Biologists use the term "biological phenotype" to refer to the external traits of an organism. For example, in dogs, a golden retriever looks very different from an English bulldog. Most of us can recognize the difference—even if we don't know the breed name—in terms of physical traits such as the shape of the snout and jaw; length of tail; type of fur. That's the phenotype of the dog.
In this exercise, we're looking at human phenotypes, and trying to understand the role of genes in producing two very different phenotypes born to the same parents, using this image from Why Phenotypic Race May Not Disappear to spark discussion.
In this exercise, we're looking at human phenotypes, because the way a person physically looks, their biological phenotype, is typically a main way we "sort" people into a category about their ancestry. Most of us refer to ancestor groupings by using the word "race." But it's important to realize that race is not a scientific concept linked to DNA. Human cultures invented the concept of racial categories, often for political reasons related to sharing power and having rights in a community—not on the basis of biological evidence.
We are concerned here with the genetic basis of a phenotype. And in this picture we have a curious example: a set of twin girls born to the same biological parents who both have very different ancestries, referred to in cultural terms as "mixed race." The twin girls look so different it's hard to believe they are sisters, or even in the same ancestor groups. But they are; they share the same genes. What's going on?
1. Humans can produce two types of twins. Identical twins arise from the same egg that splits into two. They are identical because the Central Dogma of Biology, "DNA makes RNA makes protein," is the same in each half of the split egg. Fraternal twins arise from an error in ovulation when two eggs are released, instead of one, by the mother. Here the Central Dogma still holds: DNA makes RNA makes protein—with a difference. What is different is that because there are different eggs involved, different genes are expressed and different proteins are switched on. These proteins produce different physical traits in the twin girls.
2. Genes can be turned on or off. When genes are on, they are said to be "expressed." An expressed gene makes a distinctive protein by spelling out a chemical code composed of combinations of the four letters, ACTG. A distinctive protein directs celluar activity to produce a distinctive physical trait.
3. Let's use these two ideas to reconstruct what happened, genetically, to these twins. The genes that code for proteins that control pigmentation in hair and skin were expressed differently in the twins, even though both girls share the same genes.
In one girl, the ancestral African genes were expressed. Once turned on, these genes spelled out a code to make proteins that control pigment, called melanin, that darkens skin and hair. Melanin production is an evolutionary advantage to provide protection from the sun and damaging effects of ultraviolet radiation in equatorial countries.
In her sister, the ancestral African genes were not expressed. Instead, the genes inherited from Northern European ancestors were expressed. These genes do not code for melanin, because Northern peoples, due to geographically reduced sunlight exposure, tend to require quicker ultraviolet ray absorption in order to produce sufficient Vitamin D, which is crucial to bone health. In evolutionary terms, light skin in the north is a biological advantage because it improves bone and immune system health, and therefore fitness; lighter-skinned people in these regions tended to survive long enough to reproduce. In the equatorial regions, the opposite is true. Darker skin gives a survival advantage. As a result, genes that code for pigment production are the genes that are passed on to the next generation because the parents who possessed those genes lived long enough to reproduce.This teacher sheet is a part of the Race and Genes lesson.