Genetic Influences in Human Development

1. Behavioral Genetics

Behavioral genetics is the scientific study of how genes and environments combine to generate behavior. Behavioral geneticists studies how individual differences arise in the present through the interaction of genes and the environment.

hint

Behavioral genetics is an empirical science. Empirical science is based on actual scientific experiments and observations exploring how or why things happen.

Behavioral geneticists study human behavior. They often use twin and adoption studies to research questions of interest.

• A twin study compares the rates that a given behavioral trait is shared among identical and fraternal twins.
• An adoption study compares the rates that a given behavioral trait is shared among biologically related relatives and adopted relatives.

think about it

If the biological child of tall parents were adopted into a family of short people, do you suppose the child’s growth would be affected? What about the biological child of a Spanish-speaking family adopted at birth into an English-speaking family? What language would you expect the child to speak? And what might these outcomes tell you about the difference between height and language in terms of nature and nurture?

terms to know

Behavioral Genetics

The empirical science of how genes and environments combine to generate behavior.

Twin Study

A behavior genetic research method that involves a comparison of the similarity of identical (monozygotic) and fraternal (dizygotic) twins.

Adoption Study

A behavior genetic research method that involves the comparison of adopted children to their adoptive and biological parents.

2. The Epigenetic Framework

As you may recall, you learned about the epigenetic framework in the first Challenge of this Unit. The term epigenetic has been used in developmental psychology to describe the impact the environment and our experiences on gene expression. It is the result of an ongoing, bi-directional interchange between these two factors.

Developmental psychologists Gilbert Gottlieb and Erik Erikson have contributed to this field in various ways. Gottlieb suggests an analytic framework for the nature versus nurture debate that recognizes the relationship between the environment, behavior, and genetic expression (Gottlieb 1998; 2000; 2002). This bi-directional relationship suggests that the environment can affect the expression of genes just as genetic predispositions can impact a person’s potentials. Likewise, environmental circumstances can trigger symptoms of a genetic disorder.

EXAMPLE

A person genetically predisposed for type 2 diabetes may trigger the disease through poor diet and little exercise.

Erikson also wrote of an epigenetic principle in his 1968 book Identity: Youth and Crisis. He emphasized that we develop through an unfolding of our personality in predetermined stages, and that our environment and surrounding culture influence how we progress through these stages. This biological unfolding in relation to our socio-cultural settings is done in stages of psychosocial development, where “progress through each stage is in part determined by our success, or lack of success, in all the previous stages” (Erikson, 1968).

In many human families, children’s biological parents raise them, so it is very difficult to differentiate whether children act like their parents due to genetic (nature) or environmental (nurture) reasons. Nevertheless, despite our restrictions on setting up human-based experiments, we do see real-world examples of nature vs. nurture at work in the human sphere - though they only provide partial answers to our many questions.

events to know

The Chernobyl nuclear disaster is a significant event in recent history. The nuclear power plant meltdown in Chernobyl, Ukraine, occurred in April 1986 and exposed everyone and everything in the surrounding region to disastrous levels of radiation. Researchers have looked at health outcomes among the workers, their families, duration of exposure, etc. There have been longitudinal studies that have followed pregnant women during the time of the disaster, the birth of their children, and maternal and child outcomes in the years after that. One finding among many is the change in genome damage among children who were exposed to extremely high levels of radiation when compared to the general public (Fucic et al., 2016). The damage on the genome is present in both children who were evacuated and continually exposed to the radiation. This is a key finding as there is an association between damage on the genome, birth anomalies, and cancer risk in this population to this day, more than thirty years later. The Chernobyl disaster, while unfortunate, provides us with immense insight into epigenetics where a single environmental radiation exposure impacted genes and their role in development of disease(s).

3. Monozygotic and Dizygotic Twins

Another option for observing nature versus nature in humans involves twin studies. To analyze the nature-nurture interaction using twins, we compare the similarity of monozygotic and dizygotic pairs.

Monozygotic (one egg identical) twins occur when a single zygote or fertilized egg splits apart in the first two weeks of development. The result is the creation of two separate but genetically identical offspring. About one-third of twins are monozygotic twins. Consider these facts:

• Monozygotic twins occur in birthing at a rate of about 3 in every 1,000 deliveries worldwide (about 0.3% of the world population).
• Monozygotic twins are genetically nearly identical and they are always the same sex unless there has been a mutation during development.
• The children of monozygotic twins test genetically as half-siblings (or full siblings, if a pair of monozygotic twins reproduces with another pair of identical twins or with the same person), as opposed to first cousins.
• In the uterus, a majority of monozygotic twins (60–70%) share the same placenta but have separate amniotic sacs (a fluid-filled sac that protects and contains the fetus in the uterus, also known as aminos.)
• In 18–30% of monozygotic twins, each fetus has a separate placenta and a separate amniotic sac.
• A small number (1–2%) of monozygotic twins share the same placenta and amniotic sac.

