How to Do Punnett Squares for 1 or 2 Traits (Monohybrid or Dihybrid)

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Plus, review key terms and concepts about genetics & inheritance
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A Punnett square simulates two organisms reproducing sexually, examining just one of the many genes that get passed on. The completed square shows every possible way the offspring could inherit this gene, and what the chances are for each result. Making Punnett squares is a good way to get started understanding the fundamental concepts of genetics, and we’re here to show you exactly how to do it! Read on to learn how to make Punnett squares for 1 gene (a monohybrid cross) or 2 genes (a dihybrid cross).

Making Punnett Squares at a Glance

Draw a 2x2 grid and label the rows with one parent’s genotype and the columns with the other. Cross-compare the genotypes in each square to show the different genetic combinations possible if the parents reproduce. Punnett squares show the probability of offspring inheriting certain traits from their parents.

Section 1 of 4:

Making a Punnett Square

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  1. Draw a box and divide it into four smaller squares. [1] Leave room above the box and to its left, so you can label it with the alleles from each parent.
  2. Each Punnett square describes how variations of a gene (alleles) could be inherited if two organisms sexually reproduce. Choose a letter to represent the alleles. Write the dominant allele with any capital letter, and the recessive allele with the same letter in lowercase. It doesn't matter which letter you choose. [2]
    • For example, consider the color of a bear’s fur. Call the dominant gene for black fur "F", and the recessive gene for yellow fur "f".
    • If you don't know which gene is dominant, use different letters for the two alleles.
    • If you’re working on a problem for an exam or assignment, use the letters used in the prompt.
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  3. Next, we need to know the genotype each parent has for that trait. Each parent has two alleles (sometimes 2 of the same one) for the trait, just like every sexual organism, so their genotype will be two letters long. [3] Sometimes, you'll already know exactly what this genotype is. Other times, you'll have to work it out from other information.
    • "Heterozygous" means the genotype has two different alleles (Ff).
    • "Homozygous dominant" means the genotype has two copies of the dominant allele (FF).
    • "Homozygous recessive" means the genotype has two copies of the recessive allele (ff). Any parent that shows the recessive trait (has yellow fur) belongs to this category.
  4. Pick either parent to label the left side of the square—traditionally, the female (mother) genotype goes here, but this is just a convention and either parent will work. [4] Label the first row of the grid with one of that parent's allele. Label the second row of the grid with the second allele.
    • For example, imagine the female bear is heterozygous for fur color (Ff). Write an F to the left of the first row, and an f to the left of the second row.
  5. Write the second parent's genotype for the same trait as labels for the columns. This is typically the male's, or father's. [5]
    • For example, the male bear is homozygous recessive (ff). Write an f above each of the two columns.
  6. The rest of the Punnett square is easy. Start in the upper left box. Look at the allele letter to its left, and the allele letter above it. Write both these letters in the empty box. Repeat for the remaining three boxes. If you end up with both type of allele, it's customary to write the dominant allele first (for example, write Ff, not fF). [6]
    • In our example, the top left box inherits F from the mother and f from the father, to make Ff.
    • The top right box inherits an F from the mother and f from the father, to make Ff.
    • The bottom left box inherits an f from both parents, to make ff.
    • The bottom right box inherits an f from both parents, to make ff.
  7. The Punnett square shows us the likelihood of creating offspring with certain allele combinations. There are four different ways the parents' alleles can combine, and all four are equally likely. This means that the combination in each box has a 25% chance to occur. If more than one box has the same result, add up these 25% chances together to get the total chance. [7]
    • In our example, we have two boxes with Ff (heterozygous). 25% + 25% = 50%, so each offspring has a 50% chance of inheriting the Ff allele combination.
    • The other two boxes are each ff (homozygous recessive). Each child has a 50% chance of inheriting ff genes.
  8. Often, you're more interested in the offspring's actual traits, not just what their genes are. Their phenotype describes the actual observable trait that the genotype codes for (like having black fur or yellow fur). [8] Add up the chance of each square with one or more dominant alleles to get the chance that the offspring expresses the dominant trait. Add up the chance of each square with two recessive alleles to get the possibility that the offspring expresses the recessive trait. [9]
    • In this example, there are two squares with at least one F, so each offspring has a 50% chance to have black fur. There are two squares with ff, so each offspring has a 50% chance to have yellow fur.
    • Read the problem carefully for more information about the phenotype. Many genes are more complex than this example. For example, a flower species might be red when it has the RR alleles, white when it has rr, or pink when it has Rr. In cases like this, the dominant allele is then referred to as an incomplete dominant allele. [10]
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Section 2 of 4:

