Evo-Ed: Integrative Cases in Evolution Education

Cases for Evolution Education

Population Genetics

Different sub-species of the beach mouse (Peromyscus polionotus) have different fur colors. These differences are best described by examining melanocyte cell function. Melanocytes are located at the base of the hair papilla and make a pigment molecule called eumelanin. Pigment is then transported from the base to the hair cortex where coloration becomes visible.

In biological research, to say that something at the molecular or genetic level is responsible, a phenotypic trait, a plausible mechanism and an observable relationship should be identified. In the cell biology and molecular genetics sections we discuss a possible mechanism.

Illustration of five oldfield mice with the range of colorations from dark to light.

Gene and alleles

There are two different alleles that can be identified for the MC1R gene that aid in eumelanin production in beach mice (Peromyscus polionotus). An allele is a variant of a gene, which is a discrete section of DNA that codes for a polypeptide (i.e. protein) or a functional RNA molecule. In beach mice the MC1R gene has two alleles: R and C. The R allele results in a functional MC1R protein that stimulates eumelanin production. This polypeptide has amino acid arginine at position #67 due to the codon CGC at positions 199, 200 and 201 within the MC1R nucleotide sequence. The C allele results in a non-functional MC1R protein that does not stimulate eumelanin production.  It has the amino acid cysteine at position #67 due to the codon TGC at positions 199, 200 and 201 within the MC1R nucleotide sequence.

Genotypes and Phenotypes

The MC1R gene is located on chromosome #16 in mice. Since every mouse cell has two copies of each autosomal chromosome, the cells of any given beach mouse will possess two copies of the MC1R gene – one on each chromosome.  Since there are two MC1R alleles there are three possible genotypes that an individual may possess: RR, RC, and CC.

Correlating Coat color to Gene mutations

Table of Color scores for all geno types, R R, R C, C C, for the following pody parts: Rstrum, Whisker, Cheek, Eyebrow, Ear, Ankle.

Hoekstra et al (2006) analyzed more than 400 mice to determine how fur color genotypes were related to fur color phenotypes. Mice were scored on a 2-point scale: 2 for very dark fur, 1 for medium, and 0 for very light. Scoring was done for seven different body parts – the whisker, the rostrum, the cheek, the eyebrow, the ear, the ventrum and the ankle. The scores were then matched to the known genotype that had been sequenced for each mouse, RR, RC, or CC.

They found that the RR mice were typically darker than the RC mice that in turn were typically darker than the CC mice.

Conclusion

Different alleles of the MC1R gene are responsible for some of the coat color differences observed among beach mouse populations. However, these alleles cannot explain all the color variation among populations and it is therefore probable that mutations in other genes and their alleles contribute to differences in coat color.