What Are the Phenotypes & Genotypes of a Mouse?

Because mice breed quickly, it's easier to study their genetics than those of many other animals. Scientists can make educated guesses about the genes that cause certain colors, health conditions and behavioral patterns. Understanding the difference between phenotype and genotype is crucial for understanding mouse genetics.

  1. Phenotype

    • The phenotype of an organism is the manifestation of its genes. For example, a brown mouse has a brown phenotype. This phenotype may indicate something about a mouse's genes. For example, if all of the descendants of a particular mouse are brown, this provides a valuable clue about the genetics of color. The mice likely possess a gene that makes them brown.

    Genotype

    • The genotype is the actual genetic makeup of a mouse. A mouse can, for example, have a gene for white fur without actually having white fur. In Mendelian genetics, a dominant gene will manifest in the phenotype if only one dominant gene is present, but both recessive genes must be present in order for a recessive gene to manifest.

      For example, if the gene for the color brown is a dominant gene and a mouse gets one gene for brown and one gene for white from its parents, the mouse will be brown.

    Recessive Genes

    • Recessive genes are important for understanding the difference between genotype and phenotype. Albinism, which results in white fur and red eyes, is a recessive gene. An albino mouse who has inherited one recessive gene and one dominant gene from its parents will not be an albino. Its genotype, however, will be called heterozygous because it has one recessive and one dominant gene.

      If this non-albino mouse then breeds with another non-albino mouse, they may produce an albino mouse if the second mouse also has a heterozygous phenotype. This is because the offspring may receive two recessive albino genes, one from each of its parents. This baby would have a homozygous genotype and an albino phenotype.

    Dominant Genes

    • Dominant genes always result in the dominant phenotype when they are combined with a recessive gene. Thus, the phenotypes that result from dominant genes are more common among mice. The gene that results in brown coloration is a dominant gene, so brown is a common color in mice.

    Genotype vs. Phenotype

    • Unless a mouse has a recessive phenotype like albinism, a mouse's genotype cannot be determined simply by looking at it. Mice with dominant phenotypes may still carry recessive genes that they pass on to their offspring.