Punnett Squares & Predicting Traits
Learn to build and read a Punnett square, find genotype and phenotype ratios for a one-trait cross, and state results as probabilities for each offspring.
What you'll do in this lesson
A voice-first session with the Crimsora tutor on Punnett Squares & Predicting Traits, then targeted practice and FRQs — with the tutor adapting to where you get stuck.
What this lesson covers
Two brown-eyed parents have a blue-eyed baby. A black guinea pig and a white guinea pig have a litter that is entirely black. Neither result is a mistake — both are exactly what the rules of inheritance predict. A Punnett square is the tool geneticists use to see those predictions on paper.
In this lesson you will learn how to set up a Punnett square for a cross involving one trait, how to fill in the boxes correctly, and how to turn the finished square into genotype and phenotype ratios. Just as importantly, you will learn what a Punnett square does not tell you. It never promises that a litter of four will contain exactly one white pup. It gives a probability for each offspring, one at a time — the same way flipping a coin four times does not guarantee two heads.
In this lesson you will learn how to set up a Punnett square for a cross involving one trait, how to fill in the boxes correctly, and how to turn the finished square into genotype and phenotype ratios. Just as importantly, you will learn what a Punnett square does not tell you. It never promises that a litter of four will contain exactly one white pup. It gives a probability for each offspring, one at a time — the same way flipping a coin four times does not guarantee two heads.
What a Punnett Square Actually Models
Every body cell of an organism carries two alleles for a trait, one inherited from each parent. When a parent makes sex cells (gametes — sperm or egg), those two alleles separate, so each gamete carries only one allele for that trait. This separation is the whole reason a Punnett square works.
A Punnett square is a simple grid that lists one parent's possible gametes across the top and the other parent's possible gametes down the left side. Each inside box shows one possible combination of a sperm allele with an egg allele — in other words, one possible genotype for an offspring.
A heterozygous parent with genotype can make two kinds of gametes: half carry and half carry . That parent gets two columns (or two rows). A homozygous parent, or , can only make one kind of gamete, so both of its columns are labeled with the same letter.
This is where a lot of early mistakes happen. Students sometimes write the whole parent genotype in a single column heading instead of splitting it into and . The headings are gametes, not parents. A gamete for one trait always carries exactly one letter.
One more habit worth building now: write the dominant allele as a capital letter and the recessive allele as the same letter in lowercase. Using and for black and white fur makes the square unreadable, because you can no longer tell at a glance which allele pairs belong to the same gene.
A Punnett square is a simple grid that lists one parent's possible gametes across the top and the other parent's possible gametes down the left side. Each inside box shows one possible combination of a sperm allele with an egg allele — in other words, one possible genotype for an offspring.
A heterozygous parent with genotype can make two kinds of gametes: half carry and half carry . That parent gets two columns (or two rows). A homozygous parent, or , can only make one kind of gamete, so both of its columns are labeled with the same letter.
This is where a lot of early mistakes happen. Students sometimes write the whole parent genotype in a single column heading instead of splitting it into and . The headings are gametes, not parents. A gamete for one trait always carries exactly one letter.
One more habit worth building now: write the dominant allele as a capital letter and the recessive allele as the same letter in lowercase. Using and for black and white fur makes the square unreadable, because you can no longer tell at a glance which allele pairs belong to the same gene.
Building the Square Step by Step
Suppose two heterozygous black guinea pigs are crossed: , where (black) is dominant over (white).
First, split each parent into gametes: and for the top parent, and for the side parent. Then label a two-by-two grid and fill each box by bringing down the column letter and across the row letter.
When you write a genotype in a box, always put the capital letter first: write , not . They mean the same thing, but consistent order makes it much easier to count matching genotypes at the end.
Now read the square twice. Read it once for genotype — the actual allele pairs. There is one , two , and one , a genotype ratio of .
Read it a second time for phenotype — the trait you can observe. Any box containing at least one produces black fur, because one dominant allele is enough. That covers , , and : three boxes. Only gives white fur: one box. The phenotype ratio is black to white.
Notice that the two ratios come from the same four boxes. A frequent error is reporting as the genotype ratio. Genotype counts letter pairs; phenotype counts appearances. In a cross like , the genotype ratio is all while the phenotype is 100 percent black — the two answers look completely different.
First, split each parent into gametes: and for the top parent, and for the side parent. Then label a two-by-two grid and fill each box by bringing down the column letter and across the row letter.
Now read the square twice. Read it once for genotype — the actual allele pairs. There is one , two , and one , a genotype ratio of .
Read it a second time for phenotype — the trait you can observe. Any box containing at least one produces black fur, because one dominant allele is enough. That covers , , and : three boxes. Only gives white fur: one box. The phenotype ratio is black to white.