Predicting Two Traits with Punnett Squares

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  1. Sketch a large square and divide it into 4 browns and 4 columns. The resulting Punnett square will be 4 x 4 with 16 squares total. [11]
    • A dihybrid cross like this is a way to compare the chances of an offspring inheriting 2 different traits. These Punnett squares work when both parents are heterozygous for both traits.
  2. Since both parents must be heterozygous for both traits, you know they’ll have both a dominant and recessive allele for both traits. In this example, we’ll look at a pea plant that reproduces with itself (it counts as both parents) and is heterozygous for round, yellow seeds: [12]
    • We can label the genotype for round seeds with the letter R. A capital R indicates the dominant allele (round) and a lowercase r represents the recessive allele (wrinkled).
    • We’ll label the genotype for the color yellow with the letter Y. A capital Y indicates the dominant allele (yellow) and a lowercase y represents the recessive allele (green).
    • Because both parents are heterozygous and display dominant traits, you can say they have the genotype RrYy.
  3. Gametes are the individual reproductive cells of the parents (sperm and egg) and contain 1 copy of each gene. Since the parents have both alleles for both genes, this means that there are 4 potential combinations they can pass on to their offspring: RY, Ry, ry, and rY. [13]
    • Essentially, you’re looking for every possible combination of alleles a heterozygous parent can pass on. These combinations will be the same for both parents.
  4. Just like with a 2 x 2 Punnett square, label the top of the square with one parent’s gametes and the left side with the other (it doesn’t matter which parent goes where). [14] For simplicity, list the gametes in the same order in both places.
    • So, if you list RY - Ry - ry - rY across the top, you’d also list RY - Ry - ry - rY going down the side.
  5. Start in the upper left corner and combine the gametes to the left and above the square to create a new genotype. In this case, since we listed RY first, there is an RY to the left and an RY above. This means the upper left square genotype will be RRYY (2 dominant R alleles and 2 dominant Y alleles). [15]
    • If we continue along the top row going left to right, we get RRYy, RrYy, and RrYY in the other 3 boxes.
    • In the second row from left to right, we get RRYy, RRyy, Rryy, and RrYy.
    • In the third row, we get RrYy, Rryy, rryy, and rrYy.
    • In the bottom row, we get RrYY, RrYy, rrYy, rrYY.
  6. In a dihybrid Punnett square, each square represents a 1 in 16 chance of that genotype occurring. In our example, the breakdown is: [16]
    • RRYY (1/16) - round, yellow seeds
    • RRYy (2/16) - round, yellow seeds
    • RrYY (2/16) - round, yellow seeds
    • RrYy (4/16) - round, yellow seeds
    • RRyy (1/16) - round, green seeds
    • Rryy (2/16) - round, green seeds
    • rrYY (1/16) - wrinkled, yellow seeds
    • rrYy (2/16) - wrinkled, yellow seeds
    • rryy (1/16) - wrinkled, green seeds
  7. In any dihybrid cross with heterozygous parents, there are 9 squares that will result in both alleles being dominant, 3 squares where the first allele is dominant and the second allele is recessive, 3 squares where the first allele is recessive and the second allele is dominant, and only 1 square where both alleles are recessive. [17]
    • This means the offspring has a 9 in 16 chance of inheriting both dominant traits, a 1 in 16 of getting both recessive traits, or a 6 in 16 chance of getting a combination of the two.
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Section 3 of 4:

Can you find unknown parent genotypes with a Punnett square?

