3. Based on the mathematical expression, predict how many distinct phenotypes would result from six genes,
each with two alleles, according to the model. _______________________________
4. Studies into the genetics of human skin color have concluded that at least 34 genes (Sturm and Duffy 2012)
have a detectable influence on skin color, but there are likely many more. Based on the expression you
developed above, and assuming that each of the 34 genes has two alleles, how many unique phenotypes
would be generated? Show your work.
PART 2: Searching for skin color genes
How do we know which genes are involved in skin color? Scientists use genome-wide association studies to
determine how differences in DNA among individuals or even populations relate to different phenotypes. These
studies involve comparing DNA sequences across the genomes of a large number of individuals in search of
variations that are consistently associated with particular phenotypes. These variations are called single-
nucleotide polymorphisms, or SNPs (pronounced “snips”). A SNP is a variation in a single nucleotide at a
particular position, or locus, in the genome. Not all single-nucleotide changes are SNPs. To be classified as a SNP,
the change must be found in more than 1% of the population. SNPs are also called markers.
Once a SNP locus has been identified, scientists can use the differences in populations to determine the gene or
genes that generate the phenotypes they see. Using this approach, many genes have been identified as having a
role in determining skin pigmentation. The genes identified as having the strongest effect on skin color are TYR,
TYRP1, OCA2, SLC45A2, SLC24A5, and MC1R. Among these, the melanocortin 1 receptor gene (MC1R) is the
major contributor to normal pigment variation. To date, scientists have identified more than 50 SNPs within the
MC1R gene. Each SNP represents a different allele, so there are more than 50 known alleles for the MC1R gene.
SNPs can also be used to gain insight into an individual’s ancestry. Scientists have compared SNPs among
populations of indigenous people, or people native to a particular place, and quantified how often a given allele
(SNP) of a specific gene is present within that population. This measure of the relative frequency of a given
allele, which is called allele frequency, is often expressed as a percentage. For example, the skin color gene
SLC24A5 has a SNP locus known as rs1426654. There are two alleles at this locus; an individual can have an
adenine (A) or a guanine (G). Studies of indigenous populations have revealed that the allele frequency of the A
allele is 100% among Europeans (making the frequency of the G allele 0%) and 2.5% among Africans (making the
frequency of the G allele 97.5%). If you were to consider just these two groups of people, you would consider a
person with two A alleles to most likely be of European ancestry, whereas a person with two G alleles most likely
would be of African ancestry. By looking at the frequencies of SNPs throughout their genome, a researcher can
make an informed hypothesis about an individual’s ancestry based on their DNA, as you will see in Part 3.
5. To search for genes involved in determining skin color, scientists look for SNPs associated with different skin
color phenotypes. SNPs are variations at a single nucleotide within the genome. How can a change in a
single nucleotide be responsible for differences in skin color or the function of a gene in general?