Notice that the two ratios come from the same four boxes. A frequent error is reporting as the genotype ratio. Genotype counts letter pairs; phenotype counts appearances. In a cross like , the genotype ratio is all while the phenotype is 100 percent black — the two answers look completely different.
Turning Boxes Into Probability
A four-box Punnett square has four equally likely outcomes, so each box represents a probability of , or 25 percent. To find the chance of any result, count the boxes that show it and divide by the total number of boxes.
For the cross: the probability of a white offspring is ; the probability of a black offspring is ; the probability of a heterozygous offspring is .
Here is the idea that matters most in this lesson: that 25 percent applies to each offspring separately. It is not a promise about a group. If these guinea pigs have four pups, all four could be black. Fertilization is a chance event, like flipping a coin — four flips can easily give four heads even though each flip is 50 percent.
Students often write "one of the four babies will be white." A more accurate statement is "each baby has a 25 percent chance of being white." The prediction describes probability, not a guaranteed count.
Probabilities do become more reliable as the number of offspring grows. A pea plant producing 800 seeds will land much closer to the 3:1 phenotype ratio than a guinea pig litter of four. This is the same reason 1,000 coin flips land near half heads while 4 flips often do not.
Also remember that past offspring do not change future ones. If the first three pups are black, the fourth still has a 25 percent chance of being white. Each fertilization event starts fresh.
For the cross: the probability of a white offspring is ; the probability of a black offspring is ; the probability of a heterozygous offspring is .
Here is the idea that matters most in this lesson: that 25 percent applies to each offspring separately. It is not a promise about a group. If these guinea pigs have four pups, all four could be black. Fertilization is a chance event, like flipping a coin — four flips can easily give four heads even though each flip is 50 percent.
Students often write "one of the four babies will be white." A more accurate statement is "each baby has a 25 percent chance of being white." The prediction describes probability, not a guaranteed count.
Probabilities do become more reliable as the number of offspring grows. A pea plant producing 800 seeds will land much closer to the 3:1 phenotype ratio than a guinea pig litter of four. This is the same reason 1,000 coin flips land near half heads while 4 flips often do not.
Also remember that past offspring do not change future ones. If the first three pups are black, the fourth still has a 25 percent chance of being white. Each fertilization event starts fresh.
Comparing Common One-Trait Crosses
Once you can build one square, patterns start to repeat. Memorizing these four crosses is less useful than being able to rebuild them quickly, but recognizing them helps you check your work.
Two of these are especially worth understanding. The cross shows that offspring can all look like one parent while carrying a hidden allele from the other. Every pup is heterozygous, so the recessive trait disappears in this generation but can reappear in the next.
The cross is called a test cross, and breeders use it on purpose. If you have a black guinea pig but cannot tell whether it is or , crossing it with a white () animal answers the question. If any white offspring appear, the unknown parent had to be , because a white pup needs a from each parent.
Working backward like this is a skill in itself. Whenever a recessive phenotype shows up in the offspring, both parents must carry at least one recessive allele — even if neither parent shows the trait.
| Cross | Genotypes of offspring | Phenotype outcome |
|---|---|---|
| all | 100% dominant | |
| all | 100% dominant, all carriers | |
| , | 50% dominant, 50% recessive | |
| , , | 75% dominant, 25% recessive |
The cross is called a test cross, and breeders use it on purpose. If you have a black guinea pig but cannot tell whether it is or , crossing it with a white () animal answers the question. If any white offspring appear, the unknown parent had to be , because a white pup needs a from each parent.
Working backward like this is a skill in itself. Whenever a recessive phenotype shows up in the offspring, both parents must carry at least one recessive allele — even if neither parent shows the trait.
Key terms
- Allele.
- One version of a gene, such as the allele for black fur or the allele for white fur. Each offspring inherits one allele for a trait from each parent.
- Genotype.
- The combination of alleles an organism carries for a trait, written as a letter pair such as , , or .
- Phenotype.
- The observable version of the trait — what the organism actually looks like, such as black fur or white fur.
- Homozygous.
- Having two identical alleles for a trait, either (homozygous dominant) or (homozygous recessive).
- Heterozygous.
- Having two different alleles for a trait, such as . The dominant allele determines the phenotype, while the recessive allele is carried but hidden.
- Gamete.
- A sex cell (sperm or egg) that carries only one allele per trait. Gamete letters are what label the top and side of a Punnett square.
- Punnett square.
- A grid that shows every possible combination of parent gametes, used to predict the probability of each offspring genotype and phenotype.
- Probability.
- The chance that a particular outcome happens, found by dividing the number of boxes showing that outcome by the total number of boxes.
