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Genetic Influences

One of the most amazing findings from modern science is that your entire body, including your complicated brain, is based on patterns built into DNA molecules. Development really begins, for the individual organism, when DNA from two parents is combined.

To what does a gene correspond, physically?

A gene corresponds, physically, to a set of DNA sequences that work together to produce a biological product such as a protein. The sequences may be widely separated on the DNA molecule.

For example, a control region can be in one location and a coding region that works with it could be a thousand base pairs distant, on the same DNA strand. Therefore the word "gene" really refers to a functional system, components that work together, not a single location.

The whole organism comes from information coded into the sequences of nucleotides on DNA molecules. Each nucleotide contains a sugar (deoxy­ribose), a phosphate group, and one of four nitrogenous bases: adenine, thymine, guanine, and cytosine.

The four bases form into pairs..."A" with "T" and "G" with "C." Each base pair (AT, TA, GC, CG) forms a "rung" on the DNA ladder. The artist's conception of DNA is familiar to all: a double spiral with linking bases, as shown below.

typical DNA diagram
An artist's conception of the DNA molecule. Under a microscope DNA looks more like a clump of spaghetti.

What are chromosomes?

Chromosomes are structures in the nucleus of a cell that contain the DNA for genetic transmission. Each chromosome is a single, huge strand of DNA, six feet long, folded up, containing about 20,000 active genes.

Chromosomes are often displayed in a karyotype, which is a photomicrograph (photograph taken through a light microscope) showing the chromosomes. An example of a karyotype is shown below.

photograph of chromosomes
A karyotype

Humans have 46 chromosomes grouped in 23 pairs. The so-called sex chromosomes are shown as the 23rd pair in the karyotype. The XX pattern is typical of females. The XY pattern shown here is typical of males. If this karyotype were from a female, the last pair would consist of identical-looking X-shaped chromosomes.

What is a karyotype?

Chromosomes come in matched pairs, except for the XY chromosomes of pair #23. In each pair, one chromosome comes from the father, one from the mother, so each person receives 50% of his or her genes from the mother, 50% from the father.

Why does a child often bear strong resemblance to a grandparent?

There is no similar rule saying 25% must come from each grandparent. In the 50% from each parent, genes from one or another grandparent may be more numerous.

As this implies, a child can receive a disproportionate share of genes from one grandparent by the luck of the draw. If the genes affect appearance, person­ality, or talents, a child may resemble one grandparent more than others.

Genes and Environment:
"There is No Versus"

Because physical and social environ­ments are intertwined in development, many psychologists do not like to hear references to "genes vs. environment." That makes it sound as if genes and environmental influences are indepen­dent, opposite forces of nature.

The reality is that one is never found without the other. As developmental psychologist John L. Locke put it, "There never was a versus" (Ratner, 1993). Complex mixes of genetic and environ­mental factors influence all behavior, without exception.

What was Locke's point? What was Eleanor Gibson's complaint?

The famous developmental psychologist Eleanor Gibson referred to the nature-nurture dichotomy as a "hobgoblin" (Gibson, 1994). She complained about colleagues who should know better asking, "Where do you stand on the nature-nurture dimension?"

Gibson points out, "There is no such dimension." The word dimension suggests a line stretching from heredity on one side to environment on the other. But heredity and environment are not on such a dimension. They are always intermixed.

Even when heredity is a powerful influence, environment is still crucial. Consider Wayne Gretzky, often called the greatest hockey player of all time. Socialization and learning were both essential to that outcome.

Gretzky was born in Canada, where hockey is emphasized. He played hockey in an organized league at a young age. He excelled and was called The Great One even as a child. He spent huge amounts of time practicing hockey as he grew up.

What are some points illustrated by the example of Wayne Gretzky?

No doubt Gretzky had natural talent. He probably had a good body build for hockey, good reaction time, and excellent motor coordination. Not every child earns a nickname like The Great One while still in grade school.

However, this introduces a complica­tion. Think of the psychological (envi­ronmental) impact on a young child of being called The Great One. Would this not stimulate a powerful motivation to excel, perhaps to try to become the best in the world?

What would have happened if Wayne Gretzky had grown up in Mexico instead of Canada? He might have been a very good athlete, but probably he would not have been a hockey player.

What if Gretzky had grown up in Can­ada but under conditions of extreme poverty, so his caretakers could not afford to enroll him in the junior hockey league? What if another hockey player was already called The Great One and Gretzky was called Number Two?

Such environmental factors might have altered the outcome of his career. Genes are important, but they do not determine what we become, they only provide a potential.