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  1. Imagine a cross between a purple and white flower, where purple (P) is the dominant allele and white (p) is recessive. You know that the genotype for the white flower must be pp since the white color is recessive. However, the purple parent could have a genotype of PP or Pp and still appear purple (we can label it as P? in a Punnett square): [18]
    • If we cross pp and P?, we end up with the following possible combinations: Pp, Pp, ?p, and ?p. This tells us that at least 50% of the offspring will be purple, but does not tell us the chance of white colored offspring. Without observing the offspring, a Punnett square alone is not enough to fill in unknown parent genotypes.
    • Now, pretend that you’ve observed a white flower offspring. The offspring must have pp as its genotype, meaning it receives a p allele from both parents. The purple parent’s genotype must be Pp.
Section 4 of 4:

Key Terms and Concepts

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  1. A gene is a piece of "genetic code" or a chunk of DNA that determines a trait in a living organism—for example, eye color. But eye color can be blue, or brown, or various other colors. These variations of the same gene are called alleles . [19]
  2. All your genes together make your genotype : the entire length of DNA that describes how to build you. Your actual body and behavior are your phenotype : how you ended up, partly because of genes but also because of diet, possible injury, and other life experiences. [20]
  3. In sexually reproducing organisms, including humans, each parent passes on one gene for each trait. The child keeps the genes from both parents. For each trait, the child might have two copies of the same allele, or two different alleles. [21]
    • An organism with two copies of the same allele is homozygous for that gene. [22]
    • An organism with two different alleles is heterozygous for that gene.
  4. The simplest genes have two alleles: one dominant and one recessive. The dominant variation shows up even if a recessive allele is also present. A biologist would say that the dominant allele is "expressed in the phenotype." [23]
    • An organism with one dominant allele and one recessive allele is heterozygous dominant . These organism are also called carriers of the recessive allele, since they have the allele but don't show the trait. [24]
    • An organism with two dominant alleles is homozygous dominant .
    • An organism with two recessive alleles is homozygous recessive .
    • Two alleles of the same gene that can combine to make three different colors are called incomplete dominants . An example of this are cream-dilute horses, where cc horses are red, Cc horses are a shade of gold, and CC horses are a light shade of cream. [25]
  5. The end result of a Punnett square is a probability. A 25% chance at red hair doesn't mean that exactly 25% of the children will have red hair; it's just an estimate. [26] However, even a rough prediction can be informative in some situations:
    • Someone running a breeding project (usually developing new plant strains) wants to know which breeding pair gives the best chance at good results, or whether a certain breeding pair is worth the effort.
    • Someone with a serious genetic disorder, or a carrier of an allele for a genetic disorder, may want to know the possibility that they'll pass it on to their children.
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  • Question
    How do I know when the allele is dominate? How do I know if the allele is recessive ?
    Community Answer
    Generally your teacher will use common examples that you know or have from the Mendelian pea experiment. Also, if the letter is capitalized it is the dominant allele - recessive allele is the lower case letter. Often the letter for the trait corresponds to the dominant allele - like tall is dominant to short so the teacher will use the letters T = tall and t = short.
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  • Question
    Why is there a 50% chance of black or yellow if black is dominant and yellow is recessive?
    Community Answer
    Because in the specific example used, one parent has two recessive alleles and the other parent has one dominant allele and one recessive allele. If you look at the resulting square, it's clear why the possible outcomes are 50/50: there's a 50% chance that the offspring will inherit recessive alleles from both the father and the mother. If both parents had one dominant allele and one recessive allele, then there would only be one possible recessive-recessive combination, and the chance of yellow in the offspring would be only 25%.
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  • Question
    How do I know which one is dominant and which is recessive?
    Community Answer
    The dominant allele (for example, B) is the one that, in a normal cross, will show up in the phenotype if either one or two of itself are present. So, if Bb and BB produce the same phenotype, you know that B is dominant because both 1 and 2 B's produce the same result The recessive allele (for example, b) is the one that needs two of itself in order to be expressed in the phenotype. So, if b is recessive and B is dominant, only bb will show the recessive allele in the phenotype.
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      • There's no special part of the genetic code that makes one allele dominant. We just see which trait is visible with only one copy of it, then call the allele that caused that trait "dominant." [27]
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      • Punnett squares were created by English geneticist Reginald Punnett (1875–1967) to show the number and variety of possible genetic combinations in Gregor Mendel’s theory of inheritance. [28]
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      • You can use any letters you want for the genotypes. If you’re working with lots of genotypes, write down which letter represents which trait.
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      References

