SNPwatch:Shuffling SNPs Have Opposite Effect in Men and Women

SNPwatch gives you the latest news about research linking various traits and conditions to individual genetic variations. These studies are exciting because they offer a glimpse into how genetics may affect our bodies and health; but in most cases, more work is needed before this research can provide information of value to individuals. For that reason, it is important to remember that the studies we describe in SNPwatch are for informational and educational purposes only. SNPwatch is not intended to be a substitute for professional medical advice; you should always seek the advice of your physician or other appropriate healthcare professional with any questions you may have regarding diagnosis, cure, treatment or prevention of any disease or other medical condition.

A report released today in the online edition of Science magazine demonstrates that two SNPs influence the amount of chromosome shuffling that takes place during the production of sperm and eggs. Surprisingly, the versions of the SNPs that increase the rate of shuffling in men actually decrease it in women, and vice versa.

Geneticists use the term “recombination” to describe the shuffling of chromosomes that takes place when gametes are made. Recombination helps to generate human diversity by mixing up a mother’s and father’s chromosomes, respectively, during the production of egg and sperm cells. The rate of recombination determines how much of this chromosomal shuffling happens.

Previous studies have shown that the amount of recombination varies between people and that this variability is at least partially heritable. The mechanism controlling variation in recombination rates, however, remains unknown.

Augustine Kong and co-workers conducted a genome-wide search in more than 6000 people of European ancestry to find variants that correlated with recombination rate. This rate was determined by looking at the chromosomes of both subjects and their children.

The researchers found that men who had at least one G at and at least one A at had higher rates of recombination. In women, the opposite was true: at least one A at and at least one G at was associated with high rates of recombination.

The report’s authors suggest that the opposite effect of these SNPs in men compared to women may help to keep the pace of change in the genome relatively constant. By balancing high rates of recombination in one sex with low rates in the opposite sex, nature may have found a way to maintain stability in the human genome and ensure the evolutionary success of the species.

Both SNPs identified in this study are located in a gene called RNF212. Scientists don’t yet know much about this gene. They do know that RNF212 is similar to genes important for recombination that are found in yeast and small worms called C. elegans.

Recombination is an important concept in genetics, but it can be hard to visualize. Read on for an illustrated example that will make things clearer.

meiosis.pngIn this example of meiosis, the process by which egg and sperm are made, we’re only going to show two pairs of chromosomes (a pair of long ones and a pair of short ones) instead of the usual 23 pairs for humans. Let’s say that we’re talking about what happened in your mother’s body when she produced eggs (the exact same process applies to dad’s sperm). We’ll designate the orange chromosomes as the ones contributed to your mom by your maternal grandma, and the purple chromosomes as those from your maternal grandpa.

Step 1: Each chromosome is copied, creating doublets that stick together.

Step 2: The pairs of doublets line up across from each other and swap random chunks of DNA. This is recombination. The recombination rate determines how many swaps take place

Step 3: The pairs of doublets separate and the cell divides.

Step 4: The doublets separate and the cells divide again. The cell that began this process with pairs of chromosomes has now been turned into 4 gametes, each containing one of each type of chromosome (a long one and a short one in this example). These cells become eggs.

Step 5: If one of these eggs is fertilized by a sperm cell (which was produced in the same way as the egg cells), the resulting cell will get one set of 23 chromosomes from the egg and the sperm, respectively, resulting in a cell that has 23 pairs of chromosomes, just like the adult cell we began with. This cell will then go on to become an embryo and eventually a child.