# 99.9% genetically the same? **URL:** \ **Category:** Factual Questions\ **Created:** [May 7, 2003, 6:12pm UTC](https://boards.straightdope.com/t/99-9-genetically-the-same/173719 "2003-05-07T18:12:54Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![John\_Mace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_mace/32/185_2.png) [@John\_Mace](https://boards.straightdope.com/u/John_Mace)\ **Post date:** [May 7, 2003, 6:12pm UTC](https://boards.straightdope.com/t/99-9-genetically-the-same/173719/1 "2003-05-07T18:12:54Z")
What does this actually mean? I often see this about all humans alive today. But if there are only 30,000 genes in the human genome (latest estimate), that leaves only 30 genes left as the difference between any two random people. I’m not talking about averages or whatever-- even if this were double, we’d still only be talking about 60 genes. Is that enough to account for the physical variations we see even in people of our same ethnic group? Doesn’t seem intuitively enough to explain the differences. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [May 7, 2003, 6:48pm UTC](https://boards.straightdope.com/t/99-9-genetically-the-same/173719/2 "2003-05-07T18:48:01Z")
It refers to the percentage of similarity of base-pairs within the DNA. Remember that each gene is made up of many thousands of base pairs, and that a large percentage of DNA is also made up of sections that do not code for genes at all (so-called “junk” DNA). Actually, most of the differences between people are in this junk DNA. Since it doesn’t code for genes, mutations there are not removed by selection, and so these difference can accumulate much more easily than in coding DNA. ---
**Author:** ![Goodbeem](https://avatars.discourse-cdn.com/v4/letter/g/958977/32.png) [@Goodbeem](https://boards.straightdope.com/u/Goodbeem)\ **Post date:** [May 7, 2003, 7:04pm UTC](https://boards.straightdope.com/t/99-9-genetically-the-same/173719/3 "2003-05-07T19:04:19Z")
So, **Colibri** it is even less than 99.9 %? If I understand you correctly, most of the 0.1 % is in the junk DNA and this leaves us with even less genes than the estimate in the OP to account for the variation between humans? ---
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [May 7, 2003, 7:12pm UTC](https://boards.straightdope.com/t/99-9-genetically-the-same/173719/4 "2003-05-07T19:12:25Z")
Hell, we have at least [95% of the same DNA as chimpanzees](http://www.apologeticspress.org/docsdis/2002/dc-02-07.htm)! ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [May 7, 2003, 7:32pm UTC](https://boards.straightdope.com/t/99-9-genetically-the-same/173719/5 "2003-05-07T19:32:15Z")
> [@](#): > > \*Originally posted by Goodbeem \* > \*\*So, **Colibri** it is even less than 99.9 %? > If I understand you correctly, most of the 0.1 % is in the junk DNA and this leaves us with even less genes than the estimate in the OP to account for the variation between humans? \*\* I don’t know what the source of the figures quoted in the OP is, and these figures tend to be tossed around rather carelessly in the popular literature, so I’m not sure what the actual figure might be. Some sources may give a percent similarity of all base-pairs including junk DNA; some may give the figure for coding regions only. This accounts for much of the variation in percentage similarity between chimps and humans that one sees quoted in the literature. However, similarity in base-pair sequence does not account for all genetic variation. Sections of the DNA can be duplicated, reversed, or transferred to a different chromosome, and these can have major effects. However, it is true that all humans are very similar genetically, regardless of what the actual percentage might be. ---
**Author:** ![John\_Mace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_mace/32/185_2.png) [@John\_Mace](https://boards.straightdope.com/u/John_Mace)\ **Post date:** [May 7, 2003, 7:34pm UTC](https://boards.straightdope.com/t/99-9-genetically-the-same/173719/6 "2003-05-07T19:34:42Z")
Yeah, the 95% similarity with chimps has been around for a long time. You used to hear 98.5%, but it seems the lower number has cropped up more recently. And there’s somehting like 90% similarity w/ mice, even 30% similarity w/ plants. You can read about this in [What It Means to Be 98 Percent Chimpanzee](http://www.amazon.com/exec/obidos/tg/detail/-/0520226151/qid=1052336292/sr=1-4/ref=sr_1_4/103-6354426-6222251?v=glance&s=books). Anyway, Goodbeem has hit on what I was really trying to get at. I forget how many base pairs there are-- on the order of a billion? Is there a way to translate this into how many genes we’re talking about, or is that just not possible to answer. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [May 7, 2003, 9:00pm UTC](https://boards.straightdope.com/t/99-9-genetically-the-same/173719/7 "2003-05-07T21:00:21Z")
> [@](#): > > \*Originally posted by CookingWithGas \* > \*\*Hell, we have at least [95% of the same DNA as chimpanzees](http://www.apologeticspress.org/docsdis/2002/dc-02-07.htm)! \*\* Note that this is a Creationist site. One problem in looking some of this stuff up is that some Creationists have decided it is a big deal whether humans and chimps are 95 vs 99% similar. The actual percentage will vary according to how you figure it. What is significant is not the percentage, but that chimps are much more similar to humans than they are to other animals. > [@](#): > > _originally posted by John Mace_ > **Yeah, the 95% similarity with chimps has been around for a long time. You used to hear 98.5%, but it seems the lower number has cropped up more recently. And there’s somehting like 90% similarity w/ mice, even 30% similarity w/ plants. You can read about this in What It Means to Be 98 Percent Chimpanzee.** As I said, the lower estimate is based on a different way of calculating the difference, not on new information on sequence divergence. > [@](#): > > _originally posted by John Mace_ > **Anyway, Goodbeem has hit on what I was really trying to get at. I forget how many base pairs there are-- on the order of a billion? Is there a way to translate this into how many genes we’re talking about, or is that just not possible to answer.