Explaining Genotypes & Phenotypes Big Bang Style

This post is a continuation of a previous post, which introduced the topic of genetics.  Today’s post picks up on a topic that was not fully addressed in the previous post: more specific information on genes and alleles.  I wanted to discuss this topic separately and more in depth as students sometimes have a difficult time grasping this concept, warranting a separate post providing the necessary detail for students. 

As previously discussed, genes, or biological information that codes for specific traits, are passed down from parents to offspring.  Offspring get two sets of genes, one from each parent.  The genetic composition of an organism is known as its genotype, whereas the physical appearance that is derived from this genetic composition is known as the phenotype.  But as genes come in pairs, and there are many different variants of genes, referred to as alleles, how exactly does the body figure out its physical appearance from these genes?  From reading the genes present in the genotype and determining which are dominant and which are recessive.

Figure 1: Penny's Boyfriend

Figure 2: Penny's Neighbors

 Dominant and recessive genes are terms used to identify how much power genes have in expressing themselves.  Dominant genes will always express themselves, whereas recessive alleles will not express themselves in the presence of dominant alleles but will express themselves if no dominant genes are present.  One way to think of these concepts is to think of dominant and recessive alleles like characters from the popular television show, The Big Bang Theory.  Dominant genes are the big jock boyfriends Penny used to date early in the series-they are strong, overpowering, and get Penny’s attention (Figure 1), whereas the recessive genes are Penny’s geeky neighbors-they are quiet, meek, socially awkward, and do not get Penny’s attention (Figure 2).  When Penny’s boyfriend and neighbor meet in the show the jock boyfriend (dominant gene) controls and overpowers the geeky neighbors (recessive genes) (Figure 3).  This same concept applies when a dominant and recessive gene end up in a gene pair together, except that genes do not beat each other up as the characters in the show do.  The dominant gene is the one that we phenotypically see in the overall appearance of the organism.  In real life, an example of this would be if a dominant brown eye gene and a recessive blue eye gene are paired together in an organism.  In this case, the organism will phenotypically express the brown eye gene despite carrying a blue eye gene, too.

Figure 3: Penny's Dominant Boyfriend Overtaking the Recessive Neighbor, thereby demonstrating what occurs in a heterozygous genotype.

This scenario that I just described, where a dominant and recessive gene is together in a gene pair, is called a heterozygous genotype.  Hetero is a Greek root that means different (e.g. heterosexual) and signifies when two genes of different strengths (one dominant and one recessive) are grouped together.  But what happens if two dominant genes or two recessive alleles are grouped together?  This situation is referred to as a homozygous genotype.  Homo is a Greek root that means same (e.g. homosexual) and signifies two genes of the same strength being grouped together (e.g. two dominant genes or two recessive genes) (Figure 4).  Phenotypically (appearance wise), this situation will show both genes-be it dominant or recessive-if the genes are the same.  When we identify these situations, we refer to individuals as being homozygous dominant (having two of the same dominant genes) or homozygous recessive (having two of the same recessive genes).  

Figure 4: Homozygous traits demonstrate the same phenotypic traits as is demonstrated by all the incarnations of Penny from the Big Bang Theory.

In some cases, however, if there are two different dominant alleles, both may equally show itself, a situation known as codominance (Figure 5).  In real life, this happens with the blood type AB, with the A and B genes both being dominant and both showing equal strength in expression.  No such situation exists for two recessive alleles because if two different recessive genes come together then one will take on a dominant role (e.g. if a blue and green eye genes are together in a gene pair, one will be phenotypically expressed).  These genes can only phenotypically being expressed when an organism is homozygous recessive (e.g. two recessive blue eye genes will produce a blue eyed organism).

Figure 5: Codominance as demonstrated by Penny and Sheldon of The Big Bang Theory.  Both characters are dressed as Wonder Woman and both are equally shown in this depiction.