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 Subject: Population genetics and heredity Category: Science > Biology Asked by: melim42-ga List Price: \$20.00 Posted: 04 Apr 2006 08:36 PDT Expires: 04 May 2006 08:36 PDT Question ID: 715343
 ```How many generations apart do two "relatives" have to be before they're no more closely related than two people chosen at random from the population? (\$20) Here's the idea: people are related to their children and their own parents by 50% (i.e., they share 1/2 of their genes). What percentage of genes do two randomly chosen humans share, and how many generations does it take to get to that point? I'm looking for an anwer like "5" or "10" or whatever is correct. Thanks! Hope you think this is as interesting a question as I do!```
 ```Hi Melim42-ga - First off, I'd like to say that I do find this question interesting and enjoyed researching it for you. Your question: How many generations apart do two "relatives" have to be before they're no more closely related than two people chosen at random from the population? Your answer is found in an algebraic expression called the "coefficient of relationship." This is " the proportion of genes that are held in common by two individuals as a result of direct or collateral relationship." http://www.genetic-genealogy.co.uk/Toc115570157.html I can't write the formula here because it uses subscripts and superscripts, but the degree of relationship (shared blood) between any two people who are descended from the same person decreases by double between generations. For siblings, as you rightly say, the coefficient is .5; for first cousins it is .25; for second cousins .125. By the 5th generation it is .03125; by the 7th it is .0076125. It is never 0, but gets close. Please refer to the table at: http://www.genetic-genealogy.co.uk/Toc115570135.html [scroll to the very bottom of the page]. When we think of the general population, we infer a coefficient of relationship of 0. That may be. But chances are that in the mist of time, a common ancestor or ancestors exist for any two people as noted in one of the comments below. As to your question about the percentage of genes that any two people share, I'd say it's quite high. After all, there are 20,000-25,000 genes in the human genome. They are not all different. We pay attention primarily to the genes whose expression we can see (the phenotype) like hair, eyes, length of nose, skin color, etc. But there are many more genes in the (hidden) genotype. These are the genes that govern metabolic functions or bone formation or most of what goes on inside of us. When a mutation occurs in one of these workhouse genes, disease often strikes. http://www.ornl.gov/sci/techresources/Human_Genome/project/info.shtml Estimates have been made of DNA elements (base pairs) that make up the genes that reside on the human chromosome. There are 50 - 250 million base pairs on each of the 24 human chromosomes. Thus far scientists have identified about 1.4 million locations on various chromosomes that show a single-base difference. It's a vast and fascinating subject. I hope I've helped you come to an understanding of some of the mysteries of life. Here are some other websites that may be of interest Formula for coefficient of relationship http://www.genetic-genealogy.co.uk/Toc115570135.html [scroll 1/4 way down the page; it starts with Rxy] The Human Genome Project http://www.ornl.gov/sci/techresources/Human_Genome/home.shtml Genotype and Phenotype Definition http://www.brooklyn.cuny.edu/bc/ahp/BioInfo/GP/Definition.html Genetic andQuantitative Aspects of Genealogy http://www.genetic-genealogy.co.uk/Toc115570157.html Birgid Schlindwein's Hyperglossary of Genetic Terms http://hal.wzw.tum.de/genglos/asp/genreq.asp?nr=631 All the best, Alanna``` Request for Answer Clarification by melim42-ga on 12 Apr 2006 08:18 PDT ```I was looking for a number of generations, like mikewa-ga's answer. Do you agree with him, and why or why not? I would also like to know if my own calculations are similar in concept to mikewa-ga's, if he will respond again. From my calcuations, I'd say 6 to 8 is a good estimate. I get 5 to 9 generations, assuming all of the "variable" genes are inherited independently and ignoring new mutations -- bad assumptions, I know. Assume 25,000 genes, 99% of which any two humans have in common. A relative can, therefore, share (or not) 250 genes with the index individual. After 9 generations, the index individual's "relatives" will have (on average) 250*(.5^9)<1 gene out of the original 250. Assume 25,000 genes, 99.9% in common. After 5 generations, the number of common genes is 25*(.5^5)<1.``` Clarification of Answer by alanna-ga on 13 Apr 2006 16:52 PDT ```I don't disagree with mikewa-ga's comment. As a Google Answers researcher, I am charged with giving you the answer plus the background information and web sites that will back up my information. If you felt you had an answer from the comments section, you did have the option of withdrawing the question. I gave you the link to a table showing the coefficient of relationship for 13 generations. You can choose the point on that table where you feel the relationship is small enough to be close to considered "random." Remember, it approaches but never is 0. I believe that mikewa-ga is referring to nucleotide bases (the components of each gene). His range hits the estimate of the Human Genome Project, which is 99.9% nucleotide bases. Those 99.9% string out on the chromosome in various combinations to form the 30,000 genes of the human genome and a lot of junk DNA. Nonetheless I would also state that 6-7 generations are sufficient for any two descendents to be considered "non-related." I'd like to add that because 99.9% of the nucleotide bases are the same in all of us, that is not to say that 99.9% of the genes are the same. A lot of the bases (at last 50%) are "dead space" or junk DNA. Here are some numbers from the Human Genome Project: " * The human genome contains 3164.7 million chemical nucleotide bases (A, C, T, and G). * The average gene consists of 3000 bases, but sizes vary greatly, with the largest known human gene being dystrophin at 2.4 million bases. * The total number of genes is estimated at 30,000 ?much lower than previous estimates of 80,000 to 140,000 that had been based on extrapolations from gene-rich areas as opposed to a composite of gene-rich and gene-poor areas. * Almost all (99.9%) nucleotide bases are exactly the same in all people. * The functions are unknown for over 50% of discovered genes. " * Less than 2% of the genome codes for proteins. * Repeated sequences that do not code for proteins ("junk DNA") make up at least 50% of the human genome. * Repetitive sequences are thought to have no direct functions, but they shed light on chromosome structure and dynamics. Over time, these repeats reshape the genome by rearranging it, creating entirely new genes, and modifying and reshuffling existing genes. * During the past 50 million years, a dramatic decrease seems to have occurred in the rate of accumulation of repeats in the human genome." http://www.ornl.gov/sci/techresources/Human_Genome/project/info.shtml```
 ```It depends entirely on your assumptions about the amount of inbreeding. Until quite recently, most humans lived in rather small communities with extensive inbreeding. If you go back 40 generations (~1000 years) you would have one trillion ancestors alive at that time if it were not for inbreeding. Obviously that makes no sense.```
 ```Estimates of the similarity of DNA between two randomly chosen people vary depending on what is being counted,, but usually are in the 99-99.9% range. Since each generation halves the variation attributed to a specific ancestor then just 6-8 generations should be enough to make it statistically similar to the general population. there are enough variables in this to provide lots of argument, but this should be in the right ballpark.```