** There are estimated to be about 3 billion base pairs in the human genome, and perhaps 40,000 genes (estimates vary). Only about 1-1.5% of the genome codes for genes. [This site](http://www.nature.com/cgi-taf/DynaPage.taf?file=/ng/journal/v21/n1s/full/ng0199supp_56.html) has some rather technical information on the subject. Variabilty is on the order of less than 1 base-pair per thousand bases, but an average gene consists of about 3,000 base pairs. Therefore a 0.1% variation in base will yield about 3 variants per gene. However, not all variants are significant - some will code for the same amino acid in a protein (known as “synonymous” variants); and not all amino acid substitutions will affect the resultant protein. From the site: > [@](#): > > They found that humans have low sequence diversity, less than 1 variant per kb, and that it varies with functional constraint: noncoding changes ( = 0.00039) are more frequent than coding changes ( = 0.00026); and coding changes are quite heterogeneous, as changes at synonymous sites ( = 0.0005) or at degenerate sites ( = 0.0011) are more prevalent than at nonsynonymous sites ( = 0.0001) or nondegenerate sites ( = 0.0003). A typical human cDNA is about 3 kb, with approximately 1 kb of synonymous sites and 2 kb of nonsynonymous sites, and expected gene diversities of 33% and 25% at synonymous and nonsynonymous sites, respectively. At the protein level, synonymous changes go undetected and only a fraction (25% of the total or 38% of nonsynonymous sites) of the remainder would lead to charge differences, giving an expected protein heterozygosity rate of 38% x 25%, or 9.4% — similar to the value of 10% observed by Harris. The gist of this is that a variation of less than 0.1% in the base-pair sequence translates into 10% of the proteins produced by the genes being different. What the functional result of this might be is difficult to predict. However, a small percentage difference in DNA can lead to a much greater percentage difference in gene function. ---
**Author:** ![John\_Mace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_mace/32/185_2.png) [@John\_Mace](https://boards.straightdope.com/u/John_Mace)\ **Post date:** [May 7, 2003, 10:46pm UTC](https://boards.straightdope.com/t/99-9-genetically-the-same/173719/8 "2003-05-07T22:46:53Z")
Colibri: Good info. Thanks. It left me wondering if the there is a very blurry line between two organisms said to have two “differnet genes” and two organisms having “the same gene with some variations at some base pair sites”. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [May 7, 2003, 11:38pm UTC](https://boards.straightdope.com/t/99-9-genetically-the-same/173719/9 "2003-05-07T23:38:44Z")
> [@](#): > > \*Originally posted by John Mace \* > \*\*Colibri: Good info. Thanks. It left me wondering if the there is a very blurry line between two organisms said to have two “differnet genes” and two organisms having “the same gene with some variations at some base pair sites”. \*\* I think part of the problem here lies in the fact that many people misuse “gene” when what they mean is “allele,” as in the “gene for blue eyes.” What we are really talking about is the gene for eye-color (actually, one of several genes that determine eye-color), which has alleles (variants) that produce blue or brown eyes. Basically, if a DNA sequence produces similar products (enzymes or structural proteins) or has similar regulatory effects in different organisms it can be considered to be the “same” gene, even if the actual products are different. E.g. humans and chimps share most of the same genes, but they differ in details. We share some genes with insects, (for basic things such as respiratory enzymes, etc.), although these will differ even more. However, many genetic sequences in humans and insects have diverged so much that they now have very different functions, and could be considered different genes. [This site](http://www.genomicglossaries.com/content/gene_def.asp) discusses gene definitions at great length. ---
**Author:** ![nth](https://avatars.discourse-cdn.com/v4/letter/n/58956e/32.png) [@nth](https://boards.straightdope.com/u/nth)\ **Post date:** [May 8, 2003, 12:09am UTC](https://boards.straightdope.com/t/99-9-genetically-the-same/173719/10 "2003-05-08T00:09:24Z")
hrm twins can look a lot different! they are supposedly 100% dna identical. i am talking about identical twins from same zygote/egg/sperm/or whatever you call it. i have met like more than 10 twins in last 15years. none of them are exact looking nevermind personalities. 1 set of twin don’t even look like each other, but they did look like each other when they were little. different diets and behavior change their looks when they grew up. i am no scientist, so don’t know what the explanation is, but another set of twins have different eye color. 😛 one blue, the other haze. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [May 8, 2003, 12:21am UTC](https://boards.straightdope.com/t/99-9-genetically-the-same/173719/11 "2003-05-08T00:21:22Z")
> [@](#): > > \*Originally posted by nth \* > \*\*hrm twins can look a lot different! > they are supposedly 100% dna identical. > > i am talking about identical twins from same zygote/egg/sperm/or whatever you call it.\*\* They are 100% identical genetically. The differences are produced during development. The expression of a genotype can be strongly influenced by environment. Genetically identical individuals will never be identical physically (unless you were able to ensure that they developed in exactly identical environments - which is not possible.) ---
**Author:** ![John\_Mace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_mace/32/185_2.png) [@John\_Mace](https://boards.straightdope.com/u/John_Mace)\ **Post date:** [May 8, 2003, 12:55am UTC](https://boards.straightdope.com/t/99-9-genetically-the-same/173719/12 "2003-05-08T00:55:08Z")
nth: You may be confusing fraternal and identical twins (wrt eye color). And I don’t know any geneticist who would claim that “personality” is genetically determined. Influenced, maybe, but not determined.