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Chromosomes that experienced an odd number of crossovers contain an even number of segments (half originating from the grandmother, the other half from the grandfather) but chromosomes with an even number of crossovers (or zero!) have an odd number of segments so on these chromosomes you must receive one more segment from one grandparent than the other. That means that the DNA we inherit from our maternal grandparents typically comes in about 64 segments, but it won’t necessarily be 32 segments from each maternal grandparent. So, on the set of 22 chromosomes received from our mothers there will have been an average of 42 crossover locations where there was a switch between DNA she inherited from one parent to the other. The previous article on crossover statistics revealed that there are an average of about 42 crossovers in female meiosis and about 27 in male meiosis. This is precisely what I have done in simulations run on my GAT-C model.īefore looking at the simulation results let’s anticipate what we expect to see. From these simulations we can measure the amount of variation that is expected to be observed in the proportions of DNA inherited from our ancestors.

in their paper titled “Escape from crossover interference increases with maternal age”) we can use this information as a basis for simulations that accurately mimic the crossover process. With the availability these days of abundant real data from direct-to-consumer genetic testing companies (such as the 23andMe data utilised by Campbell et. The statistics relevant to the recombination process were discussed in detail in a previous article (Crossovers: Frequency and Inheritance Statistics – Male Versus Female Matters).

The level of variation is driven by the number (and location) of crossover events that occur when the ova and the sperm cells are created. These figures vary because our parents didn’t necessarily pass on to us equal portions of the DNA that they received from their parents. For instance, we receive an average of one quarter of our DNA from each of our four grandparents and an average of one-eighth of our DNA from each of our eight great-grandparents etc. Beyond parent-child relationships we tend to talk about averages. chromosomes) provided by our mothers in the ova and the 22 segments/chromosomes provided by our fathers in the sperm cell. This is delivered to us in the form of the 22 segments (i.e. When we consider the DNA that we inherit from our ancestors the only quantity that we can be certain of is that we receive half of our autosomal DNA from each parent. Philip has answers for us as a result of his simulations. How does this work, and does it make a difference whether we inherit our grandparent’s DNA through males or females? It’s the total of the DNA of both grandparents that adds up to 50% for the couple. We get more or less than exactly 25% from each person in a grandparent pair. We know that on average, we’re going to inherit 25% of our DNA from each grandparent – but we also know in reality that’s not what happens. Philip Gammon, our statistician friend has been working with crossover simulations again in order to tell us what we might expect relative to how much DNA we actually inherit from grandparents and great-grandparents.