Worked example
In pea plants, tall () is dominant over short (). A heterozygous tall plant is crossed with a short plant. Draw the Punnett square, give the genotype and phenotype ratios, and state the probability that a single seed grows into a short plant.
Step 1 — Write the parent genotypes. Heterozygous tall is . Short is a recessive phenotype, so the short plant must be (a short plant cannot carry a , or it would be tall). The cross is .
Step 2 — Find the gametes. The parent makes two kinds: and . The parent makes only one kind, , so both of its rows are labeled .
Step 3 — Fill the grid.
Step 4 — Count genotypes. Two boxes are and two boxes are . The genotype ratio is , which simplifies to heterozygous to homozygous recessive. Notice that no appears anywhere, because the short parent has no to give.
Step 5 — Count phenotypes. Boxes with at least one are tall: two boxes. Boxes that are are short: two boxes. The phenotype ratio is tall to short.
Step 6 — State the probability. Two of the four boxes are short, so . Each seed from this cross has a 50 percent chance of growing into a short plant. This does not mean exactly half of any particular group of seeds will be short — it is the chance for each seed on its own.
Step 2 — Find the gametes. The parent makes two kinds: and . The parent makes only one kind, , so both of its rows are labeled .
Step 3 — Fill the grid.
Step 5 — Count phenotypes. Boxes with at least one are tall: two boxes. Boxes that are are short: two boxes. The phenotype ratio is tall to short.
Step 6 — State the probability. Two of the four boxes are short, so . Each seed from this cross has a 50 percent chance of growing into a short plant. This does not mean exactly half of any particular group of seeds will be short — it is the chance for each seed on its own.
Practice questions
Two black mice are crossed. Black () is dominant over brown (). Both parents are heterozygous. What is the probability that a single offspring will be brown?
- 0%
- 25%
- 50%
- 75%
Answer: 25%
The cross is . The four boxes are , , , and . Brown is recessive, so an offspring is brown only with genotype — one box out of four, or . The 75 percent choice is the chance of a black offspring, and 50 percent is the chance of a heterozygous offspring, so it helps to reread exactly which outcome the question asks for.
A gardener crosses a homozygous dominant purple-flowered plant () with a white-flowered plant (). Predict the genotypes and phenotypes of the offspring, and explain why the white trait seems to vanish in this generation.
Answer: All offspring are (100 percent heterozygous) and all have purple flowers. The white allele has not disappeared — every offspring carries one , but the dominant masks it in the phenotype.
The parent can only make gametes and the parent can only make gametes, so every one of the four boxes reads . Because is dominant, a single copy is enough to produce purple flowers. The recessive allele is still present and can reappear whenever two of these offspring are crossed with each other, since gives a 25 percent chance of . A complete answer separates the genotype result from the phenotype result — they are very different here.
A rabbit breeder has a black rabbit and does not know whether its genotype is or . She crosses it with a white () rabbit and gets six black babies and no white ones. Can she be certain the black parent is ? Explain using probability.
Answer: No. If the parent were , each baby would still have a 50 percent chance of being black, so six black babies in a row is possible by chance. The result makes more likely, but it does not prove it.
This question tests the core idea that a Punnett square gives probability per offspring, not a guaranteed count. For a cross, the chance that all six offspring happen to be black is multiplied by itself six times, which is small but not zero. Getting even one white baby would have proven the parent was , because a white rabbit needs a from each parent. Absence of the recessive trait is weaker evidence than its presence.
FAQ
- What is the difference between genotype ratio and phenotype ratio?
- Genotype ratio counts the actual allele pairs in the boxes, and phenotype ratio counts how many boxes produce each observable trait. For a cross the genotype ratio is ( to to ), while the phenotype ratio is (dominant to recessive), because and look the same.
- If a Punnett square says 25 percent, does that mean one out of every four babies will have the trait?
- No. It means each individual offspring has a 25 percent chance. A litter of four could have zero, one, two, three, or even four offspring with the trait, just like four coin flips do not always give exactly two heads. Real ratios match the prediction more closely when the number of offspring is very large.
- How do I know which letter to use for the alleles?
- Use the first letter of the dominant trait, capitalized, and the same letter in lowercase for the recessive trait. If tall is dominant, use for tall and for short. Using two different letters makes it impossible to tell that the alleles belong to the same gene.
- Can two parents that show a dominant trait have offspring with the recessive trait?
- Yes, if both parents are heterozygous. Each parent carries a hidden recessive allele and can pass it on, so a cross produces offspring 25 percent of the time. This is exactly why two brown-eyed parents can have a blue-eyed child.
Learn this with a teacher, not a page
The Crimsora tutor teaches Punnett Squares & Predicting Traits live — explaining on a whiteboard, asking you questions, and adapting to where you get stuck.