Genetic Variation

After a sperm fertilizes an egg, it forms a zygote. The egg and the sperm each contain 23 chromosomes, and so the zygote has 23 pairs of chromosomes. Therefore, each child inherits half of their genetic information from each biological parent. Chromosomes are long strings of genetic material known as deoxyribonucleic acid (DNA). Each chromosome contains multiple genes, which are formed by specific sequences of DNA (Figure 5.1a). A single gene often has multiple variations, which we refer to as alleles. So, if there was a specific gene that coded for hair color, different alleles of that gene will determine the hair color of an individual. Thus, scientists investigate how our genotypes (or genetic makeup) determine our phenotypes, such as hair color, eye color, skin color, whether we have a cleft chin, or other psychological traits (see Figure 5.1b).

Most traits are controlled by multiple genes (i.e., they are polygenic), but there are a few traits that are controlled by one single gene, such as having wet or dry earwax in humans or the length of fur in a cat (McDonald, 2013). We inherit one “earwax” allele from each parent and there two possible variants of the allele – the dominant form (B) and the recessive form (b). When someone has two copies of the same allele, they are said to be homozygous for that allele (BB or bb). When someone has a mixture of alleles for a given gene, they are said to be heterozygous (Bb). Having wet earwax is a dominant trait (B), so if we have that dominant allele in our genotype we will have wet earwax, regardless of the other allele we inherit (BB or Bb). Having dry earwax is a recessive trait, which means that we will only have dry earwax if we are homozygous for that recessive allele (bb).

If you AND your partner both have wet earwax, you might be thinking that it is inevitable that your offspring will have wet earwax too. But, actually it’s not that simple. It depends on the specific alleles that you and your partner carry. If one of you is homozygous (BB), then all your offspring will have wet earwax. It gets a little more complicated, however, if you and your partner are both heterozygous for this gene (Bb). If we use a Punnett square to look at the four possible combinations of genes (Figure 5.2), we see there is a 75% chance your offspring will have wet earwax and a 25% chance they will have dry ear wax (Figure 5.2). We recognize that earwax is rather a strange trait to talk about, but we did not want to perpetuate the common myth that traits such as eye color, cleft chins, presence of earlobes, and tongue rolling are determined by a single gene. These traits are polygenic and occur on a continuum.

Some genetic disorders come from inheriting two recessive alleles. For example, phenylketonuria (PKU) is a metabolic condition where individuals lack an enzyme that normally converts harmful amino acids in foods into harmless byproducts. If this condition goes untreated, people with PKU can experience significant deficits in cognitive function, seizures, and an 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.

Where do genes that contribute to diseases like PKU come from? It has been shown that PKU is due to a gene mutation. This is where the DNA sequencing in a specific gene is slightly altered. Gene mutations can arise spontaneously but once present, it can be passed down to future generations. While many mutations can be harmful or lethal, once in a while, a mutation benefits an individual by giving that person an advantage over those who do not have the mutation. Recent research shows that people who have long lives despite an unhealthy lifestyle, may be benefiting from gene mutations that protect from heart disease and high cholesterol (Williams, 2016).

Theories of Evolution

Theories of evolution aim to explain how genetic variation among a population changes over time. Charles Darwin, an English biologist, proposed an influential evolutionary theory called the theory of natural selection of evolution. He suggested that the individuals within a species that were best suited to their particular environments were more likely to survive and reproduce and pass on their genes to future generations. This is often referred to as the survival of the fittest. Let’s consider the story of the peppered moth in the UK. In the early 19^th century, these moths were predominantly light in color, which provided them with good camouflage from predatory birds. However, during the industrial revolution, the trees in and around large cities became covered in black soot and the light colored moths were virtually eradicated because they were eaten by birds. However, there were also some rarer, dark-colored moths that were now much harder to see when they landed on the trees. Thus, they were more likely to thrive and mate, and pass their dark coloring genes to their offspring. So, over time, there were more and more dark colored peppered moths. Darwin assumed that each successive generation would evolve to be better adapted for the environment. We certainly see evidence for evolution through natural selection among multiple species. However, scientists now acknowledge that evolution simply means changes in genetic variations, and not necessarily improvements. Darwin’s studies focused on plants and animals, but modern-day scientists have found genetic linkages to a number of behavioral characteristics, ranging from basic personality traits to sexual orientation to spirituality (for examples, see Mustanski et al., 2005; Comings et al., 2000). Genes are also associated with temperament and a number of psychological disorders, such as depression and schizophrenia.

Darwin’s theories on evolution took the world by storm, because it was in direct opposition to the popular religious doctrine of the time. Most believed that people were created by God. However, Darwin believed that humans evolved from other species, Unfortunately, like many other White men of his time, Darwin mistakenly believed that race was genetically determined and Anglo-Europeans were “genetically superior” to people of color. Darwin’s theory of natural selection was abused by eugenicists to provide a “scientific rationale” for racism, sexism, and xenophobia. We now know that race is a social, not a biological construct. There is actually more genetic variation within a given racial category than there is between racial categories, so two white people typically share more genetic overlap with a person of color, than they do with each other.

Modern biology acknowledges that most human characteristics are controlled by multiple genes. Each of us represents a unique interaction between our genetic makeup and our environment. For example, intelligence is a complex construct that has a polygenetic component to it, but it is also highly dependent on education, nutrition, and other environmental factors, such as stress. People have questioned whether evolution through natural selection still applies to humans in our modern world given the advances in modern medicine. However, humans have not stopped evolving. Evolution does not necessarily result in improvements; evolution simply means that there are changes in genetic variation over time.

Gene-Environment Interactions

Environmental factors can turn some genes on and others off, we call these epigenetic changes. Epigenetics helps to explain why two identical twins (who have identical genes) show an incredible amount of variability in their phenotypes. For example, one twin could develop a mental illness such as schizophrenia, while the other never does. Their unique environmental interactions determine how their genetic information is expressed, which in turn gives rise to different phenotypes for various traits. Tienari and colleagues found that among adoptees whose biological mothers had schizophrenia, those who were raised in a disturbed family environment were about 6 times more likely to develop schizophrenia (or related disorder) than those raised in a healthy environment. Adoptees with a biological mother who had schizophrenia and who raised in a disturbed environment were also much more likely to develop schizophrenia than participants with no biological family history of schizophrenia, regardless of type of childhood environment (Tienari et al., 2004). This research highlights that both genetic vulnerability and environmental stress are necessary for schizophrenia to develop, and that genes alone do not tell the full tale (Figure 5.3).

We also know that the genes can influence the environment, which in turn can amplify the effects of any genetic predispositions we have, this is often referred to as the multiplier effect. For example, imagine a child who, due to their genes, is naturally agile and athletic. Their parents notice this at an early age and they set up a mini-basketball court and teach their child how to dribble and shoot. The child is encouraged by their enthusiasm and likes to practice. The parents sign their child up for basketball camp and a local team. Thus, the child’s genes have influenced their environment. As a consequence, the child is likely to be a better player than other children.

Text Attribution:

Materials in this section came from:

Grose-Fifer, J., Spielman,R., Dumper,K., Jenkins, W., Lacombe, A.,  Lovett,  M. & Perlmutter, M. (2023). Introduction to Psychology (A critical approach). CUNY. https://pressbooks.cuny.edu/jjcpsy101/. Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.https://pressbooks.cuny.edu/jjcpsy101/