Before we get to the Punnett square practice, let’s first learn a little key vocabulary that will help us solve the problems.
It’s been understood for centuries that children resemble their parents, but modern science has continued to seek to unravel the secrets of inheritance. We all have traits, or characteristics, that we received genetically from our parents. Genetics is the study of heredity and how traits are passed from parents to offspring and heredity is the process by which those traits are passed.
In genetics, only sexually reproducing organisms are investigated. We already know that in sexual reproduction, each parent gives one half of their DNA to the new offspring. Which half though? That’s what genetics is all about. We can’t say for certain, but when we know the parents genetic information, we can use that to predict the probable genetic outcomes of the child too. A parent or offspring’s genetic make up is known as their genotype. The genotype is usually made up of two alleles for each trait (one from each parent). The physical traits you can observe in an organism is their phenotype. Phenotype is the result of the genotype.
We know that we inherit two pieces of information for each trait because we have two parents. These forms of a gene are known as alleles. Most traits have two or more forms, or alleles. Alleles are usually represented with capital and lower case letters, such as B and b. A capital letter indicates a dominant allele. Dominant means that this allele is powerful and can mask the presence of other alleles. Genotypes of BB or Bb will express a dominant trait. A lowercase letter indicates a recessive allele. Recessive means this allele is weaker, and will only be expressed when no dominant allele is present. Only genotypes of bb will express a recessive trait.
Lastly there is just a few more terms to know and understand. When speaking about an organism’s genotype, scientists will often use the words heterozygous and homozygous. Heterozygous means that the organism has two alleles which are different, such as Bb. Hybrid is another word used to describe a heterozygous organism. Homozygous, on the other hand, means that the organism has two alleles which are the same, BB or bb. Purebred and true-breeding are other words used to describe homozygous organisms.
Punnett Square Vocabulary Practice
Using the paragraphs above, determine the word that matches with each definition.
_____________ 1. Gene that is expressed only in the homozygous state
_____________ 2. Gene that is always expressed
_____________ 3. Different forms of genes for a trait
_____________ 4. Genotypes where the alleles are the same
_____________ 5. Genotypes where the alleles are different
_____________ 6. The genetic makeup of an organism
_____________ 7. The physical characteristics or an organism
_____________ 8. the study of heredity
Punnett Squares Sample
Take a look at the sample question below before solving the following questions:
The first step in solving a Punnett square practice question is to determine the parents’ genotypes and which trait is dominant and which is recessive. The question will always give enough clues to make this possible, but in this question, the parent genotypes were given and the question also informed us that green pea pods are dominant (and therefore represented with a capital letter) while yellow pea pods are recessive (and will therefore be represented with a lowercase letter).
The second step is to put the parents’ genotypes on the outside of the Punnett square. In the image above, the father is represented in blue while the mom is represented in pink. Either parent can be written on either side of the Punnett square. It doesn’t matter!
Next, fill in the interior boxes of the Punnett square by writing the two parent alleles which are directly to the left and directly above each box. Typically, the capital letter is written first.
Finally, calculate the possible genotypes and phenotypes of the offspring by counting the interior boxes of the Punnett square. Genotypes and phenotypes are usually listed with the most dominant variation first and gradually descending to the most recessive. Even genotypes and phenotypes which are not represented in the interior of the Punnett square are recorded, but they are listed as 0%.
If one offspring expresses a specific genotype or phenotype, the fraction is 1/4 and the percentage is 25%. If there are 2, it is 2/4 or 50%. If there are 3, it is 3/4 or 75%. If all of the offspring express a certain genotype or phenotype, write 4/4 or 100%.
Solving Punnett Squares Practice
For each of the following Punnett square practice questions, list all possible genotypes and phenotypes of the offspring.
1. Purple flowers are dominant to white flowers. PP x pp
2. Brown eyes are dominant to blue. bb x bb
3. Tall plants are dominant to short. Tt x TT
4. Predict the genotypic and phenotypic outcomes for the offspring of a cross between two heterozygous tall pea plants.
5. In pea plants, purple flowers are dominant to white. A purebred purple flower is crossed with a white flower. Predict the genotypic and phenotypic outcomes.
6. A heterozygous black fur rabbit is crossed with a white fur rabbit. Predict the genotypic and phenotypic outcomes.
7. A heterozygous round pea is crossed with a purebred round pea. Predict the genotypic and phenotypic outcomes.
Punnett Squares Teaching & Practice Resources
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