The Punnett quadrilateral simulates two organisms that reproduce sexually, and examines one of the many genes that parents pass on to their offspring. A complete quadrilateral shows each possible heritable gene, and the probability of each. This is why the Punnetian quadrilateral is a great way to understand basic genetic concepts.
Step
Part 1 of 2: Making a Punnett Square
Step 1. Draw a 2 x 2 rectangle
Draw a rectangle, then halve its length and width so that it becomes four small rectangles. Leave some space above and to the left of the rectangle so that it can be labeled.
Read the background information below if you have trouble understanding the following steps
Step 2. Name the alleles involved
Each Punnett quadrilateral describes the way in which different genes (alleles) are inherited when two organisms successfully reproduce. Select a letter to represent the allele. Write the dominant allele in capital letters, and the recessive allele with the same alphabet but in lowercase. You are free to choose any alphabet.
- For example, you could use the letter "B" for a black feather dominant gene, and the letter "b" for a yellow feather recessive gene.
- If you don't know the dominant gene, use different letters for the two alleles.
Step 3. Check the genotypes of both parents
Next, you need to know the genotype of each parent that has the trait. Each parent has two (sometimes the same) alleles for a related trait, just like every sexual organism so that the genotype consists of two letters. Sometimes, the genotype is already given a question, although usually you will need to look for it from other information:
- "Heterozygous" means the organism has two different alleles (Bb).
- "Homozygous dominant" means the organism has two copies of the dominant allele (BB).
- "Homozygous recessive" means the organism has two copies of the recessive allele (bb). All parents who exhibit a recessive trait (yellow plumage) fall into this category.
Step 4. Label the row with the genotype of one of the parents
Choose one parent, usually the female (the mother), but you can also choose the father. Label the first row of the grid with the first allele of the parent. After that, label the second row of the grid with the second allele.
For example, a female bear is heterozygous for fur (Bb). Write B to the left of the first line, and b to the left of the second line
Step 5. Label the column with the genotype of the other parent
Write the genotype of the second parent for the trait according to the row label, usually using the male parent, aka the father.
For example, male bears are homozygous recessive (bb). Write b above each column
Step 6. Write down the inherited letters of each row and column
From here on out, the Punnett quadrilateral is easy to work with. Start in the first box (at the top left). Look at the letters on the left and above. Write the two letters in the box, and repeat for the remaining three squares. When an organism inherits both types of alleles, the dominant allele is usually written first (that is, write Bb instead of bB).
- In this example, the top left box receives a B from the mother and a b from the father, to produce a Bb.
- The top right box receives a B from the mother and a b from the father to produce Bb.
- The lower left box accepts b from both parents to generate bb.
- The bottom right box accepts b from both parents, to produce bb.
Step 7. Interpret the Punnett quadrilateral
The Punnett quadrilateral shows the probability of having offspring with a certain allele. There are four different combinations of the combined alleles of the parents, and the odds of all four are equal. That is, the combination in each square has a 25% chance of occurring. If more than one square has the same result, add up these 25% odds to get the total odds.
- In this example, we have two boxes with combination Bb (heterozygous). Calculate 25% + 25% = 50% so that each offspring has a 50% chance of inheriting the combined Bb allele.
- The other two boxes each contain bb (homozygous recessive). Each offspring has a 50% chance of receiving the bb gene.
Step 8. Describe the phenotype
Oftentimes, you are more interested in a child's true nature, not just their genes. This problem is easy to solve in most basic situations, which is usually the reason the Punnett quadrilateral is used. Add up the probabilities of each quadrilateral with one or more dominant alleles to find the probability that the offspring will inherit the dominant trait. Add up the probabilities of each box with two recessive alleles to find the probability that the offspring will inherit the recessive trait.
- In this example, there are two squares with at least one B so that each offspring has a 50% chance of having black feathers. There are two boxes with bb so that each offspring has a 50% chance of having yellow feathers.
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Read the questions carefully for more information on phenotypes. Most genes are more complex than this example. For example, a flower species can be red when it has the MM allele, and white if it has mm, or pink when it has Mm. In this case, the dominant allele refers to imperfect dominance.
Part 2 of 2: Background Information
Step 1. Understand genes, alleles, and traits
Genes are pieces of the “genetic code” that determine traits of living organisms, such as eye color. However, the eyes of organisms can be blue, or brown, or a variety of other colors. This variation of the same gene is called allele.
Step 2. Understand the genotype and phenotype
All genes together make up genotype, which is the entire length of DNA that describes how your body is built. Your body and behavior are actually phenotype; you are shaped not only by genes, but also by diet, injury, and other life experiences.
Step 3. Study gene inheritance
In sexually reproducing organisms, including humans, each parent inherits one gene for each trait. Children receive genes from both parents. For each trait, a child can have two copies of the same allele, or two different alleles.
- Organisms with the same two alleles are named homozygous for that gene.
- Organisms with two different alleles are named heterozygous for that gene.
Step 4. Understand dominant and recessive genes
The simplest genes have two alleles: one dominant and one recessive. Dominant variation will appear even though the gene also has a recessive allele. Biologists will refer to it as the dominant allele "reflected in the phenotype."
- An organism that has one dominant allele and one recessive allele is heterozygous dominant. This organism is also called carrier (carrier) a recessive allele because it has a related allele, but the trait is not visible.
- An organism with two dominant alleles is homozygous dominant.
- An organism with two recessive alleles is homozygous recessive.
- Alleles of the same gene can combine to produce three different colors named imperfect domination. An example of this case is a mixed beige horse, namely the KK horse is red, the KK horse has a golden shade, and the KK horse has a bright beige color.
Step 5. Know the benefits of the Punnett quadrilateral
The final result of a Punnett quadrilateral is a probability. A 25% chance of having red hair doesn't mean exactly 25% of children will have red hair; this is only an estimate. However, even rough predictions can be helpful in certain situations:
- A person running a breeding project (usually developing new plant breeds) wants to find out which breeding pair is most likely to get the best results, or whether a particular pair is worth breeding.
- A person with a serious genetic disorder, or a carrier of the allele of a genetic disorder who wants to know the chances of passing the gene on to his or her child.
Tips
- You can use any letter, it doesn't have to be just F and f.
- There is no specific part of the genetic code that makes one allele dominant. We only look at the trait that is visible with only one copy of it, then name the allele that causes that trait to be "dominant".
- You can study the inheritance of two genes at once using a 4 x 4 grid, and code the four alleles for each parent. You can increase it to any number of genes (or genes with more than two alleles), but the box will quickly get large.
