There are a number of common misconceptions when it comes to genetics, heredity, determinant traits, and genetic testing just to name a few. Below you will find a list of some of the most common misconceptions in the field of genetics. Explore each link to learn more.
- Genes are the sole determinants of traits
- Single genes code for most traits
- Dominant traits are the most common traits in a population
- The limiting factor to getting genetic information is the speed and/or cost of genome sequencing
- All mutations are harmful
- Once a mutation is discovered, it can be “fixed”
- All genetic tests are equally reliable and precise
- Only certain people have “disease genes”
- If a couple has a “one-in-four” risk of having a child with a disease, and their firstborn has the disease, the next three children will have a reduced risk.
- Only genetically modified food crops have genes.
1. Genes are the sole determinants of traits
Genes are typically not the only determinants of traits. Although a few traits, such as blood type, are determined strictly by genetics, most traits are influenced both by genes and the environment in which we live. We do not inherit a disease, instead we inherit susceptibility factors that increase risk for a disease. For example, recent studies suggest 50 to 60 percent of alcoholism risk is genetic (National Institute on Alcohol Abuse and Alcoholism). This means the other 40 to 50 percent of the risk is environmental. A person may be genetically predisposed to alcoholism due to previous family history. However, certain healthy lifestyle choices, like minimizing alcohol intake, will help prevent that individual from becoming dependent on alcohol. The environment can have the same effect on those predisposed to obesity. Obesity has a genetic component caused by the action of multiple different genes. However studies report dramatically different results on the degree of genetic contribution. If obesity runs in a family, but an individual chooses to make healthy eating choices and exercise regularly, he or she can prevent obesity. Genes do not set in stone what traits we will and will not possess. Ultimately, a person making healthy lifestyle choices and educating themselves on which diseases they are genetically predisposed to can help reduce the risk of certain diseases.
2. Single genes code for most traits
Multiple genes, not just a single gene, determine most traits in humans such as height, eye color, and skin color. When a trait is controlled by more than one gene it is called polygenic. Traits that are coded for by multiple genes do not have distinct classes and instead exhibit a range, which is why we see a continuum of height and skin color. Even eye color ranges in shades of brown, blue, green and hazel. Current research has shown that nearly 100 genes could affect skin color (Barsh, 2003)! For example, we all contain genes that control the production of melanin, the pigment most commonly associated with skin color. However, we differ in the amount of melanin produced as well as the type (black, brown or reddish). Many of us learned incorrectly in high school that brown hair is dominant to blond hair. Often this lesson was accompanied by a Punnett square filled with lowercase and uppercase letter b’s. We now understand that inheritance of pigmentation is much more complex and cannot be explained in a simple single-gene Punnett square.
3. Dominant traits are the most common traits in a population.
When people hear the word “dominant”, often they incorrectly believe that the majority of the population expresses this trait. Describing a trait as dominant does not mean it is the most common; it means that it is expressed over the recessive trait. For example, tongue rolling is a dominant trait, controlled by the dominant version of a particular gene (R). Individuals with one or two copies of R will exhibit tongue rolling. Only individuals that have two recessive versions of the gene (r) will lack the ability to tongue roll. But what does this tell us about the relative commonness or rareness of tongue rolling in a population?
How frequently a trait is observed in a population is not related to whether or not it is dominant or recessive. Instead, it is a reflection of how frequently the gene responsible for causing a trait is found in people. For example, polydactyly, the presence of extra fingers and/or toes, can be caused by a dominant mutation. However, polydactyly only occurs in 0.31-6.18 births in 1,000, depending on ethnic background. This means it is very rare for a person to have the gene mutation that causes polydactyly, even though it is dominant.
4. The limiting factor to getting genetic information is the speed and/or cost of genome sequencing
New technology allows the human genome to be sequenced very quickly and efficiently. The human genome project, which started in 1990, took 13 years to sequence the first human genome at a cost of $2.7 billion. Today a human genome can be sequenced in a few days for less than $10,000. Speed and cost is no longer a limiting factor in getting genetic information. Now that we have the information, the biggest challenge is analyzing the genome. Sequence data amounts to a very long string of the letters A,T,G, and C, which correspond to the building blocks of DNA. Scientists that specialize in analyzing this data are known as bioinformaticists. They use computer programs to try and determine which genes code for which traits and how genes can influence one another. Ultimately we hope to understand the role of all of the approximately 22,000 genes in the human genome.
5. All mutations are harmful
Absolutely not! Most mutations that occur in our DNA sequence are changes in single nucleotides that do not cause harm to the individual. Recent research suggests that each of us inherits approximately 60 new mutations that are not present in our parents (Conrad et al., 2011). These mutations account for some of the .1% difference between the genomes of two individuals. Some mutations occur in areas that do not code for genes, which means there are no changes in the individual. When mutations do occur in a gene, they may not result in a significant change in the protein encoded by that gene. Likely the mutation will go unnoticed.
