Getting Into Your Genes: Explaining Mendelian Genetics & Punnett Squares

Genetics is the study of the mechanisms of inheritance, which broadly encompasses all sorts of topics.  Mendelian Genetics simply refers to the mechanism of inheritance between parent and offspring.  Tthe focus of today’s blog post: exploring and explaining Mendelian Genetics and (what many students loathe, although I love) Punnett Squares. 

Genetics is the study of the mechanisms of inheritance, but what exactly does that mean?  It means that people who study genetics are concerned with how organisms become what they are.  You may have heard it over and over again, but each individual is made up a series of DNA and genes.  DNA, which is short for the tongue twitching term deoxyribonucleic acid, is simply a molecule that carries our genetic code.  Our genetic code, or blue print of what we are supposed to look like and function (i.e. carnivores, herbivores, human, animal, plant, etc.), is coded for via our genes, which make up the building blocks of who and what we are.  You can think of DNA and genes in layman’s terms in the following way:

Figure 1: DNA is the blueprint and Genes are the pieces.  Image Source: Schroeder

The DNA is the instruction manual, while the genes are the pieces for that the instruction manual.  It is sort of like a model airplane or car or IKEA furniture (and just as frustrating sometimes) (Figure 1).  Depending on what type of DNA (instruction manual) you have, you may have a model for an animal or a plant, and more specifically a fish vs a goat or a rose vs poison ivy.  The genes are the pieces of the organism that dictate individual variations in the organism.  Let us take a rose for an example: the genes (or in simple terms, building blocks) dictate the type of fragrance, color, size, number of petals, etc. of the rose.   

Now Mendelian Genetics refers to mechanisms of inheritance between parent and offspring, and the term was coined by its discoverer, Gregor Mendel.  Now Gregor Mendel was not some award winning scientist.  He was simply a monk who had an interest in science and who spent his spare time gardening.  Through gardening and harvesting peas he began to notice that the plants had different colors of flowers (purple vs. white) and different seed coats (smooth vs. wrinkled).   He was inspired by this observation to breed the peas in his garden.  He soon discovered that the specific colors and seed coats (both different traits) he observed were passed down differently from parent to offspring, and these specific traits, which we now know as genes, were passed down through parent to offspring based on what the parents had available to them in their genetic code.  In other words, whatever genes the parents have are what will be passed down to the offspring, meaning a parent can only pass down what they have and not anything they do not have.  

Now each individual carries two sets of genes for each trait that they have, and the reason for this is because a male and female parent must contribute genes to produce offspring, which is from the female parent providing the egg and the male parent providing the sperm.  The sperm and egg cells (referred to in biological terms as gametes) carry copies of the parent’s genes that will be passed to the offspring, and the union of the sperm and egg produce a whole organism or an offspring.  Please see Genotypes vs. Phenotypes for more information regarding expression and genetic composition.

Now because of the pioneering work of Gregor Mendel we can predict, based on the physical appearance and genetic composition of parents, what the offspring will look like, and this is done through a process referred to as Punnett Squares.  Punnett Squares are used to determine potential inheritance of offspring of two parents, and basically it takes the genetic composition of each parent and demonstrates the potential genetic composition outcomes of the offspring.  It does this by demonstrating a 4 x 4 probability, thereby producing a 25% chance per square, of what the offspring could look like. 

Figure 2: Genes/Jeans of Parents (Image Source: Shutterstock)

To demonstrate, let us go back to the beginning and think about what happens to produce offspring.  The male and the female each have their own set of genes, which are represented as pairs of blue jeans in Figure 2.  I chose this analogy because you have to remember that each individual has two genes per trait, and blue jeans have two legs.  The male and the female meet and they decide (knowingly or unknowingly) to reproduce.  To produce the offspring their bodies have produced their corresponding gametes (males produce sperm and females produce eggs).  Each of these gametes contains copies of one gene from those pairs of genes from each parent.  But since both parents carry two genes per traits, how do we know which gene is passed on to the offspring?  Through a Punnett Square analysis!

We know that each parent can only supply one gene to their offspring, so we can think of the gametes (sperm or egg) as being one pant leg, each representing one of the genes contained in the pair for each trait.  But we still do not know which pant leg the offspring will get.  So what we do is we put each pant leg in the Punnett Square as shown in the Figure 3:

Figure 3: Punnett Square 1.0.  Note: Males are typically at the top of the Punnett Square, whereas females are at the side, which is how it is presented here and why there is a blue vs red line (for male and female, respectively). This is not standard and you can invert the male and female placement.

Each of the mini squares (of which there are four in total) represents a 25% chance of what the offspring will look like because each parent can potentially give each offspring one of the two genes each parent carries, leading to a possibility of four outcomes because each parent can only give one gene to their offspring.  Now when Punnett Squares are done for professional purposes one does not actually use jeans to represent genes.  Instead, genes are represented by letters, so an actual Punnett Square would look like the following (Figure 4):

Figure 4: Punnett Square 2.0

Figure 4: Punnett Square 2.0.  For an explanation of the letters please go to the Genotypes vs. Phenotypes post.

And that, ladies and gentlemen, is how we predict what potentially the offspring will look like.  Now it is important to remember a couple of things about Punnett Squares: They only demonstrate the probability of what your offspring could look like for an individual trait.  They cannot predict specifically how exactly your offspring will look like.  As noted in the previous post, Brief Review of Genetics, a few factors are at play when it comes to physical expression of traits, specifically those that are environmental in nature.  But Punnett Squares are useful in predicting what your offspring may look like or what genes they may have. 

References:

Feder, Kenneth and Park M. Human Antiquity: An Introduction to Physical Anthropology and Archeology, McGraw-Hill.