Punnett Square Calculator Hair Color

Punnett Square Calculator for Hair Color

Predict the probability of offspring hair color using Mendelian genetics (Brown vs. Blonde).

Brown (Homozygous Dominant – BB) Brown (Heterozygous – Bb) Blonde (Homozygous Recessive – bb) Capital 'B' is the dominant allele for dark hair.

Brown (Homozygous Dominant – BB) Brown (Heterozygous – Bb) Blonde (Homozygous Recessive – bb) Lower 'b' is the recessive allele for light hair.

Genetic Cross Results

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Phenotype Probabilities:

Brown Hair

0%

Blonde Hair

0%

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function calculateHairGenetics() { var p1 = document.getElementById("parent1").value; var p2 = document.getElementById("parent2").value; var p1_a1 = p1[0]; var p1_a2 = p1[1]; var p2_a1 = p2[0]; var p2_a2 = p2[1]; // Set Labels document.getElementById("p1_a1").innerText = p1_a1; document.getElementById("p1_a2").innerText = p1_a2; document.getElementById("p2_a1").innerText = p2_a1; document.getElementById("p2_a2").innerText = p2_a2; var results = [ sortGenotype(p1_a1 + p2_a1), sortGenotype(p1_a1 + p2_a2), sortGenotype(p1_a2 + p2_a1), sortGenotype(p1_a2 + p2_a2) ]; document.getElementById("cell_00").innerText = results[0]; document.getElementById("cell_01").innerText = results[1]; document.getElementById("cell_10").innerText = results[2]; document.getElementById("cell_11").innerText = results[3]; var brownCount = 0; var blondeCount = 0; var genoCounts = { "BB": 0, "Bb": 0, "bb": 0 }; for (var i = 0; i < 4; i++) { genoCounts[results[i]]++; if (results[i].includes("B")) { brownCount++; } else { blondeCount++; } } document.getElementById("brown-pct").innerText = (brownCount / 4 * 100) + "%"; document.getElementById("blonde-pct").innerText = (blondeCount / 4 * 100) + "%"; var summary = "Genotype Breakdown:"; summary += "BB (Homozygous Dominant): " + (genoCounts["BB"] / 4 * 100) + "%"; summary += "Bb (Heterozygous): " + (genoCounts["Bb"] / 4 * 100) + "%"; summary += "bb (Homozygous Recessive): " + (genoCounts["bb"] / 4 * 100) + "%"; document.getElementById("genotype-summary").innerHTML = summary; document.getElementById("results-area").style.display = "block"; } function sortGenotype(pair) { var arr = pair.split(""); if (arr[1] === "B" && arr[0] === "b") { return "Bb"; } return pair; }

How Hair Color Genetics Work

Predicting hair color is a fascinating application of biology. While human hair color is actually polygenic (influenced by multiple genes), we can use a simplified Punnett Square to understand the fundamental relationship between dark hair and light hair.

The "B" and "b" Alleles

In basic Mendelian genetics for hair color:

• B (Dominant): The allele for brown or dark hair. Because it is dominant, having just one "B" allele will result in dark hair.
• b (Recessive): The allele for blonde or light hair. To have blonde hair, an individual usually needs two recessive alleles (bb).

Understanding Genotypes

A genotype is the genetic makeup of an individual. There are three primary genotypes in this model:

1. BB (Homozygous Dominant): The person has two brown hair alleles. They will have brown hair and can only pass on the brown allele.
2. Bb (Heterozygous): The person has one brown and one blonde allele. They will have brown hair (because B is dominant) but are "carriers" of the blonde trait.
3. bb (Homozygous Recessive): The person has two blonde alleles and will have blonde hair.

Example Calculation

If two parents are both heterozygous (Bb), their Punnett square would look like this:

• 25% chance of BB (Brown hair)
• 50% chance of Bb (Brown hair, carrier of blonde)
• 25% chance of bb (Blonde hair)

In this scenario, even if both parents have brown hair, there is a 1-in-4 chance their child will have blonde hair!

Why is hair color more complex in real life?

It is important to note that this calculator uses a simplified single-gene model. In reality, multiple genes like MC1R and OCA2 influence the specific shade of brown, red, or blonde. This is why you see such a vast spectrum of hair colors ranging from platinum blonde to raven black and various shades of auburn.