Domestic budgies have come a long way from their wild green and yellow relatives. Looking at your own flock, you may wonder how breeders are able to plan and predict such a diverse array of patterns and colors. How is it possible to anticipate the colors of chicks just from looking at their parents?
The answer is the Punnett square: a handy graphic representation tool used to make predictions about the traits of offspring.
You may already be familiar with the term “Mendelian genetics.” German monk Gregor Mendel developed his keystone theory of genetic inheritance in the mid-1800s, working with the humble pea plant in his monastery’s gardens. These findings have stood the test of time–in fact, there’s a good chance you learned about Mendel’s ideas (like heritability and dominant vs. recessive traits) at some point in a science class.
Reginald Crundall Punnett, an early 20th-century scientist at the University of Cambridge, also found inspiration and insight in Mendel’s ideas. Like the monk himself, Punnett was fascinated by the genetics of sweet pea plants–specifically, the way traits like color occurred in predictable ratios among the offspring of breeding pairs. In his efforts to create a simple way to visualize these ratios, the Punnett square was born!
The Punnett square is a grid that helps us visualize the possible distribution of genotypes among the offspring of any two parents. Here, genotype refers to a specific version of a gene. In pea plants, for example, if leaf color is controlled by a certain gene, then some genotypes (i.e, some versions of this gene) may result in green leaf color, while others result in yellow leaf color. The outward, physical manifestation of a genotype (in this case, green or yellow leaf color) is referred to as phenotype.
Most animals (like budgies and humans) and some plants (like pea plants) are diploid, meaning they have two complete sets of chromosomes in their cells– one set from each parent. This also means they have two copies of each gene and each copy could be one of several versions of that gene.
Certain versions of the gene are dominant, meaning just one copy of the gene will result in the organism having that phenotype. Other versions of the gene are recessive, meaning they will be overridden by the dominant allele so we will only see the recessive phenotype when no dominant allele is present.
Because of this, each genotype in a Punnett square is represented by two letters, with one “letter” being inherited from each parent. The letters represent alleles, the versions of a gene that make up the specific genotype. Having two of the same alleles for a gene is called homozygotic, while having two different alleles is called heterozygotic.
Don’t worry if you’re not completely comfortable with these terms yet. Punnett squares are very visual, and you don’t need advanced genetic knowledge to understand them. Let’s set up a classic Punnett square to illustrate the pea plant color example we mentioned above.
You’ll see that we’ve drawn our grid and added the genotypes of both parent plants: one along the vertical edge of the square and one along the horizontal edge. The capital G allele encodes for dominant green color, while the lowercase g allele encodes for recessive yellow color.
Parent one has gg (homozygous) genotype, meaning it has two copies of the recessive yellow allele. Because it does not have a copy of the dominant green allele (G), parent one appears yellow (i.e., it has a yellow phenotype).
Parent two has a Gg (heterozygous) genotype, so it has one copy of the dominant green allele (G) and one copy of the recessive yellow allele (g). Although this isn’t a hard and fast rule, it can be helpful to imagine dominant alleles as “muting” recessive alleles. So in parent two, the dominant green allele (G) “mutes” the recessive yellow allele (g), resulting in a green phenotype. Now let’s fill out the rest of our Punnett square.
In our completed Punnett square, each inner section of the grid represents a possible offspring genotype. You can see that we’ve ended up with two possible Gg genotypes and two possible gg genotypes. In this pairing between plants one and two, we can estimate that 50% of offspring will have a Gg genotype and appear green, while 50% will have a gg genotype and appear yellow.
Although budgie genetics are more complicated than the pea plant example above, we can still use this basic concept to understand how certain colors and patterns are passed from parent to chick. We’ll use the example of yellow base vs. white base to illustrate. Below, we’ve included a crash course explaining what the terms yellow and white base mean, but feel free to skip ahead if you’re already familiar.
Example 1: Will offspring be yellow or white?
For our Punnett square example, we should know that the yellow base allele–we’ll represent it with a capital Y– is dominant, and the white base allele–lowercase y–is recessive.
Let’s say that we’d like to breed two budgies. We know our female is yellow base with a YY genotype, and our male is white base and must have a yy genotype. After we fill out our square, it should look something like this:
From this square, we can see that the only possible genotype for our chicks is Yy, so all of the offspring from this pairing will have the yellow base phenotype!
Example 2: Homozygous or Heterozygous?
The Punnett square can also be used to figure out the genotype of a parent. Let’s say you have a white base (yy) male and a yellow base female, but don’t know whether she has a heterozygous Yy or homozygous YY genotype. If she has a YY genotype, your Punnett square would look exactly like the example above, with the odds of having white base offspring being zero percent. If she has a Yy genotype, it would look like this:
This pairing would result in 50% white base, 50% yellow base. If these birds mate and have any white base offspring, you could confidently say that your female has a heterozygous Yy genotype.
Example 3: Yellow x Yellow = White???
The Punnett Square can also help us map out less likely genetic outcomes. For example, if two heterozygous yellow base budgies mate, there is a chance they would produce a white base budgie. How? Because each heterozygous budgie has two copies of the yellow gene, one dominant (Y) and one recessive (y), they could produce three different combinations–homozygous dominant yellow base (YY), heterozygous yellow base (Yy), or homozygous recessive white base (yy).
This means that in this type of pairing, we would expect to see 75% of the offspring with a yellow base phenotype. Of that 75%, 25% is homozygous dominant (YY) and 50% is heterozygous dominant (Yy). The remaining 25% of the offspring would be have a white base and be homozygous recessive (yy). So, if you ever see a white base budgie from two yellow base parents, you can know that both of those parents are heterozygous for the yellow base gene!
Wrapping Up
Budgies can have so many variations in color and pattern beyond just a yellow or white base. The genetic science behind some of these variations is more complex than the simple dominant vs. recessive inheritance patterns we’ve illustrated here, but with a good grasp on the basics, you should have an easier time tackling more complex Punnett squares when the need arises.
Has this intro to budgie color genetics piqued your interest? Stay tuned for our upcoming deep dive on color variations and their underlying genetics. In the meantime, check out our other articles at The Budgie Academy and connect with us on TikTok, Facebook, and Instagram to stay in the loop!
Literature Cited
Harveson, B. (2016, June 14). Gregor Mendel and His Peas – the Origin of Modern Genetics. University of Nebraska-Lincoln. https://cropwatch.unl.edu/2016/gregor-mendel-and-his-peas-%E2%80%93-origin-modern-genetics
Reginald Crundall Punnett: DNA from the Beginning. http://www.dnaftb.org/5/bio.html#:~:text=Reginald%20Punnett%20and%20William%20Bateson
Martin, T. A Guide to Colour Mutations and Genetics in Parrots. 2002.
Cooke, T. F. et al. Genetic Mapping and Biochemical Basis of Yellow Feather Pigmentation in Budgerigars. Cell 171, 427-439.e21 (2017).
1 thought on “The Genetics of Budgie Color: Punnett Squares”
Pingback: Your Guide to Budgie Colors: What Color is Your Budgie? - The Budgie Academy