Punnett Squares
Although Mendel’s work was largely ignored for forty years after it was published, its rediscovery in the early 1900’s sparked a revolution in the study of heredity and the establishment of genetics as a distinct field of study. Among the early genetics researchers was Reginald Punnett, who expanded Mendel’s studies to other organisms. Reginald Punnett devised what we now call the Punnett square, which is a tool to visually depict all possible progeny and their expected ratios from a controlled cross[1].
An example: In Mendel’s crosses, each of Mendel’s F1 offspring were monohybrid (Aa). The F1 self-cross Aa x Aa gave F2 progeny in a phenotypic ratio of 3 dominant: 1 recessive, or a genotypic ratio of 1 AA : 2Aa: 1aa. A Punnett square depicts this cross in a table.
Across the top row of the table, the possible gametes that an individual can produce are written. An individual with genotype Aa can produce a haploid gamete with either the A allele or the a allele:
| No Data | A | a |
Down the first column of the table, the possible gametes of the second parent are written, each in its own row:
| No Data | A | a |
| A | ||
| a |
Then, the remainder of the table is filled in by filling each box in with the gametes that correspond to each row and column:
| No Data | A | a |
| A | A | a |
| a | A | a |
⇓
| No Data | A | a |
| A | A A | A a |
| a | A a | a a |
These four boxes represent the combinations of alleles that are possible among offspring. These are the genotypes of potential offspring. You’ll see that there is one box with genotype AA, two boxes with genotype Aa, and one box with aa. This is a visual representation of the 1AA : 2 Aa : 1 aa genotypic ratio Mendel observed. Note that, by convention, for a heterozygote we usually write the dominant allele first.
Test Your Understanding
The Punnett square gives us a tool to predict the possible offspring of any cross, given the genotypes of the parent. It also, however, can be used in reverse, to determine the genotype of a parent.
An example: Individuals with a dominant phenotype can either be homozygous dominant (AA) heterozygous (Aa). When the second allele is unknown, the genotype can be written A_, with the underscore indicating that uncertainty. How could we determine that unknown allele? One way to do this is to perform a testcross.
A testcross is a cross that includes a parent with the recessive trait. The recessive parent is called the tester. An individual with the recessive phenotype must be homozygous recessive, so the phenotypes of the offspring will reveal the genotype of the unknown parent. If any offspring have a recessive phenotype, the unknown allele must be a since we see recessive offspring.
| No Data | A | _ |
| a | dominant | recessive |
| a | dominant | recessive |
Conclusion from the phenotypes:
the genotypes must be
⇓
| No Data | A | a |
| a | Aa | aa |
| a | Aa | aa |
A testcross can be used with multiple genes at a time. So, for example, an individual with a phenotype of two dominant traits (A and B) and a genotype of A_B_ could be testcrossed with an individual of genotype aabb.
Testcrosses give characteristic offspring ratios. A monohybrid testcross (Aa x aa) is a testcross of a monohybrid individual. Note that this is different from a monohybrid cross (Aa x Aa).
A monohybrid testcross always gives offspring in a 1:1 ratio of dominant to recessive. Or, in other words, ½ of the offspring are dominant, and ½ are recessive. So, if a cross gives two different phenotypes in a 1:1 ratio, we can usually conclude that it was a monohybrid testcross and our unknown parent is heterozygous.
Test Your Understanding
A dihybrid testcross (AaBb x aabb) gives four potential combinations of phenotypes: AB, Ab, aB, and ab. These combinations can be called phenotypic classes. For a dihybrid testross, the four phenotypic classes will all be present in roughly equal numbers. This is a 1:1:1:1 ratio. In the next section of the text, looking at Mendel’s Second Law, we will look at the biological principles behind this ratio.
Test Your Understanding
- Mendelism / by R.C. Punnett. Wellcome Collection https://wellcomecollection.org/works/u5wbhqtr/items. ↵