      1. ↑ https://pressbooks.umn.edu/classroompartners/chapter/punnett-squares/
      2. ↑ https://pressbooks.umn.edu/classroompartners/chapter/punnett-squares/
      3. ↑ https://www.nature.com/scitable/definition/genotype-234/
      4. ↑ https://mathbench.umd.edu/modules/prob-stat_punnett-squares_intro/page16.htm
      5. ↑ https://mathbench.umd.edu/modules/prob-stat_punnett-squares_intro/page16.htm
      6. ↑ https://pressbooks.umn.edu/classroompartners/chapter/punnett-squares/
      7. ↑ https://pressbooks.umn.edu/classroompartners/chapter/punnett-squares/
      8. ↑ https://www.genome.gov/genetics-glossary/Phenotype
      9. ↑ https://pressbooks.umn.edu/classroompartners/chapter/punnett-squares/
      1. ↑ https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/incomplete-dominance
      2. ↑ https://www.biologycorner.com/bio2/genetics/notes_dihybrid.html
      3. ↑ https://www.biologycorner.com/bio2/genetics/notes_dihybrid.html
      4. ↑ https://www.biologycorner.com/bio2/genetics/notes_dihybrid.html
      5. ↑ https://biology.arizona.edu/mendelian_genetics/problem_sets/dihybrid_cross/03t.html
      6. ↑ https://www.biologycorner.com/bio2/genetics/notes_dihybrid.html
      7. ↑ https://biology.arizona.edu/mendelian_genetics/problem_sets/dihybrid_cross/03t.html
      8. ↑ https://www.biologycorner.com/bio2/genetics/notes_dihybrid.html
      9. ↑ https://flexbooks.ck12.org/cbook/ck-12-biology-flexbook-2.0/section/3.6/primary/lesson/punnett-squares-bio/
      10. ↑ https://www.technologynetworks.com/neuroscience/articles/gene-vs-allele-definition-difference-and-comparison-331835
      11. ↑ https://flexbooks.ck12.org/cbook/ck-12-biology-flexbook-2.0/section/3.6/primary/lesson/punnett-squares-bio/
      12. ↑ https://www.technologynetworks.com/neuroscience/articles/gene-vs-allele-definition-difference-and-comparison-331835
      13. ↑ https://sites.stat.washington.edu/thompson/Genetics/1.3_genotypes.html
      14. ↑ https://sites.stat.washington.edu/thompson/Genetics/1.3_genotypes.html
      15. ↑ https://sites.stat.washington.edu/thompson/Genetics/1.3_genotypes.html
      16. ↑ https://vgl.ucdavis.edu/test/cream
      17. ↑ https://pressbooks.umn.edu/classroompartners/chapter/punnett-squares/
      18. ↑ https://learn.genetics.utah.edu/content/disorders/inheritance/
      19. ↑ https://www.dnaftb.org/5/bio.html

      About This Article

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      To make a Punnett square, start by drawing a box that's divided into 4 equal squares. Then, label the rows with one parent's genotype and the column's with the other parent's genotype. Then, label each square, starting with the letter to the left of the square followed by the letter above the square. To learn how to interpret your Punnett square, scroll down!

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