Rat Genetics 101

Whether you’re curious about why some people have brown eyes or how garden peas inherit their colors, basic genetics helps explain it all. In this guide, we’ll cover:

  1. Key Basic Terms

  2. Why We Use Letters for Genes

  3. How to Read a Punnett Square

  4. Beyond the Basics

This is Genetics 101—no biology degree required!

Key Basic Terms

  • Gene: A “recipe” in your DNA that tells your body how to make on trait (like eye color).

  • Allele: One version of a gene. You inherit two—one from each parent.

  • Genotype: The pair of alleles you carry (written as letters, like Aa).

  • Phenotype: The trait you actually see (like blue eyes or brown eyes).

  • Dominant: An allele that shows its trait when at least one copy is present (capital letter, e.g., “A”).

  • Recessive: An allele whose trait appears only when you have two copies (lowercase “a”).

  • Homozygous: Having two identical alleles (AA or aa).

  • Heterozygous: Having two different alleles (Aa).

  • Carrier: A heterozygote (Aa) for a recessive trait—you don’t show it, but you can pass it on.

  • Locus: The specific spot on a chromosome where a gene lives.

  • Chromosome: A long strand of DNA in your cells—humans have 23 pairs, other creatures have different numbers.

  • Polygenic: A trait controlled by many genes working together (like height).

  • Incomplete Dominance: When one allele doesn’t fully mask the other, giving a blend of traits.

  • Co-dominance: When both alleles show up equally (like blood type AB in humans).

  • Penetrance: The % of people with a genotype who actually show the expected trait.

  • Expressivity: How strongly a trait is expressed—two people with the same genes might look a bit different.

Why We Use Letters for Genes

Writing out full gene names every time would be a mouthful. Instead, geneticists use letters:

  • Pick a Letter: Usually the first letter of the trait (e.g., “P” for purple flowers).

  • Uppercase vs. Lowercase:

    • Uppercase (P) = dominant version

    • Lowercase (p) = recessive version

  • Combine Them:

    • PP = two dominant alleles

    • Pp = one dominant + one recessive

    • pp = two recessive alleles

This shorthand helps us quickly see who carries which versions.

How to Read A Punnett Square

A Punnett square is a simple chart that shows possible offspring genotypes. Let’s use a classic example: pea plants with purple (P) vs. white (p) flowers.

Two parent plants, both heterozygous (Pp):

  1. Draw a 2×2 grid.

  2. Label the top with one parent’s alleles (P and p).

  3. Label the side with the other parent’s alleles (P and p).

  4. Fill in each box by combining top + side:

  5. Calculate the results

    PP (25%) → homozygous dominant (purple)

    Pp (50%) → heterozygous (purple, but carrier of white)

    pp (25%) → homozygous recessive (white)

That means 75% purple flowers and 25% white.

Beyond the Basics

Not every trait follows this simple pattern. Other inheritance types include:

  • Polygenic Traits: Many genes add up to a single trait (e.g., human height, skin color).

  • Incomplete Dominance: Neither allele fully hides the other, producing a blend (e.g., red + white flowers = pink).

  • Co-dominance: Both alleles show up equally (e.g., human blood type AB).

  • Epistasis: One gene can mask another (e.g., a gene that turns off pigment might hide color genes).

These patterns can get complex, but the core idea remains: genes and alleles interact in predictable ways.