The Heritability Index

Twin research is important to scientists studying genes. There are two types of twins: fraternal (dizygotic) and identical (monozygotic) twins. The term "fraternal" may falsely imply that dizygotic twins must be males, when they could be females, or a male/female pair. So the technical term dizygotic (coming from two eggs or zygotes) is preferred.

Dizygotic twins come from two different sperm and two different eggs and share 50% of their genetic material, like any brother or sister. Identical twins, by contrast, come from one zygote (fertilized egg) that splits and grows into two individuals. Alone among humans, identical twins share 100% of their genetic material with somebody else.

What are influences that can cause identical twins can develop differently?

This does not mean identical twins develop in identical ways, however. For one thing, they may be mirror images of each other.

This can be determined by examining the thumb prints. If they swirl in opposite directions, then the identical twins are the mirror image type.

That can affect behavior because, to a limited degree, the two hemispheres of the brain are specialized for different tasks. One identical twin may be especially good with language, for example, while the other is better with math.

Epigenetics (environmental effects on expression of the DNA) may also lead to differences of genetic expression in identical twins. Epigenetic effects typically occur in the womb, after identical twin embryos are separated, and they may result in different genetic systems being activated.

Finally, it should go without saying that twins may choose to emphasize differences. Some twins like to dress alike and act alike, others emphasize their differentness.

Sometimes twins disagree about which of those options is preferable. (That happened with the identical twins and famous advice columnists Ann Landers and Dear Abby, according to one of their daughters.) Different choices can lead to different lifestyles, looks, health, and mannerisms.

However, other things being equal, identical twins are much more likely to make similar life choices or experience similar life events. For all their differences and occasional acrimony, Ann Landers and Dear Abby looked alike, chose the same profession, and were both highly successful with it.

The distinction between dizygotic and monozygotic twins is used to calculate a measure of genetic influence called a heritability index. The heritability index is the amount of statistical variability (in test scores or other measurements) account­ed for by genes.

In other words, the heritability index tells you how much you could improve the accuracy of a prediction about one identical twin, given a measurement of the same variable obtained from the other identical twin, as compared to typical fraternal twins measured for the same trait. If traits are highly correlated in identical twins, but not so much in fraternal twins, then a trait has a high heritability index.

What does the heritability index tell us?

The heritability index is what people are usually referring to when they make statements such as, "the genetic component of homo­sexuality is somewhere between 30% and 70%." The component referred to here is a statistical concept. It is like saying, "the expected error in your prediction of sexual orientation is reduced by 30-70%, if you know the sexual orientation of the identical twin."

How can something like religiosity produce a high heritability index, when it is plainly learned?

Heritability indexes can be calcu­lated for any psychological trait that can be measured. For example, Waller, Kojetin, Bouchard, Lykken and Tellegen (1990) showed genes contributed about 50% to measures of religiosity (the tendency to be religious).

The researchers explored this variable because Plomin, an expert on behavior genetics, suggested that religiosity and political beliefs probably would not be related to genes. Taking this as a challenge, Waller and colleagues compared the religious and political views of identical and dyzygotic twins. They concluded that religiosity actually had a high heritability index.

Does this mean you inherit a tendency to be religious or not religious? Not specif­ically. It means religiosity correlates with temperament, personality, or other factors influenced by genes.

To make a point about the limitations of the heritability index, Plomin, Corley, DeFries, and Fulker (1990) calculated a heritability index for television-watching. As they suspected, it had a high herit­ability index. They wrote:

Of course, there are no genes for television viewing just as there are no genes for performance on IQ tests or for height....

We do not inherit genes that code for vocabulary words or for height, and we cannot inherit genes that code for television viewing. Genes only code for sequences of amino acids...

Finding genetic influence on indi­vidual differences in children's television viewing means that some unspecified genetic differences among children indirectly affect the extent to which children watch television.

What point did Plomin and colleagues make with the example of television viewing?

The moral of the story is that high heritability indexes do not demonstrate that a behavior is "coded in the genes." A high heritability index merely indicates that, for the variable being measured, identical twins are much more similar than dyzygotic twins.


Gibson, E. J. (1994). Has psychology a future? Psychological Science, 5, 69-76.

Plomin, R., Corley, R., DeFries, J. C., & Fulker, D. W. (1990). Individual differences in television viewing in early childhood: Nature as well as nurture. Psychological Science, 1, 371-377.

Ratner, N. B. (1993). [book review]. Learning to speak. Science, 262, 260.

Waller, N. G., Kojetin, B. A., Bouchard, T. J., Lykken, D. T., & Tellegen, A. (1990). Genetic and environmental influences on religious interests, attitudes, and values: A study of twins reared apart and together. Psychological Science, 1, 138-142.

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