Punnett Square: Dominant and Recessive Traits
Every human on earth is a combination of two sets of genes: your mother’s and your father’s. These genes created a blueprint for you, and they make you unique. Genes lead to different traits, or characteristics, such as brown eyes or blue eyes. Parents passing on their genes to their offspring is called heredity. In genetic terms, you are a hybrid of both of your parents: a combination of their many different genes.
What’s the probability of receiving a particular gene? The Punnett Square is a tool that allows you to see the different gene combinations that are possible when two parents of any species create offspring.
When looking at the model of inheritance which the Punnett Square illustrates (referred to as Mendelian inheritance), you are observing combinations of dominant alleles and recessive alleles. An allele is a version of a gene (the eye color gene can consist of blue, brown, green, gray, and hazel alleles). Dominant genes mask recessive genes. For example, brown eyes are the dominant gene for eye color, and blue eyes are recessive, so when the genes for brown and blue eyes are combined in offspring, there is a 75% chance that the offspring will have brown eyes. This is why the majority of people in the world have brown eyes.
Problem: How does a Punnett Square work?
- Piece of paper
- Colored pencils
- Invent two parents, one male and one female, that have different characteristics. For example, make one parent have blonde hair and blue eyes and the other have brown hair and brown eyes. You can make a drawing with your colored pencils to help you keep track of all the characteristics you’re combining.
- Let’s make the Punnett Square for eye color. If the father is heterozygous for brown eyes, this means he has one brown eye allele and one blue eye allele. Write Bb at the top of the square. The capital B is for brown eyes and the lower case b is for blue eyes. Write bb (a homozygous blue eye allele) on the side of the square – these are the mother’s genes.
- Now tape a capital and lower case B on each side of two different pennies. These represent the gene that the child gets from each parent.
- Flip each penny. What do you get? If you get two capital B’s, the child will have brown eyes. If you get two lower case b’s, the child will have blue eyes. Do you remember what happens if you have a capital B (brown) gene and a lower case b (blue) gene?
- How likely is it that the child will have blue eyes?
- Try the experiment with other characteristics as well. You can create a whole fictitious family with different gene combinations.
The Punnett Square shows you how different gene combinations lead to different characteristics like eye color. In the given scenario (one parent is heterozygous for brown and blue eye alleles and one is homozygous for blue eyes), their child has a 50% chance of having blue eyes. If both parents had one blue eye allele and one brown eye allele, their child would have a 25% chance of having blue eyes.
The Punnett Square, named after British Geneticist Reginald C. Punnett, is a good tool for thinking about dominant and recessive alleles, but it isn’t a perfect scientific model. It only works if the genes are independent of one another (situations where having a certain gene doesn’t change the probability of having another). There are also many different genes that combine to produce a characteristic like eye color, not just one. That’s why there are many different patterns and shades of brown, blue, green, hazel, and gray eyes.
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