see the number of chromosomes and what they look like in the karyotype in the cell, which can be viewed under a light microscope. The resulting physical characteristics of the offspring (called the phenotype) are determined by the interaction of genetic material supplied by the parents (called the genotype). A person’s genotype is the genetic makeup of that individual. Phenotype, on the other hand, refers to the individual’s inherited physical characteristics, which are a combination of genetic and environmental influences.

Karyotypes of Chromosomes

Most traits are controlled by multiple genes, but some traits are controlled by one gene. A characteristic like cleft chin, for example, is influenced by a single gene from each parent. In this example, we will call the gene for cleft chin “B,” and the gene for smooth chin “b.” Cleft chin is a dominant trait, which means that having the dominant allele either from one parent (Bb) or both parents (BB) will always result in the phenotype associated with the dominant allele. When someone has two copies of the same allele, they are said to be homozygous for that allele. When someone has a combination of alleles for a given gene, they are said to be heterozygous. For example, smooth chin is a recessive trait, which means that an individual will only display the smooth chin phenotype if they are homozygous for that recessive allele (bb).

Imagine that a woman with a cleft chin mates with a man with a smooth chin. What type of chin will their child have? The answer to that depends on which alleles each parent carries. If the woman is homozygous for cleft chin (BB), her offspring will always have cleft chin. It gets a little more complicated, however, if the mother is heterozygous for this gene (Bb). Since the father has a smooth chin — therefore homozygous for the recessive allele (bb) — we can expect the offspring to have a 50 percent chance of having a cleft chin and a 50 percent chance of having a smooth chin.

Recessive and Dominant Genes in Cleft Chin Trait

Sickle-cell anemia is just one of many genetic disorders caused by the pairing of two recessive genes. Another disease, phenylketonuria (PKU), is a genetic condition in which individuals lack an enzyme that normally converts harmful amino acids into harmless byproducts. If someone with this condition goes untreated, he or she will experience significant deficits in cognitive function, seizures, and increased risk of various psychiatric disorders. Because PKU is a recessive trait, each parent must have at least one copy of the recessive allele in order to produce a child with the condition.

Recessive and Dominant Genes in Phenylketonuria

So far, we have discussed traits that involve just one gene, but few human characteristics are controlled by a single gene. Most traits are polygenic: controlled by more than one gene. For example, height is a polygenic trait. There are actually three genes with six alleles that control height. If you are dominant on all the alleles for height then you will be tall. If you are recessive on all, you will be short. But if you have a combination of dominant and recessive, you will be somewhere in between in height. Skin color and weight are two other common traits that are polygenic, or controlled by several alleles in our genetic code.

Polygenic Chart

Where do harmful genes that contribute to diseases like PKU come from? Gene mutations provide one source of harmful genes. A mutation is a sudden, permanent change in a gene. While many mutations can be harmful or lethal, once in awhile, a mutation benefits an individual by giving that person an advantage over those who do not have the mutation.

The theory of evolution, as first proposed by the scientist Charles Darwin, asserts that individuals best adapted to their particular environments are more likely to reproduce and pass on their genes to future generations, a phenomenon he called “natural selection.” In order for evolution to occur, there must be competition for natural resources among plant and animal species. But even more important, there must be variability in genes (and resultant traits) that allow for variation in adaptability to the changing natural environment. If a population consisted of identical individuals with exactly the same genes, any dramatic change in the environment would affect everyone in the same way, and there would be no variation in selection. In contrast, diversity in genes and associated traits allows some individuals to perform slightly better than others when faced with environmental change. As noted before, a mutation in a gene can also help an individual adapt better. This creates a distinct advantage for individuals best suited for the new environment, and they are most likely to reproduce and pass on their genes to the next generation.

The Five Types of Mutation

Gene-Environment Interactions

Genes do not exist in a vacuum. Although we are all biological organisms, we also exist in an environment that is incredibly important in determining not only when and how our genes express themselves, but also in what combination. Each of us represents a unique interaction between our genetic makeup and our environment, and “range of reaction” is one way to describe this interaction. Range of reaction asserts that our genes set the boundaries within which we can operate, and our environment interacts with the genes to determine where in that range we will fall. For example, if an individual’s genetic makeup predisposes her to high levels of intellectual potential and she is reared in a rich, stimulating environment, then she will be more likely to achieve her full potential than if she were raised under conditions of significant deprivation. According to the concept of range of reaction, genes set definite limits on potential, and environment determines how much of that potential is achieved. Some disagree with this theory and argue that genes do not set a limit on a person’s potential.

Another perspective on the interaction between genes and the environment is the concept of genetic-environmental correlation. Stated simply, our genes influence our environment, and our environment influences the expression of our genes. Not only do our genes and environment interact, as in range of reaction, but they also influence one another bidirectionally. For example, the child of an NBA player would probably be exposed to basketball from an early age. Such exposure might allow the child to realize his or her full genetic, athletic potential. Thus, the parents’ genes, which the child shares, influence the child’s environment, and that environment, in turn, is well suited to support the child’s genetic potential.

In another approach to genetic-environmental interactions, the field of epigenetics looks beyond the genotype itself and studies how the same genotype can be expressed in different ways. In other words, researchers study how the same genotype can lead to very different phenotypes. As mentioned earlier, gene expression is often influenced by environmental context in ways that are not entirely obvious. For instance, identical twins share the same genetic information. (Identical twins develop from a single fertilized egg that splits, so the genetic material is exactly the same in each; in contrast, fraternal twins develop from two different eggs fertilized by different sperm, so the genetic material varies as with non-twin siblings.) But even with identical genes, there remains an incredible amount of variability in how gene expression can unfold over the course of each twin’s life.

Sometimes, one identical twin will develop a trait and the other will not. In one example, Jennifer has moderate weight, but her identical twin Karen is obese. While Jennifer was careful about what she ate and mostly ate healthy food, Karen ate fatty foods and was not careful. Although these individuals share an identical genotype, their phenotypes differ as a result of how that genetic information is expressed over time. The epigenetic perspective is very different from range of reaction, because here the genotype is not fixed and limited.

Gene-Environment Interaction on Identical Twins

Genes affect more than our physical characteristics. Indeed, scientists have found genetic linkages to a number of behavioral characteristics, ranging from basic personality traits to sexual orientation to spirituality. Genes are also associated with temperament and a number of psychological disorders, such as depression and schizophrenia. So while it is true that genes provide the biological blueprints for our cells, tissues, organs, and body, they also have significant impacts on our experiences and our behaviors.

Susan Snycerski on Environment and Genetics