When it comes to punnett square practice worksheets, my students used to fall into two camps: either they have the skills mastered to determine the genetic probability of a cross or they tend to struggle with the concept. During the past few years I have spent a lot of time building a better system of teaching this topic to help my students master punnett squares.
What Is A Punnett Square?
In its simplest terms a punnett square (also known as a genetic square) is a diagram that can be used to predict the probability of the offspring of a genetic cross between two parents. Once a punnett square is created for a genetic cross, it can be used to answer a variety of questions. This is where my students have felt overwhelmed in the past. My solution has been to use a systematic approach where students follow the same series of steps for each problem so they can collect the needed data to answer the genetics questions being asked.
The first step in any punnett square problem is to identify the genotypes of the parent. This is where students need to have a good understanding of the vocabulary terms being used in your genetics unit. Students need to be able to read a genetic question and identify which version of a trait is dominant and which is recessive (this is usually clearly stated in the question). They also need to be able to distinguish between homozygous and heterozygous as well as genotype and phenotype.
Once the parental genotypes are determined, students can separate the alleles of each parent and build the punnett square with the appropriate number of possible outcomes (for example a monohybrid cross has 4 possible outcomes while a dihybrid cross has 16). Once the possible genotype outcomes are filled into the squares, students can determine the possible genotypic and phenotypic ratios for the parental cross.
Monohybrid Cross Worksheet
The simplest type of punnett square used to make genetic predictions is a single trait, or monohybrid cross. In lectures at the start of my genetics unit I tend to use a lot of examples of punnett squares that have easy to identify traits. Distinguishing a yellow seed from a green seed or a tall plant from a short plant builds confidence for my students who have told themselves that they aren’t good at science prior to my class. My monohybrid cross worksheet is an assignment I like to start in class as a whole class activity and then let my students work independently or in small groups to complete the problem set.
Dihybrid Cross Worksheet
Once students have shown mastery of monohybrid crosses I like to introduce my dihybrid cross worksheet. This is also the time when I discuss the law of independent assortment with multiple traits. While students might not know what “independent assortment” means, many have learned the F-O-I-L (first, inside, outside, last) or distributive property in math class. Again, I like to use plenty of examples to reinforce the topic. My dihybrid cross worksheet gives my students plenty of practice and allows them to work together as they are gaining a better understanding of probability and genetics.
What Is Incomplete Dominance?
The third type of punnett square practice worksheets I teach my students are those that fall under the category of non-mendelian genetics. I pair this topic with my genetics notes that explain that not all traits follow Mendel’s Laws of Inheritance. I teach my students that polygenic traits exist (such as height) but when it comes to punnett squares, I focus on both incomplete dominance and codominance.
Incomplete Dominance vs Codominance
While the terms incomplete dominance and codominance often get interchanged, they are not the same thing. Incomplete dominance is when the hybrid trait is an intermediate of the two homozygous conditions. A common example is when a red snapdragon flower is crossed with a white snapdragon flower, the offspring are a pink snapdragon.
By contrast, codominance is when the heterozygous condition displays both allele variations simultaneously. The most common example of codominance is human blood type. A person with the blood type AB possesses both the Type A characteristics and the type B characteristics.
After the lecture, I like to have my students work through my incomplete dominance practice worksheet. As with the examples above, I have students focus on the same procedure steps to ensure they are building confidence with the repetition of completing the punnett squares to answer the genetics problem sets.
Sex-Linked Trait Worksheet
The final type of punnett square practice worksheets I teach my students coincide with the teaching of human genetics. While sex-linked traits bring in the variable of gender to genetics, it also is a great time to review the prior three punnett square practice worksheets that my students have completed. After my students complete the sex-linked trait worksheet, I teach pedigrees as a way to connect genetics and heredity.
Final Thoughts…
Students can gain a lot of confidence with punnett square practice worksheets, but don’t stop there. In my class I also have students do a punnett square lab and a pedigree lab as a formative assessment in my genetics unit. With plenty of opportunities to work through punnett square practice problems, students can gain the skills and confidence needed to succeed in your genetics unit.