Mutations can even be beneficial to an individual. For example, a mutation might encode for a gene that causes resistance to a disease. In this case, the gene will become “selected for” in a population over time. Individuals in the population who carry the mutation will survive and pass it on to their offspring. Mutations help us adapt as our environment changes over time. An example of a “good” mutation occurs in CCR5, a gene that encodes for a protein found on the surface of certain immune system cells. The HIV virus attaches to CCR5 as a way to enter a host cell, eventually leading to AIDS if left untreated. However, approximately 10% of individuals of European descent have a version of CCR5 that contains a deletion (Martinson et al., 1997). This mutation does not allow the HIV virus to bind properly to a host cell, thus delaying the entry of HIV into the cell and reducing the individual’s risk of developing AIDS.
6. Once a mutation is discovered, it can be “fixed”
Once a mutation has occurred in the genome, it cannot be “fixed”. Because we do not currently have the technology to fix an error in DNA, after identifying a disease causing mutation we must then discover a drug or other intervention that will counteract the malfunctioning gene. An example of this is Huntington’s disease. Huntington’s disease is caused by incorrect repetition of the sequence CAG within the huntingtin gene (Collins, 1999). Even though we can identify what causes Huntington’s disease, there is still no cure.
Scientists are currently working on gene therapy, which is an experimental technique used to treat or prevent diseases. This technique may allow doctors to treat a disorder by inserting a gene into a patient’s cells instead of using drugs or surgery to treat them. Most research has been done on mice, but in time scientists hope to perfect this technique to allow it to be used in treatment of diseases (What is Gene Therapy, 2012).
7. All genetic tests are equally reliable and precise
Even though there are many genetic tests available on the market today, not all are equally reliable and precise. Advances in sequencing technology mean genetic testing has become widespread and easily available, in some cases without the involvement of a medical professional. If you are considering genetic testing, it is important to research what specifically you would like to find out and what test will give you the most accurate results. For example, some tests produce results with near 100% certainty, such as presymptomatic testing for Huntington’s disease. Other tests may give you a percentage risk when stating your predisposition to a certain disease. Therefore, it is important to interpret the results of genetic tests correctly. A genetic test may reveal that you have a 15% chance of getting a certain disease. This result is alarming at first, but upon further investigation you discover the general public has a 13% chance of getting that same disease. This means that although your risk is higher than average, it is not significant enough to make drastic life changes to avoid getting the disease. For example, the average population risk of Alzheimer’s disease is approximately 7%. Certain versions of the APOE gene can double the risk of getting Alzheimer’s disease. However, a doubled risk still only amounts to approximately 14%. Genetic tests that show predispositions to a disease do not reveal with certainty that you will develop a disease in the future, instead they show that you are more likely than average to get the disease. If you decide to pursue genetic testing it is important to go over the results with a doctor, genetic counselor or other health care professional trained in interpreting genetic tests. (Donnelly, 2011).
8. Only certain people have “disease genes”
Genetic diseases are caused by mutations in genes that all of us have. For example, there has been a lot of coverage in the press about the “breast cancer gene”, BRCA1. In fact, all of us (men and women) have the BRCA1 gene. The normal function of BRCA1 is to produce a protein that helps to repair DNA damage. Individuals that have a version of BRCA1 that does not function properly are more likely to accumulate damage in their DNA because it is not getting repaired efficiently. This damage may lead to cancer.
9. If a couple has a “one-in-four” risk of having a child with a disease, and their firstborn has the disease, the next three children will have a reduced risk.
The “one-in-four” risk does not mean that one in every four children that a couple has will have a certain disease. The “one-in-four” risk means that each child has a 25% (one-in-four) chance of having a disease. This can be depicted in a Punnett square, which shows the probability of inheritance. For example, let’s say a man and a woman are each carriers of sickle cell anemia, which would make them heterozygous, possessing one dominant and one recessive allele (Ss). A Punnett square can be created to determine the probability of inheritance that their child will have that disease. Here is what it would look like:
From this Punnett square we can see that this couple’s child has a 25% chance of being homozygous dominant (SS), 50% chance of being heterozygous (Ss), and 25% chance of being homozygous recessive (ss). Since sickle cell anemia is a recessive disease, only the ss offspring will be affected. It is important to note that just because one child expresses the disease does not mean that future children of the same couple will have a decreased risk of inheriting the disease. The probability of inheritance is the same for all children of the couple.
10. Only genetically modified food crops have genes
All food crops, whether genetically modified or not, have genes. Just like humans, plants have DNA, which determines which phenotypes will be expressed based on the genotypes of the plant. In fact, Gregor Mendel, the father of genetics, first discovered how genes function during his study on pea plants! Genetically modified food crops differ from regular food crops because they have genes from other organisms introduced into their genome. Specifically, the most common modifications are genes from bacteria used to make crops herbicide and pesticide resistant. This allows farmers to spray their fields with herbicide to kill weeds without affecting the crops, or to reduce the use of pesticides since the crops are already pest resistant. Therefore, the only genetic difference between GMO crops and conventional crops is the addition of 1 or 2 genes. There has been much debate on the issue surrounding the labeling of foods that contain GMO plant products.