• About two-thirds of twins are dizygotic.
• Dizygotic twins share the same amount of genetic material as would any two children from the same mother and father.
• Older mothers are more likely to have dizygotic twins than are younger mothers.
• Couples who use fertility drugs are also more likely to give birth to dizygotic twins. Consequently, there has been an increase in the number of fraternal twins in recent years (Bortolus et al., 1999).
• In-vitro fertilization (IVF) techniques are also more likely to create dizygotic twins. For IVF deliveries, there are nearly 21 pairs of twins for every 1,000.
• Fraternal twins each have their own placenta and own amniotic sac.

big idea

Identical twins (monozygotic) come from one egg dividing and usually share the same placenta. Fraternal twins (dizygotic) come from two separate eggs, each with its own placenta.

Monozygotic twins can be categorized into four types depending on the timing of the separation and duplication of cells. The categorization of monozygotic twins depends on various types of chorionicity (the number of placenta) and amnionicity (the number of amnions or inner membranes that encircle the fetus).

Monozygotic twins are a result of when and how the fertilized egg divides. This is known as placentation (the formation or arrangement of the placenta in a woman's uterus). The following diagram shows four ways monozygotic twins are formed, depending on the timing of the separation and duplication of cells.

1. Morula with cleavage at days 1 to 3 and results in dichorionic / diamniotic formation (separate placenta, separate amnion).
2. Blastocyst with cleavage at days 4 to 8 which results in monochorionic / diamniotic formation (sharing the same placenta but having separate amnions).
3. Implanted blastocyst with cleavage at days 8 to 13 results in monochorionic / mono amniotic (shared placenta and shared amnions).
4. Formed embryonic disc with cleavage at days 13 to 15 which in this figure leads to conjoined twins (shared placenta, shared amnions, and bodies fused).

terms to know

Monozygotic

Derived from a single ovum.

Amniotic Sac

A fluid-filled sac that protects and contains the fetus in the uterus.

Dizygotic

Derived from two separate ova.

Chorionicity

The number of placenta.

Amnionicity

How the baby’s amniotic sac looks.

Placentation

The connection between the fetus and the mother’s uterus.

3a. Conjoined Twins

Conjoined twins are monozygotic twins whose bodies are joined together during pregnancy. This occurs when the zygote starts to split after day 12 following fertilization and fails to separate completely. This condition occurs in about 1 in 50,000 human pregnancies, however, actual incidence is estimated at 1 in 250,000 births (Spitz, Kiely, & Pierro, 2018).

Most conjoined twins are now evaluated for surgery to attempt to separate them into separate functional bodies. The degree of difficulty rises if a vital organ or structure is shared between twins, such as the brain, heart, pelvis, or liver.

key concept

Conjoined twins are usually categorized based on the site of adjoining or the area of the body which the twins share. These are the eight categories of conjoined twins and the shared organs and/or body parts (Spitz, Kiely, & Pierro, 2018):

1. Parapagus (lower abdomen, umbilical region, pelvis, and genitourinary)
2. Thoracopagus (upper abdomen, thorax, and heart)
3. Omphalopagus (upper abdomen with no sharing of the heart)
4. Cephalopagus (head and thorax)
5. Ischiopagus (pelvic bones, gastrointestinal tract, and genitourinary)
6. Pygopagus (sacrum, pelvic bones, and coccyx)
7. Craniopagus (skull, sinuses, and meninges)
8. Rachipagus (neural tube)

did you know

The first incident of symmetrically conjoined twins was illustrated in a stone carving that is dated to 80 BC.

term to know

Conjoined Twins

Monozygotic twins whose bodies are joined together during pregnancy.

