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Genetics and Inheritance:Why owning a pair of genes matter

Author: Hayden Huang
Editor: Guan-Jong Chen
SCICOM MIT

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Introduction:
If both of your parents have dimples (small dents in the cheek, which become prominent when you smile), do you think you ll have dimples or not? In general, you are likely, but not guaranteed, to have dimples. This is because having dimples is a dominant trait. Genetics is the study of how certain features are transferred from generation to generation. Not all characteristics are passed through inheritance. For example, if you have black hair but dye your hair green, that doesn t mean your children will have green hair.

How it works:
Your body is made up of individual living units called cells. These cells almost always contain DNA (deoxyribonucleic acid, but don t worry about the name for now). Your DNA contains genes, which are instruction manuals for building proteins that are used for building your body and keeping it running correctly. There are many proteins necessary for basic life thought to number over 20,000 basic proteins from these genes. Your DNA contains instructions for making all these protein, and when stretched out, human DNA is about 1 meter (3 feet) long. Each person carries two versions of the genes for each protein, which can be both the same (no dimples and no dimples) or different (dimples and no dimples). Additionally, not all proteins are exactly the same from person to person, which explains why people don t all look alike. One exception to this rule is identical siblings, such as identical twins, who share identical DNA, and thus likely have nearly identical proteins and generally look alike (and sometimes, act alike as well).

Since each person carries two versions of the same gene, a natural question is to ask what happens when the two versions have different results, such as someone having both dimples and non-dimples versions. In some cases, there is a stronger version of the gene that wins. To see how this might happen, picture a glass of water. If you add a spoonful of black ink to it, the water will become dark. Adding a second spoonful of ink won t make it much darker. Thus, a glass of water with nothing added will be clear, while a glass of water with either one or two spoons of ink will be dark, in which case ink would be a dominant protein. Similarly, dimples are the result of a protein that helps the muscles that cause dimples to appear. Having one gene that expresses this is enough to get this effect.

When geneticists talk about these traits, they use the terms dominant and recessive to signify which gene is expressed in the person. Dominant traits means that if you have one or two copies of the gene, that gene will be expressed (like the gene for dimples). Recessive traits are those that require both genes to be the same in order for the genetic trait to show up (like not having dimples).

When a cell divides, the DNA is duplicated and both new cells receive identical copies of the DNA. Thus, the two new cells will be genetically identical to the old cell. However, when two people conceive a child, the child will inherit only one copy from each parent. Thus, the child will have two copies, like each parent, but the combination will be different. The exact combination of genes the child has will determine what the child looks like, among other things.

Why this is important:
Why are geneticists (and other people) interested in genetics and inheritance? Not only is it interesting to see what traits are carried over from generation to generation, but many diseases are linked to genetics, as opposed to being caused by bacteria or viruses. As a result, it s important to determine which conditions might be linked to genetic factors. Usual clues include the condition being limited to families. Dominant traits and recessive traits have different patterns dominant traits will generally show up in each generation, in half or more of the children, while recessive traits tend to skip generations, and only show up in a quarter of the children.

An example of a genetic condition is sickle-cell anemia, a condition where a red blood cell protein called hemoglobin is made differently from most people. This change can cause the red blood cells to change shape under certain conditions from a round doughnut shape to an elongated, more crescent shape. Interestingly while such conditions can sometimes disappear over time, sickle-cell tends to persist. One popular explanation is that while having both genes for this condition can lead to disease, having only one gene for this condition (with the other gene being the non-sickling variant) does not lead to significant anemia (making this a recessive condition) and leads to resistance against malaria. Because in many regions of the world where sickle-cell is prevalent, malaria is also present, the gene for this protein confers enough protection for the population as a whole and as a result, the anemia levels do not drop.

Things to keep in mind:
What was presented here is a slightly simplified version of DNA, proteins and inheritance. For example, radiation can cause mutations in the DNA. When your DNA is copied, it sometimes makes errors. While your cells have mechanisms to repair this, these mechanisms are not perfect. Other things happen that can cause changes in DNA, but in general, DNA is remarkably consistent.

Some traits are controlled by multiple genes. While dark hair and eyes tend to be dominant, they are not controlled by a single gene, thus it s not always predictable what color eyes and hair your child will have simply by looking at the hair color of the parents. This is also reflected in the large number of hair color variation (black, blond, brown, red, all with different shades).

Finally, much work still needs to be done, particularly in determining whether certain types of behavior and personality might be genetically influenced. While some people are convinced that soft traits such as humor, aggressiveness, etc. are influenced by genetics, solid evidence is lacking. Further, the degree to which these traits might be determined by inheritance is not known at all.

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