Genetics

Lesson 5.3

Monohybrid Cross

5.3.1 Punnett Squares reading

A large part of understanding genetics is understanding how genes (traits) are expressed (shown). Located on our chromosomes are sections of DNA called genes. Each gene holds the information for a specific trait. That trait may be something physical like hitchhiker's thumb, Widow’s peaks or Sickle Cell Anemia. The gene may also hold the information for things that make us human or simply alive like having 2 kidneys, making digestive enzymes or that you have a thumb.

For each gene there are alleles. Alleles are the options that are available for each gene. For example if we are looking at the gene for earlobe type, the two alleles are attached or unattached (free). (see figure 5.9) The alleles for widow’s peaks are either present (you have it) or absent (you don’t.) (see figure 5.10)

Which trait is expressed is determined by what type of trait it is. Traits may be simple dominance traits or they may be incomplete dominance or sex-linked. No matter which type of trait they are we can use a genetic tool called a Punnett Square to help us predict which allele a person will have.

Before we can actually do any Punnett squares we must first master a few vocabulary terms. There are two ways that we can describe or say a person’s genetic traits, either by phenotype or genotype. A phenotype is the word (often allele type) used to describe the trait, generally it is what the trait physically looks like (phenotype/physical: P/P). For example, if the trait we are talking about is eye color, the phenotypes would be green, blue and brown. If the trait was Hitchhiker’s Thumb (HT) the phenotypes would be HT present or HT Absent. (see figure 5.11) 

Figure 5.11

Thumbs up by David Benbennick / CC BY-SA 3.0

Geldzähldaumen by j987654 / Public Domain

The other way to describe a trait is by genotype. The genotype is the two letter abbreviation used to signify a trait. The letter we use does not matter, it is the meaning of the letter that counts. We will oftentimes use a letter from the beginning of the trait or allele name, but that is not always the case. A genotype will look like this Aa, AA, AB, etc.

In doing a Punnett square, two different genotypes are paired together to determine the predicted outcome of their cross (what would happen if they were to have babies.) The genotypes for the two parents goes on the outside of the cross and the offspring’s (children’s) genotypes will be calculated in the center four squares. (see figure 5.12)

Figure 5.12

One parent’s genotype is placed on either the top or left hand side of the Punnett square. One letter of each parental genotype is placed over or next to each box. Those letters are then moved down and across into the four boxes. These new genotypes produced in the center boxes are predicted genotypes for the offspring. (see figure 5.13)

Figure 5.13

Using the new offspring genotypes, we can then make a prediction as to what trait or allele the offspring will have. The four boxes represent the whole (100%) of this individual. Because of this, in most Punnett squares each box represents a 25% chance. A video tutorial of how to set up and complete a Punnett square can be seen here.

The 4 part method:

Throughout this lesson, you will be asked to answer Punnett square questions using the the 4 part method. The 4 part method is simple a way to organize a genetics question and to help you read and understand the results of a genetic cross.

The four parts to this method are

1. label the alleles,
2. label the cross,
3. do the Punnett square,
4. list all possible results.

In figure 5.14 you will find an example problem with all four parts labeled. A video tutorial of the 4 part methods can also be viewed here.

Figure 5.14