Our skin produces its own natural sunscreen – a brown pigment called melanin. The amount produced is determined by a combination of genetics and sun exposure.

Why this is important…Not all traits are inherited according to Mendel’s laws. Variation in human skin color is the result of interesting inheritance patterns.

Foundations

Melanin is a brownish pigment that colors our skin, hair and eyes. More melanin results in darker tones. The amount of melanin produced in each of these structures is inherited, but it is more complex than the rules for inheritance described by Mendel.

Mendel showed that each individual inherits two copies of each gene. He described different versions of these genes (alleles) as either dominant or recessive. Since then, we have learned that not all dominant traits mask recessive traits completely. We have also learned that some characteristics are controlled by more than one gene, which results in more variable phenotypes.

We also need to recognize something you intuitively know – the environment influences our traits as well. Environmental factors like sunlight, disease, injuries, diet, and stress can influence how our genes express themselves. You are not just a body carrying out a genetic program!

Key terms

Melanin – brown or red pigment produced by specialized cells

Incomplete dominance – inheritance pattern where individuals with two different alleles (heterozygotes) show traits that are intermediate between dominant and recessive traits

Polygenic inheritance – inheritance pattern where a single, genetically-controlled characteristic is controlled by more than one gene pair

Gene locus – location, or address, of a gene on a chromosome (plural: gene loci)

Details

Skin color is determined by how much melanin is produced in our outer layer of the skin (epidermis). All humans have the same density of pigment producing cells – melanocytes – but the activity level of these cells is different. More active melanocytes produce more melanin, which is incorporated into the cells of the epidermis as they divide, making them darker. The same process colors hair and the iris of the eyes.

Four overlapping hands. Each one has different skin tone.

Figure 2-8. Skin color. Different skin tones are the result of different amounts of melanin produced in the dermis. ("Four hands overlaying" by Vicki Nunn is in the Public Domain)

Melanin is a natural sunscreen. It protects the DNA of stem cells in the epidermis from damage by ultra-violet light. Dark skin is an advantage in sunny environments. Early humans had dark skin, but several thousand years ago, probably in northern Europe, a mutation occurred that resulted in less pigment production and lighter skin. Since intense sunlight was not a danger here, this was not a problem. In fact, it was an advantage in less sunny areas to allow more ultra-violet light through the skin because ultra-violet light helps skin make vitamin D. Therefore, people with lighter skin in these environments had an advantage that became more common. This provides an example of natural selection in humans.

Inheritance of skin color is more complex than traits studied by Gregor Mendel. There are three reasons for this and the following sections will explain how each work.

Dark skin is not completely dominant over light skin (incomplete dominance)
Skin color is determined by more than one gene (polygenic inheritance)
Sunlight causes skin to tan (environmental effects)

Incomplete dominance

Incomplete dominance is a change to Mendel’s Theory of Dominance. In the characteristics he studied, dominant or recessive alleles resulted in just two phenotypes. Alleles that are incompletely dominant do not entirely mask the recessive trait, which results in a blending effect. This means there can be three phenotypes (Figure 2-9).

The way these alleles are passed from one generation to the next works the same as Mendel described. The only thing that changes is how the phenotypes are interpreted since the dominant allele does not completely hide the recessive allele.

Diagram of pink and white flowers being interbred. Three generations shown along with genes, symbolized by pink or white dots.

FIGURE 2-9. Incomplete dominance. In some flowers this results in three traits. Individuals with dominant and recessive alleles have traits that are in the middle of the other two. This is seen with deep pink, light pink and white flowers. ("Incomplete dominance" by Sciencia58 is in the Public Domain, CC0)

Stop & Think #7: Red flowers are dominant over white flowers, but red is incompletely dominant. If a red-flowered plant is crossed with a pink-flowered plant, what offspring will result? Predict the percentage for each genotype and phenotype of offspring.

Stop & Think #8: In a garden, there are red, pink and white-flowered snapdragons. You want only white snapdragons to come up next year. Which plants would you have to eliminate from the garden to get only seeds that will grow into white-flowered snapdragons?

Polygenic inheritance

Skin color is also controlled by more than one gene. Genes that control it are located on different chromosomes (that is, different gene loci). If it were controlled by only one gene with incomplete dominance, there would be only three colors of skin. Obviously, there are more than three skin colors!

Stop & Think #9: If eye color is controlled by several genes, could parents who are blue-eyed and brown-eyed have a child with blue eyes? Explain.

There are probably three gene pairs that code for skin color. This means there are 6 alleles controlling skin color all at once, instead of just two. Each dominant allele inherited increases melanin production. Dark-skinned individuals inherited more dominant alleles than light-skinned individuals.

We don’t usually work inheritance problems for so many genes at once. For the sake of illustration, however, the genotype for a person with moderately dark skin might look like this: Aa Bb Cc. A very dark-skinned individual would have a genotype like this: AA BB CC.

Environmental effects

Tanning also causes variation in skin color. Exposure to sunlight causes melanocytes to increase melanin production making our skin darker. This provides more protection from ultra-violet light. Brilliant solution!

Stop & Think #10: What health problems are faced by light-skinned individuals living in very sunny environments? What about dark-skinned individuals living in less-sunny environments?

In addition to the alleles we inherit, the environment causes even more variation. This is true for hair color as well. Sunlight and chemicals can bleach hair, or we can color it artificially.

Our environment has an important influence on the rest of our body. Diet influences characteristics like height, weight and heart health. Exercise influences mental health, weight, muscle mass, and lung capacity. Our genes do not provide an unchangeable blueprint for who we are.

Stop & Think #11: Describe several other environmental factors that influence your phenotype. (Think beyond the examples given above.)

Learning objectives

Solve inheritance problems involving traits showing incomplete dominance.
Explain polygenic inheritance and give several examples in humans.
Explain how the environment can influence physical traits and give examples.

Stop & Think Questions Explained

7. Answer to practice problem: Since the red-flowered plant must have 2 dominant alleles (AA) and the pink flowered plant must have a mix of alleles (Aa), the offspring would probably be:

Genotypes: 50% AA and 50% Aa
Phenotypes: 50% red and 50% pink

8. Removing red and pink-flowered plants would get rid of all dominant alleles. All seeds would have only recessive alleles, resulting in white flowered plants.

9. If the brown-eyed parent passes on mostly recessive alleles, children can have blue eyes.

10. Light-skinned individuals are prone to skin cancer. Dark-skinned individuals may not produce enough vitamin D if they live in cloudy environments.

11. Besides sunlight, diet, and exercise, many aspects of the environment can affect us. These can include injuries that leave scars, pigments used for hair dye or tattoos, and medication. I’m sure you can think of others.