If you cross two heterozygous (yy pea plants, what proportion of the offspring will be heterozygous)

The short answer is no, the heterozygous pea plant in question will only have yellow seeds. However, the offspring that might result from selfing this plant could have green or yellow seeds — but each one will have only one seed color, never a mix.

Table of Contents Show

Slipping into your favorite genes
The baby has your seed color
Two genes diverged in a yellow pea plant
Prediction Party with Reginald C. Punnett
“It’s complicated”

Let’s dig into why this is!

Slipping into your favorite genes

To get to our answer, we’ll want to cover a few genetics ground rules.

As you may be aware, traits like seed color are determined by the genes in a plant’s DNA.

In the case of Mendel’s peas, there are two different versions, or alleles, for seed color: yellow (Y) and green (y). Every plant will have two copies of each gene. We call this combination of alleles the plant’s genotype.

The seed color we can observe is called the phenotype. Since seed color in pea plants is determined entirely by genetics (in fact, determined entirely by one gene!), you can actually perfectly predict the phenotype from the genotype. This isn’t always the case, but let’s mind our peas and q’s first!

The baby has your seed color

In pea plants, seed color is an example of complete dominance. This means the dominant allele only needs to be present in one copy to give rise to the trait. A recessive allele must be present in two copies. For seed color, the yellow allele (Y) is dominant while the green allele (y) is recessive.

Combining that together, you get:

Genotype	Phenotype
YY	Yellow
Yy	Yellow
yy	Green

Now we can see that there are three possible genotypes for seed color. Two give us yellow seeds, while one gives us green seeds.

Two genes diverged in a yellow pea plant

So now that we’ve got our feet under us with what dominant and recessive alleles are, let’s move onto thinking about crosses.

The first thing we need to remember is that the egg and sperm will each have only one copy of any given gene.

This is necessary because we want to make sure that plants always have two — and only two — copies of each gene. Otherwise, we’d double the number of alleles every generation! And that would make things get wonky pretty quickly.

So eggs and sperm only have one copy of the seed color gene. But here’s the catch. Their seed color is determined by the DNA of the plant that made those eggs and sperm. For a heterozygote (Yy), that means that all the seeds will be yellow — even though some of them have the yellow allele and some have the green allele.

Prediction Party with Reginald C. Punnett

Like I mentioned at the beginning of the article, seed color is determined entirely by one gene. This means we can predict the seed color of the offspring resulting from crossing a heterozygous yellow-seeded plant to itself.

To illustrate crosses like this, we can use a diagram called a Punnett square.

To set up a Punnett square, you write the possible alleles for the eggs or sperm of each parent, one across the top and the other down the side.

So, for our heterozygous yellow pea plant, the eggs will have either the Y allele or the y allele

And since we’re selfing this plant, the same goes for the sperm.

Now we can fill in the combinations that would arise from each pairing of sperm and egg to figure out the possible outcomes of this cross.

Now we can see from our Punnett square that some of the offspring will end up with yellow seeds and others will end up with green seeds. Exactly which allele ends up in each seed is random.

If we did this experiment enough times in real life, we would see something very close to a 1:2:1 ratio of plants with the YY:Yy:yy genotype. And we would see something very close to 3:1 for yellow to green seeded plants!

Our observed ratios would only be “very close” to our predictions because there’s always slight variability in random events. Think about how you might get 2 tails in a row flipping a coin, even though you’d expect to get one heads, one tails.

And there you have it. A heterozygous yellow pea plant will only ever have yellow seeds, but its offspring could have yellow or green seeds!

“It’s complicated”

I’d like to finish off by mentioning that this logic and Punnett squares don’t work for all traits. Alleles can have complicated relationships with each other, some traits are determined by many genes, and some are influenced by the environment.

Traits that are passed seamlessly from parent to offspring are called Mendelian traits (after Gregor Mendel). In addition to completely dominant relationships (like pea seed color), alleles for Mendelian traits can be incompletely dominant or codominant.

Incomplete dominance means that neither allele is dominant over the other. In these cases, the heterozygote will have an intermediate phenotype compared to the homozygotes.

Four o’clock flowers are an example of this. The offspring of a red and white flowered plant will be pink!

The other Mendelian relationship two alleles for the same gene can have is codominance. For alleles that are codominant, both are independently expressed, meaning that both phenotypes are observed.

This is the case for speckled chickens. Offspring from a black parent and white parent will have a mixture of black and white feathers.

Mendelian traits are inherited in a manner that adheres to the Punnett square logic, but some have more complicated allelic relationships. (Images via Shutterstock)

While Mendel discovered many foundational concepts of genetics and inheritance, we know now that many traits are much, much more complicated.

Most traits that we might wonder about on a day to day basis are impossible to apply this logic to. That includes traits like height, aptitude for a particular skill, and personality. Traits like these are influenced by many genes plus the environment, and not inherited in a simple manner from parent to offspring. So unlike our pea plants, you’re completely out of luck trying to predict them based on parental traits.

