last updated:

Feb 10, 2021


[Brain Image]    

PSY 340 Brain and Behavior

Class 03: Genetics and the Evolution of Behavior

QUESTION: When we use the word "gene" what do we are we saying? What is a "gene"?

ANSWER: Two different (though related) meanings

(1) An inherited biological unit or factor that determines some physical trait  [= Mendelian genetics]


(2)  Inherited molecule(s) or sequences of DNA which give the code(s) or instructions for the production of specific proteins within cells [= Molecular genetics

Mendelian Genetics (Origins)

Some Examples
Dominant Trait
Recessive Trait

Tongue Rolling
Ear Lobe
Hangs Free
Hand Clasping
Left Thumb on Top
Right Thumb on Top
Hitchhiker's Thumb
Straight Thumb
Bent Thumb
Other Traits
No Dimples

No Freckles

Normal Vision

Normal Vision

Low sensitivity to Poison Ivy
High Sensitivity to Poison Ivy

MendelThe German Catholic monk, Gregor Mendel (1822-1884), in the 19th century worked out rules of inheritance through genes. Though he did not know what they were made of, his research demonstrated that some type of unit or structure passes intact from generation to generation. And, these units cause a living organism to have a particular trait.

Molecular Genetics (DNA): Origins

                  Watson & Crick

We now know that these genes are located on stretches of the chromosomes (long strands of genes) which are made of the double-stranded ("the double helix") molecule DNA (Deoxyribonucleic Acid). In the most famous NATURE article published in 1953 (see above), James Watson and Francis Crick (who died in 2004) described the basic molecule, DNA by which genetic information is transmitted from parents to offspring. Later researchers determined how these long strands of DNA actually work. They won the 1962 Nobel Prize in Physiology or Medicine for their work.

The strands of DNA act as templates or models for the creation of ribonucleic acid (RNA) molecules (which are single strands). RNA, in turn, serves as the template or model to make (synthesize) protein molecules (see figure on left below).

Function of DNA   Chromosomes     [DNA Strand

Chromosomes & Genes

Human beings have 23 pairs of chromosomes (see above)

The Human Genome. Each of us has a "genome," that is, all of the genes found in our DNA that code for building proteins. At one time, researchers thought that humans probably would have 50,000 to 150,000 genes. Then, the first results of the Human Genome Project were reported in 2001 and scientists were shocked to find far fewer genes than they had predicted. At first, the estimate was 35,000 genes. Then, in 2008 scientists at MIT and Harvard said humans only have about 20,500 genes (Broad Institute of MIT and Harvard, 2008). And, in 2014 an international research group reported that there "may be as few as 19,000 human protein-coding genes" (Ezkurdia et al, 2014)

Trait TransmissionHeritable (Simple) Traits (Mendelian Genetics)

Most genes are passed across generations in pairs: one from the father and one from the mother. If we receive identical genes from each parent, we are homozygous for that gene. If we receive different (unmatched) genes from our parents, we are heterozygous for that gene.

Some physical characteristics, e.g., eye color, are determined by genes which can be dominant, recessive, or intermediate. A dominant gene causes its trait to be expressed whatever the other gene might be while a recessive gene causes its trait only if the other gene in the pair is also recessive. If both parents have a dominant gene for a trait, their offspring will receive identical genes for that trait, i.e., they are homozygous for the trait. However, if their parents have one dominant and one recessive gene themselves for the trait, the parents are considered heterozygous for the trait. The results of transmission are seen in the diagram on the right. This shows the transmission of eye color (brown vs. blue). Some genes are also considered to be intermediate between dominance & recession.

Note: DOMINANT ≠ MOST FREQUENT. A recessive gene may be widely present across populations and a dominant gene relatively infrequently present.

Liu et al
            (2010) Hue vs. SaturationIntermediate Traits. There are some physical traits and medical illnesses that are the results of a single gene as noted above: for example, sickle-cell anemia, cystic fibrosis, muscular dystrophy, Huntington disease, and Tay-Sachs disease. These are called Mendelian disorders.

But, what about the notion of an "intermediate" (not dominant and not recessive gene)? Note that many traits which we once thought were controlled by a single dominant vs. recessive gene are now known to be the results of multiple genes, e.g., at least 10 genes are involved in eye color and more than 180 genes in how tall people grow up to be.

 How did scientists find this out? The image to the right shows a figure from the study of Liu et al (2010) who discovered the multiple number of genes involved in eye color. What they did was to photograph the iris (the part of our eye that is colored) of many thousands of people with a high-definition camera. Then they looked at how there are many subtle differences in the hue (the actual color) and the saturation level (how pure the color is) of these eyes and compared their findings to the full genome of these individuals. Thus, they found that the actual "color" of the eye--in all its subtle differences--are the results of multiple genes contributing to the differences. [Note that I lightened the actual image from Liu et al, 2010, in order to illustrate the notion of gradations of color (hue vs. saturation).]

