Understanding the Basics of a Punnett Square
A common way to predict the outcome of simple genetic crosses is to make a Punnett Square, a method originally proposed by the British geneticist Reginald Punnett.
To construct a Punnett square, you must know the genotypes of the parents. What follows is a step-by-step description of the Punnett square approach using a cross of heterozygous tall plants.
Setting Up a Punnett Square
- Step 1: Write down the genotypes of both parents. In the following example, a heterozygous tall plant is crossed to another heterozygous tall plant. The plant providing the pollen is considered the male parent and the plant providing the eggs, the female parent.
Male Parent: Tt
Female Parent: Tt
- Step 2: Write down the possible gametes that each parent can make. A gamete contains only one copy of each gene.
Male Gametes: T or t
Female Gametes: T or t
- Step 3: Create an empty Punnett square. The number of columns equal the number of male gametes, and the number of rows equals the number of female gametes.
Example: 2 rows & 2 columns. Place the male gametes across the top of the Punnett square and the female gametes along the side.
- Step 4: Fill in the possible genotypes of the offspring by combining the alleles of the gametes in the empty boxes.
- Step 5: Determine the relative proportions of genotypes and phenotypes of the offspring. To determine the phenotypes, you must know which allele is dominant. For plant height, T (tall) is dominant and t (short) recessive. The genotypes TT and Tt are tall, whereas the genotypes tt is short.
A Testcross Can Be Used to Determine an Individual’s Genotype
When a trait has two variants, one of which is dominant over the other, we know that an individual with a recessive phenotype is homozygous for the recessive allele.
Test crosses involve breeding the individual in question with another individual that expresses a recessive version of the same trait.
If all offspring display the dominant phenotype, the individual in question is homozygous dominant; if the offspring display both dominant and recessive phenotypes, then the individual is heterozygous.
If the individual being tested produces any recessive offspring its genotype is heterozygous. If all the offspring are phenotypically dominant, its genotype is homozygous.