Probability of Inheritance

The value of studying genetics is in understanding how we can predict the likelihood of inheriting particular traits. This can help plant and animal breeders in developing varieties that have more desirable qualities. It can also help people explain and predict patterns of inheritance in family lines.

One of the easiest ways to calculate the mathematical probability of inheriting a specific trait was invented by an early 20th century English geneticist named Reginald Punnett
. His technique employs what we now call a Punnett square. This is a simple graphical way of discovering all of the potential combinations of genotypes that can occur in children, given the genotypes of their parents. It also shows us the odds of each of the offspring genotypes occurring.

Setting up and using a Punnett square is quite simple once you understand how it works. You begin by drawing a grid of perpendicular lines:

Next, you put the genotype of one parent across the top and that of the other parent down the left side. For example, if parent pea plant genotypes were YY and GG respectively, the setup would be:

Note that only one letter goes in each box for the parents. It does not matter which parent is on the side or the top of the Punnett square.

Next, all you have to do is fill in the boxes by copying the row and column-head letters across or down into the empty squares. This gives us the predicted frequency of all of the potential genotypes among the offspring each time reproduction occurs.

In this example, 100% of the offspring will likely be heterozygous (YG). Since the Y (yellow) allele is dominant over the G (green) allele for pea plants, 100% of the YG offspring will have a yellow phenotype, as Mendel observed in his breeding experiments.

In another example (shown below), if the parent plants both have heterozygous (YG) genotypes, there will be 25% YY, 50% YG, and 25% GG offspring on average. These percentages are determined based on the fact that each of the 4 offspring boxes in a Punnett square is 25% (1 out of 4). As to phenotypes, 75% will be Y and only 25% will be G. These will be the odds every time a new offspring is conceived by parents with YG genotypes.

An offspring's genotype is the result of the combination of genes in the sex cells or gametes (sperm and ova) that came together in its conception. One sex cell came from each parent. Sex cells normally only have one copy of the gene for each trait (e.g., one copy of the Y or G form of the gene in the example above). Each of the two Punnett square boxes in which the parent genes for a trait are placed (across the top or on the left side) actually represents one of the two possible genotypes for a parent sex cell. Which of the two parental copies of a gene is inherited depends on which sex cell is inherited--it is a matter of chance. By placing each of the two copies in its own box has the effect of giving it a 50% chance of being inherited.

If you are not yet clear about how to make a Punnett Square and interpret its result, take the time to try to figure it out before going on.

Are Punnett Squares Just Academic Games?

Why is it important for you to know about Punnett squares? The answer is that they can be used as predictive tools when considering having children. Let us assume, for instance, that both you and your mate are carriers for a particularly unpleasant genetically inherited disease such as cystic fibrosis
. Of course, you are worried about whether your children will be healthy and normal. For this example, let us define "A" as being the dominant normal allele and "a" as the recessive abnormal one that is responsible for cystic fibrosis. As carriers, you and your mate are both heterozygous (Aa). This disease only afflicts those who are homozygous recessive (aa). The Punnett square below makes it clear that at each birth, there will be a 25% chance of you having a normal homozygous (AA) child, a 50% chance of a healthy heterozygous (Aa) carrier child like you and your mate, and a 25% chance of a homozygous recessive (aa) child who probably will eventually die from this condition.

If both parents are carriers of the recessive allele for a disorder, all of their children will face the following odds of inheriting it: 25% chance of having the recessive disorder 50% chance of being a healthy carrier 25% chance of being healthy and not have
the recessive allele at all

If a carrier (Aa) for such a recessive disease mates with someone who has it (aa), the likelihood of their children also inheriting the condition is far greater (as shown below). On average, half of the children will be heterozygous (Aa) and, therefore, carriers. The remaining half will inherit 2 recessive alleles (aa) and develop the disease.

If one parent is a carrier and the other has a recessive disorder, their children will have the following odds of inheriting it: 50% chance of being a healthy carrier
50% chance having the recessive disorder

It is likely that every one of us is a carrier for a large number of recessive alleles. Some of these alleles can cause life-threatening defects if they are inherited from both parents. In addition to cystic fibrosis, albinism, and beta-thalassemia are recessive disorders.

Some disorders are caused by dominant alleles for genes. Inheriting just one copy of such a dominant allele will cause the disorder. This is the case with Huntington disease, achondroplastic dwarfism, and polydactyly. People who are heterozygous (Aa) are not healthy carriers. They have the disorder just like homozygous dominant (AA) individuals.

If only one parent has a single copy of a dominant allele for a dominant disorder, their children will have a 50% chance of inheriting the disorder and 50% chance
of being entirely normal.

Punnett squares are standard tools used by genetic counselors. Theoretically, the likelihood of inheriting many traits, including useful ones, can be predicted using them. It is also possible to construct squares for more than one trait at a time. However, some traits are not inherited with the simple mathematical probability suggested here. We will explore some of these exceptions in the next section of the tutorial.