by Andrew Dombrowski
Genetics is core to biology, because it is the field that describes how traits are inherited from parents to offspring and how those traits are expressed. For that reason, a solid understanding of genetics provides a good foundation for PCAT biology in general. However, when studying genetics, it is easy to get confused by what sometimes seems like a whirlwind of similar-sounding terms.
In this blog post, we’ll walk through some of the most commonly confused terms in genetics, starting at the core level of what happens in the cell and then broadening our scope to talk about inheritance patterns.
1. One common point of confusion occurs in meiosis. Students often wonder what the difference is between sister chromatids and homologous chromosomes. To simplify this, let’s step back and look at the chromosomes that are present in standard diploid cells in your body (that is, before meiosis takes place in germ cells).
For each somatic chromosome, you have one copy from your mother and one copy from your father. Let’s say we’re talking about chromosome 14, to pick a random example: we can conceive of these two copies as 14maternal and 14paternal. These are homologous chromosomes, because they contain the same basic genes and structure, but may not be completely identical (that is, one might have a mutation). When a copy is made of either 14maternal or 14paternal, the result is a pair of identical sister chromatids (14maternal and 14maternal or 14paternal and 14paternal).
2. Another point of confusion on the cellular level has to do with how the information contained in DNA is used to build proteins. Transcription occurs when an mRNA strand complementary to the template strand of a DNA sequence is synthesized. Translation occurs when ribosomes process mRNA to form proteins with the help of tRNA.
The way to keep these terms straight is to think of transcription as a way of transferring a message within the same ‘language’ of nucleic acids, much like how a scribe in a hospital will transcribe a patient’s history from verbal words to the electronic medical record. You’re going from one format to another, but you’re keeping the message in the same ‘language.’ In contrast, translation is going from one language to another—in this case, from the language of nucleic acids (mRNA) to that of proteins (amino acids).
3. On the level of the organism, confusion often arises between the terms genotype and phenotype. Genotype refers to the information contained within an organism’s DNA, for example, whether a certain mutation or variant of a gene is present. In contrast, the term phenotype refers to the physical manifestation of the genotype. A common misconception is to think of the phenotype as describing the organism’s appearance. While this is true for many examples used in biology textbooks (like Mendel’s pea plants), many important phenotypes are invisible, as in metabolic/signaling disorders like familial hypercholesterolemia (an autosomal recessive condition that leads to extremely high cholesterol levels and early-onset cardiovascular disease).
4. The terms gene and allele can also be challenging to keep track of. The term ‘gene’ refers to the area of DNA that codes for a given protein (or set of proteins, given the possibilities enabled by alternate splicing), while ‘allele’ describes a specific variant. To take a familiar example from Mendel’s pea plants, the gene for flower color will have alleles for purple and white.
5. Part of the reason why Mendel’s pea plants are often used as examples to teach genetics is because they involve simple genotypes that always result in a predictable phenotype. Reality is not always so simple, and the terms penetrance and expressivity describe ways in which there is not always a perfect one-to-one mapping between genotype and phenotype. Penetrance describes the percentage of individuals with a given genotype who manifest the phenotype. For classic examples like those in Mendel’s pea plants, the penetrance is 100%, but in reality, incomplete penetrance can take place. An example is an inherited disorder known as osteogenesis imperfecta (OI), in which some individuals with the disease-causing alleles do not manifest the disease, for reasons that remain imperfectly understood. Expressivity, in contrast, describes the degree to which a phenotype is manifested. An example is feline polydactyly (or cats with extra toes), which is clustered in various groups of cats, including some in Key West, Florida – Hemingway cats are quite well known for this. Some cats affected by this condition will have more extra toes than others due to the interaction of the genotype with complex regulatory and signaling processes in development. This is a textbook example of varying expressivity.
We hope this helps, and encourage you to check out the various PCAT resources that Next Step offers, including our PCAT practice tests (one of which is free!), and our PCAT tutoring. We wish you the best of luck in your journey towards pharmacy school!
We wish you the best of luck!