The Smelly Genetics of Love

People May Prefer Partners with Different MHC Genes
October 8, 2008 You had me at hello. Love at first sight. A new study suggests that these sayings may need to be updated. You had me once I could smell you. Or love at first sniff. In this study, researchers found that how someone smells may play a big role in falling in love. People aren't looking for perfumes, though (sorry L'Oreal). Instead the idea is that they smell potential mates to get information about their MHC genes. What they're looking for are people who have different MHC genes from themselves. These people apparently smell better than folks with similar MHC genes. And couples with different MHC genes will be more likely to have children with stronger immune systems.
Odor and MHC Genes
Scientists have known for a while that other animals like mice, rats, fish and lizards prefer mates that have a certain smell. And that the certain smell partly comes from the other animal's MHC genes. Animals prefer a potential mate that has different MHC genes from themselves. It's been less clear whether humans do the same. Some studies show people do and some studies show people don't. Famous sweaty T-shirt studies have shown that women like the smell of men with different MHCs than their own. Basically scientists have men wear a shirt for awhile. Then women smell a bunch of shirts and say which ones they like best. They usually pick the shirts from men with different MHC genes. One idea about why people look for mates with different MHC genes has to do with the future health of their children. People with varied MHC genes have strong immune systems. So children of couples with different MHC genes will be more likely to have stronger immune systems. And be more likely to survive. The new study directly tested this idea by looking at the DNA of 30 couples from Utah and 30 couples from Nigeria. What they found was that the couples from Utah tended to have different MHC genes from each other. This fits with the idea that having different MHC genes is an important part of picking a mate. The Nigerian couples had more similar MHC genes to each other. This is inconsistent with the idea but there are some possible reasons for this. Perhaps social factors play a bigger role than genes in Nigeria. Or maybe having too different of MHC genes is not ideal either. The Nigerians already have much different MHC genes from each other compared to the couples from Utah. So instead of choosing mates with extreme differences in MHC genes, they chose more similar mates. These studies, when combined with others done in the past, begin to give us a handle on how humans find their mates. It also shows us how much our DNA is involved in our behaviors. And how much we can learn from looking at our DNA. MHC Genes are an Important Part of the Immune System Like any gene, each MHC gene has the instructions for making a protein. In this case, they have the instructions for making an MHC protein. MHC proteins play an important role in the immune system. They are the proteins that tell a body which cells are infected and need to be killed. They tag cells for destruction. MHCs take small bits of protein (or peptides) from the inside of a cell and display them on the outside of the cell. This is a cell's way of showing whether it is healthy or not. Immune cells, called T cells, roam around the body and check out the MHCs on display. In healthy cells the MHCs show peptides that the T cells recognize. This means that the cell is healthy and the T cell moves on. But unhealthy cells are a different matter. For example, if a cell is infected by a virus, the MHCs will display peptides from the virus. This signals that the cell is sick. When a T cell sees the viral peptides and doesn't recognize them, it kills the cell and the virus inside. This only happens if the MHC can actually display the virus bits to the outside of the cell. If it can't, then the T-cell will ignore the infected cell. And the infection will spread. MHCs can't display every peptide. In fact, each MHC binds to only a set of peptide sequences. This means that people with many different MHCs can bind more peptides. This is important because different germs create different peptides. So the more peptides a cell can recognize, the more germs an immune system can fight. This is why having many different versions of the nine MHC genes is so important. Each different version can produce different MHCs. So the more varied a person's MHC genes, the more varied are the viruses and bacteria he or she can recognize. And kill. Remember, people have two copies of their MHC genes -- one from their mom and one from their dad. So people with the most diverse MHC genes will actually have 18 different versions. The researchers hypothesized that people with different MHC genes would be more attracted to each other. To test this idea, researchers didn't have study participants smell t-shirts. Instead, the researchers looked directly at their MHC genes. To do this, they used something called the HapMap.

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Smell may play a big
role in why couples
fall for each other.
The top cell can display
the viral peptide and
so will be destroyed
The HapMap Project
The HapMap project was designed to find lots of differences between people at the DNA level. They were specifically looking for small changes in DNA letters. Human DNA is made up of only four different letters -- A, G, C, and T. Each human has around 6 billion of these letters arranged in a certain order. All of these letters together are a person's genome. Most of the letters or DNA sequence is the same from person to person. About every 1,200 letters there is a difference. Each one of these spots is called a Single Nucleotide Polymorphism, or SNP. The HapMap project looked at 270 different people to find SNPs all over their genomes. As of 2007, they have found over 3 million significant SNPs. The main reason researchers started this project was to create a tool for scientists to find genes involved in disease. This has been very successful so far. Scientists can also use this data to tell how different two people are at the genetic level. To do this, scientists compare one person's SNPs to another person's. Similar people will have similar letters at their SNPs. Different people will have different letters at their SNPs. The same thing can be done to see how similar parts of the genome are. This is what the MHC geneticists did. They took married couples from HapMap and looked at their SNPs including the 9,010 SNPs in the MHC genes. Specifically, they looked at 30 European American married couples from Utah and 30 African married couples from the Yoruba population in Nigeria. They found that the Utah couples had dissimilar MHCs. But the Nigerian couples had no preference for very different MHC genes. MHC Genes are not the Whole Story The Utah data suggests that sometimes mates choose each other based on having dissimilar MHCs. This is strengthened because the Utah couples' DNA were more similar almost everywhere else. The MHC region was one of the few to actually be significantly different. In other words, the Utah couples didn't simply choose each other because they were different everywhere. The data suggests that they chose each other because their MHCs were specifically different. There are many possible reasons for the Nigerian couple results. One of course is that the model is wrong. People do not choose each other based on differences in their MHC genes. Another possibility is that sometimes social factors trump biological ones. For example arranged marriages might match couples together who have less MHC diversity. Another possibility is that having too diverse of MHCs doesn't result in the healthiest child. In other words, maybe having too different of MHC genes is not good. Perhaps there is an optimal amount of MHC diversity. It turns out that the Nigerian group had more MHC diversity to begin with than the Utah group. So maybe the Nigerian mates chose similar MHCs to not have too much diversity. Whereas, the Utah group chose mates with different MHCs to increase their diversity. Another T-shirt study had similar results. This time the T-shirts came from men of different ethnicity than the women. These women could have chosen from very dissimilar MHCs but they didn't. They preferred men with more similar MHCs than the extremely different ones. This study shows that at least in some human populations MHCs play a role in mate choice. Obviously more research is needed. It will be interesting to learn how important MHC diversity is. And what other factors can trump it.
The DNA of each
couple was sequenced
and compared to find
out how similar their
MHC genes were.