3b. Vanishing Twins

Researchers suspect that as many as 1 in 8 pregnancies start out as multiples, but only a single fetus is brought to full term because the other fetus has died very early in the pregnancy and has not been detected or recorded. Early obstetric ultrasonography exams sometimes reveal an “extra” fetus, which fails to develop and instead disintegrates and vanishes in the uterus. There are several reasons for the “vanishing” fetus, including it being embodied or absorbed by the other fetus, placenta or the mother. This is known as vanishing twin syndrome. Also, in an unknown proportion of cases, two zygotes may fuse soon after fertilization, resulting in a single chimeric embryo, and, later, fetus.

term to know

Vanishing Twins

A fetus that fails to develop during pregnancy and disintegrates.

4. Twin Studies

Let’s now look at how nature and nurture apply through twin studies, using the features of height and spoken languages. Unsurprisingly, identical twins are almost perfectly similar for height. The heights of fraternal twins, however, are like any other sibling pairs: more similar to each other than to people from other families, but hardly identical. This contrast between twin types gives us a clue about the role genetics plays in determining height.

Now consider spoken language. If one identical twin speaks Spanish at home, the co-twin with whom she or he is raised almost certainly does too. But the same would be true for a pair of fraternal twins raised together. In terms of spoken language, fraternal twins are just as similar as identical twins, so it appears that the genetic match of identical twins does not make much difference.

IN CONTEXT

Twin studies and adoption studies are two instances of a much broader class of methods for observing nature-nurture called quantitative genetics—the scientific discipline in which similarities among individuals are analyzed based on how biologically related they are. We can do these studies with siblings and half-siblings, cousins, and twins who have been separated at birth and raised separately (Bouchard, Lykken, McGue, Segal, & Tellegen, 1990). Such twins are very rare and play a smaller role than is commonly believed in the science of nature–nurture (Plomin, DeFries, Knopik, & Neiderhiser, 2013).

It would be satisfying to be able to say that nature-nurture studies have given us conclusive and complete evidence about where traits come from, with some traits clearly resulting from genetics and others almost entirely from environmental factors, such as child-rearing practices and personal will; but that is not the case. Instead, everything has turned out to have some footing in genetics. The more genetically-related people are, the more similar they are - for everything: height, weight, intelligence, personality, mental illness, etc.

big idea

Adopted children resemble their biological parents even if they have never met them, and identical twins are more similar to each other than are fraternal twins. And while certain psychological traits, such as personality or mental illness (e.g., schizophrenia), seem reasonably influenced by genetics, it turns out that the same is true for political attitudes, how much television people watch (Plomin, Corley, DeFries, & Fulker, 1990), and whether or not they get divorced (McGue & Lykken, 1992).

term to know

Quantitative Genetics

Scientific and mathematical methods for inferring genetic and environmental processes based on the degree of genetic and environmental similarity among organisms.

REFERENCES

Bortolus, R., Parazzini, F., Chatenoud, L., Benzi, G., Bianchi, M. M., & Marini, A. (1999). The epidemiology of multiple births. Human Reproduction Update, 5, 179-187.

Bouchard Jr, T. J., Lykken, D. T., McGue, M., Segal, N. L., & Tellegen, A. (1990). Sources of human psychological differences: The Minnesota study of twins reared apart. Science, 250(4978), 223-228.

Erikson, E. H. (1968). Identity: Youth and crisis. New York: Norton.

Fucic, A., Aghajanyan, A., Druzhinin, V., Minina, V., & Neronova, E. (2016). Follow-up studies on genome damage in children after Chernobyl nuclear power plant accident. Archives of toxicology, 90(9), 2147-2159.

Gottlieb, G. (1998). Normally occurring environmental and behavioral influences on gene activity: From central dogma to probabilistic epigenesis. Psychological Review, 105, 792-802.

Gottlieb, G. (2000). Environmental and behavioral influences on gene activity. Current Directions in Psychological Science, 9, 93-97.

Gottlieb, G. (2002). Individual development and evolution: The genesis of novel behavior. New York: Oxford University Press

McGue, M., & Lykken, D. T. (1992). Genetic influence on risk of divorce. Psychological Science, 3(6), 368-373.

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(6), 371-377.

Spitz, L., Kiely, E., & Pierro, A. (2018). Conjoined twins. In Rickham's Neonatal Surgery (pp. 457-474). Springer, London.

Newton, R. (2022, October 28). 2.2 genetics and Heredity. Human Growth and Development. Retrieved February 9, 2023, from pressbooks.pub/mccdevpsych/chapter/2-2-genetics-and-heredity/

Lazzara, J. (2020, June 26). Genetics and prenatal development. Lifespan Development. Retrieved February 9, 2023, from open.maricopa.edu/devpsych/chapter/chapter-2-genetics-and-prenatal-development/