Genetic Changes (Molecular Genetics)

Heredity & Environment

Most variations in behavior are results of combined influences of many genes (= polygenetic inheritance) and environmental conditions. Behaviors always develop as the joint result of the interaction of these two factors.

Heritability or the heritability index (H2): estimate of how much variation or differences in a characteristic across a population are due to differences in heredity. Range from 0 (no influence) to 1 (totally heritable influence). Note this is a measure of the variability of the characteristic, NOT a measure of the average level of the characteristic.  It is generated by comparing fraternal and identical twins and how much they both share a particular trait.

A different way of understanding the heritability index: "“…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.” (Dewey, 2017-2018, italics added). So, if we had a heritability index of 0.70 for a particular trait and we know that one identical twin has that trait, our error of predicting that trait in the other identical twin is reduced by 70%.

Consider a characteristic such as high blood pressure (e.g., 120/70 or 145/95 or 110/60). Let us say that we measure the blood pressure among two different groups (populations) such as African-American and Italian American adults and, then, calculate an estimate of heritability. Suppose we come up with the following hypothetical (made-up) data

Population Average BP 50% fall within the range of Heritability (H2)
African American
145/87 127/77 to 150/93 0.70
Italian Americans 137/81 120/73 to 148/89 0.40

What do these data tell us?

Thus, be very very careful when you read stories about the heritability of intelligence across different races. Those data only tells about the proportion of variation within each group that comes from genetics, not why the average level of intelligence may be at a certain level.

Research on heritability comes from these research approaches:

Problems in Heritability Research
Environmental Modification
The Evolution of Behavior

Corrections to WRONG IDEAS about evolution

  • Use or disuse of a body structure or behavior does not pass down to the next generations (= Lamarckian evolution is wrong though the findings of epigenetics (above) has begun to challenge this notion).
  • Genes create adaptive responses to particular environments, that is they increase fitness (= the likelihood that one will pass one one's genes to a new generation). Thus, what is fit in one environment might not be in a different one.
  • Evolution spreads genes and is not of benefit either to an individual or a species. As Richard Dawkins notes, genes are "selfish" and use us to spread themselves!

Brain Evolution

              O. WilsonEvolutionary Psychology (aka SOCIOBIOLOGY) founded by Dr. Edward O. Wilson (Harvard U. Biologist)

  • Simple altruism seems unlikely to confer an advantage.
  • Reciprocal altruism (I help you if you help me) requires recognition (and, thus, a good set of sensory organs & brain). It is possible that some animals will "cheat" and not actually help the other. So, among many animals, reciprocal altruism may not be a complete explanation. If a particular human culture, though, adds very strong penalties for cheating (= ethical standards), this will increase the effectiveness of reciprocal altruism.
  • Kin selection: altruism leads to helping kin which passes at least some genes along to a new generation. Data tend to show that forms of altruism which benefit family members do show up all over the animal (and vegetative) world.
  • Group Selection: altruistic groups (as groups and not just individuals) do better overall than less cooperative groups (a controversial theory). This theory shows strength as long as the altruism takes place within the group itself and (as we pointed out above) non-cooperators are expelled from the group.


Broad Institute of MIT and Harvard (2008, January 15). Human Gene Count Tumbles Again. ScienceDaily. Retrieved January 15, 2008, from http://www.sciencedaily.com/releases/2008/01/080113161406.htm

Dewey, R. A. (2017-2018). Chapter 10. Development. In Psychology: An Introduction [Online textbook]. https://www.psywww.com/intropsych/ch10-development/genetic-influences.html

Ezkurdia, I., Juan, D., Rodriquez, J. M., Frankis, A., Diekhans, M., Harrow, J., Vazquez, J., Valencia, A., & Tress, M. L. (2014). Multiple evidence strands suggest that there may be as few as 19 000 human protein-coding genes. Human Molecular Genetics, 23(22), 5866-5878. doi:10.1093/hmg/ddu309

Jonsson, H., Magnusdottir, E. et al. (2021). Differences between germline genomes of monozygotic twins. Nature Genetics. https://doi.org/10.1038/s41588-020-00755-1

Liu, F.l, Wollstein, A., Hysi, P. G., Ankra-Bad, G.,...Kayser, M. (2010). Digital quantification of human eye-color highlights genetic association of three new loci. PLoS Genetics, 6, e1000934.

Plomin, R., DeFries, J. C., Knopik, V. S., & Neiderhiser, J. M. (2016). Top 10 replicated findings from behavioral genetics. Perspectives on Psychological Science, 11(1), 3-23. DOI:10.1177/1745691615617439

Plomin, R., & Deary, I. J. (2015). Genetics and intelligence differences: Five special findings. Molecular Psychiatry, 20, 98-108. doi:10.1038/mp.2014.105

Rogers, K. (2012, January 16). Epigenetics: A turning point in our understanding of heredity. Scientific American Guest Blog. Retrieved January 25, 2012 from http://blogs.scientificamerican.com/guest-blog/2012/01/16/epigenetics-a-turning-point-in-our-understanding-of-heredity/

This page was first posted January 19, 2005.