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A step-by-step guide to using Punnett squares to predict the traits of offspring.
Why do two brown-eyed parents sometimes have a blue-eyed baby? It’s not magic—it’s a mathematical puzzle hidden inside our cells that you can solve with a simple grid.
Before we build our square, we need to understand the 'players.' Every trait, like eye color or height, is determined by alleles (different versions of a gene). We represent these with letters. A dominant allele is written with a capital letter (e.g., ) and can mask other traits. A recessive allele is written with a lowercase letter (e.g., ) and only shows up if the dominant one is missing. Your genotype is the specific combination of letters you have, such as , , or . If you have two of the same letters, you are homozygous; if they are different, you are heterozygous.
Quick Check
If a plant has the genotype , is it homozygous or heterozygous?
Answer
Heterozygous, because it has two different alleles (one capital, one lowercase).
A Punnett square is a visual tool used to calculate the probability of an offspring inheriting a specific trait. To start, draw a square and divide it into four equal quadrants. We place the alleles of the parents on the outside of the box. One parent’s alleles go across the top, and the other parent’s alleles go down the left side. It is vital to separate the letters: if a parent is , one goes over the first column, and the goes over the second column.
Let's cross a homozygous dominant tall plant () with a homozygous recessive short plant (). 1. Draw your 2x2 grid. 2. Place Parent 1 ( and ) above the two columns. 3. Place Parent 2 ( and ) next to the two rows. Your grid should now have letters on the outside, ready to be 'dropped' and 'slid' into the center.
Quick Check
Where do the alleles of the two parents go on a Punnett square?
Answer
One parent's alleles go across the top, and the other parent's alleles go down the left side.
To find the possible genotypes of the children, you fill in the four inner boxes. For each box, look at the letter above it and the letter to its left. Drop the top letter down into the box and slide the side letter across into the box. By convention, we always write the capital letter first (e.g., write , not ). Each of the four boxes represents a chance for the offspring. If two boxes contain , there is a chance the offspring will have that genotype.
Let's cross two heterozygous 'carriers' for a trait (). 1. Top axis: and . Side axis: and . 2. Top-left box: (top) + (side) = . 3. Top-right box: (top) + (side) = . 4. Bottom-left box: (top) + (side) = . 5. Bottom-right box: (top) + (side) = . Result: , , and .
Once the square is full, we can determine the phenotype, which is the physical appearance. If (Brown eyes) is dominant over (blue eyes), then and will both result in brown eyes. Only will result in blue eyes. In our previous example, even though there are three different genotypes, there are only two phenotypes: Brown eyes and blue eyes. This explains why two brown-eyed parents can have a blue-eyed child—they both must be 'carrying' the hidden allele!
A scientist has a purple flower but doesn't know if it is or . They cross it with a white flower (). If even ONE offspring turns out white, what was the parent? 1. If parent was : all offspring are (100% purple). 2. If parent was : half are (purple) and half are (white). 3. Since a white flower appeared, the parent must have been heterozygous ().
In a Punnett square, what does a single internal box represent?
If you cross (tall) with (short), what percentage of the offspring will be short?
A heterozygous genotype () will always show the dominant phenotype.
Review Tomorrow
In 24 hours, try to draw a Punnett square for a cross between and from memory and calculate the percentage of recessive offspring.
Practice Activity
Find a trait in your family (like attached earlobes or a widow's peak) and try to work backward to guess your parents' genotypes!