We introduce the topic of evolution by asking the question
How is human skin color inherited, and how has human skin color evolved in human history?
Consider the skin pigmentation of biracial individuals. Can these phenotypes be interpreted
as a simple example of dominance? Do you think that dark pigmentation is due to a dominant
allele or a recessive allele? So think about it. Each of these celebrities is lighter than
one of their parents. Do you think light skin could be dominant?
And consider the case of a family with fraternal twins, one dark-skinned and one light-skinned.
Both parents are biracial, yet one of their twins has a very different skin tone than
either of their parents, or their brother. First, let’s discuss the inheritance of
skin color in humans. In the story of Desiree’s Baby, the son
of Desiree and Armand had darker skin than either of the parents. Since both Desiree
and Armand are light-skinned, again as in the celebrity photos we saw before, can we
consider the light-skinned allele to be dominant? Before we decide on what genotype determines
light skin color, based on anecdotal evidence, let’s think about a much larger sample size.
This well-known work by Byron Kim, shows the diversity of human skin color tones. Each
panel is 10 inches by 8 inches, and the painting fills an entire wall. The colors of the panels
represent 275 different skin tones of people Kim met. The title of his painting, “Synecdoche,”
means “a part standing for the whole,” so here the panels represent skin tones of
mankind. You can visit the link on this slide to hear the artist’s own words about this
painting. If skin color is expressed by a dominant allele in a single gene pair, we
would not observe such a diversity of skin color variations. The only skin colors observed
would be dark and light. And if skin color was a case of incomplete dominance, we would
observe only three skin tones: light, dark, and medium. What other possibility is there?
The trait must be polygenic - a trait determined by more than one gene.
To get the variety of skin tones as seen in the painting, there must must be several genes
for skin color, but we will limit it to 3 genes, A, B, and C. Remember each
gene has 2 alleles, or alternate forms - a dominant allele (uppercase A,B,C) coding for
dark skin, and a recessive allele ( lowercase a,b,c) coding for light skin.
The genotypes for each can be homozygous dominant, represented by two capital letters, as in
AA, BB, or CC; heterozygous, as in Aa, Bb, or Cc; or homozygous recessive as in aa, bb,
or cc. Here are three possible genotypes that could
be inherited. The individuals with the darkest skin would have all dominant alleles, Individuals
with intermediate skin color would have combinations of dominant and recessive alleles, and those
very light skin would have all recessive alleles. Remember each individual would have 3 genes, each with
2 alleles, for a total of 6 alleles. Diploid means each gene has 2 of each gene.
In this 64-box Punnett square from the case study, Desiree’s Baby, we can see that the
trihybrid parents (that is, parents with unlike alleles for all three genes under consideration,
or AaBbCc) could produce 8 different gametes, And have many possible offspring with many
different skin tones. Note that some of these combinations are repeated.
With these three genes there are many combinations of letters, producing 7 different skin color
shades, represented by the different colors in this table, each having a different number
of dominant alleles. Note that the offspring might have no dominant alleles shown in the
table as ) dominant alleles, and would be very light in color. The lower the number
of recessive alleles, the lighter the skin. Not all of them have the same probability
of occurring. The number above the genotype in each box refers to the number of dominant
alleles this potential offspring would have. The offspring are most likely to have the
same skin color as both their parents (that is 3 dominant alleles out of 6), but the probability
of having 3 out of 6 dark skin color alleles is only 20 out of 64. The offspring can also
be darker than the parents (that is have more than 3 dominant alleles), as seen in 22 of
the 64 boxes. Finally, the offspring can have a lighter skin color than either of their
parents, as seen in 22 out of 64 as well. There is only a 1/64 chance that an offspring
would have very light or very dark skin, a 22/64 chance they would have lighter skin
than their parents (that is 1 + 6 + 15), 22/64 chance they would have darker skin than their
parents (again 1 + 6 + 15), and 20/64 chance they would have similar skin color as their
parents. This may be confusing, let’s take a look
at it on a graph. In this bell-shaped curve, we can see that the most likely genotype for
the offspring to inherit is the same genotype of the parents – 3 dominant alleles. They
are less likely to have more dark alleles or more light alleles than their parents.
We will explore some possible genotypes for each of the characters in the Desiree case
study in class.