Download Common Misconceptions in Genetics

Common Misconceptions
http://knowgenetics.org/common_misconceptions/
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 huntington 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. 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.
8. 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.
9. 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.