|MadSci Network: Genetics|
It is possible to generate mathematical models in population genetics, and hundreds of researchers have created thousands of models for all types of population genetics questions. Dr. Dorak's Basic Poplulation Genetics Page contains some formulas and links to many more.
All mathematical models of this type require us to simplify the real world to get rid of most of the messy details, and simplify the problem. For example the simplest model would assume that there was random mixing of the population, that the average family size is 2 parents plus some number k children per family, that 50% of children are female, etc. And the model might not include a factor for people migrating into the population from external populations.
In reality, we can only be sure of the numbers you are interested in if we sample everyone in England and find out how many of them are from populations that have been living outside of England for tens of thousands of years, such as Australian aboriginals, natives of South America, Africans, etc. We can only make rough estimates of the date of the most recenta apparent common ancestor of all humans, and then make estimates of average time span per generation to compute the value for all 6 billion people on Earth today.
There are many other useful methods for learning about relatedness of populations of people (or any mammal, or plants or any organism). A good book that describes some DNA sequence analysis methods and how they have been applied to several interesting topics is: _Tears of the Cheetah_ by Stephen J. Gould.
Try the links in the MadSci Library for more information on Genetics.