# Biologist/Geneticists: Why aren't sibs identical? **URL:** \ **Category:** Factual Questions\ **Created:** [February 26, 2007, 12:28am UTC](https://boards.straightdope.com/t/biologist-geneticists-why-arent-sibs-identical/393657 "2007-02-26T00:28:51Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![WordMan](https://avatars.discourse-cdn.com/v4/letter/w/e8c25b/32.png) [@WordMan](https://boards.straightdope.com/u/WordMan)\ **Post date:** [February 26, 2007, 12:28am UTC](https://boards.straightdope.com/t/biologist-geneticists-why-arent-sibs-identical/393657/1 "2007-02-26T00:28:51Z")
I think my question is straightforward to understand, I hope. If a female and male of a species mate and have kids, what keeps any progeny from being identical? I base this on the assumptions that: a) the DNA contributed by the female is, well, identical - meaning that a female parent has DNA that defines her design and that DNA is consistently represented in each of her eggs. b) the DNA contributed by the male is, well, identical - same as above, except that the male’s DNA is consistently represented in his sperm. c) DNA splits and recombines based on consistent rules - so if the female’s and male’s DNA split and re-combine together in progeny in a consistent manner, you will always end up with DNA location #1 of the female combining with location #1 of the male, and so on down the billions of juntion points along the chain. So - is one (or all) of these assumptions off? Is the DNA “deposited” into eggs and/or sperm variable, or do recombinations occur inconsistently? Or is there some other reason progeny vary so radically? Thanks! ---
**Author:** ![zelie\_zelerton](https://avatars.discourse-cdn.com/v4/letter/z/59ef9b/32.png) [@zelie\_zelerton](https://boards.straightdope.com/u/zelie_zelerton)\ **Post date:** [February 26, 2007, 12:34am UTC](https://boards.straightdope.com/t/biologist-geneticists-why-arent-sibs-identical/393657/2 "2007-02-26T00:34:23Z")
I’m not any kind of geneticist so don’t bother reading my post if you want an answer. Just here to try to clarify my understanding of the OP. As I understand it you are saying that every ova has the same genetic material as every other ova? Ditto for the male so why not get a bunch of offspring which are the same as each other? If that is what you are saying I can see some major flaws right off but I’m sure I’m not reading you right. Could you clarify for me please. ---
**Author:** ![ultrafilter](https://avatars.discourse-cdn.com/v4/letter/u/3d9bf3/32.png) [@ultrafilter](https://boards.straightdope.com/u/ultrafilter)\ **Post date:** [February 26, 2007, 12:35am UTC](https://boards.straightdope.com/t/biologist-geneticists-why-arent-sibs-identical/393657/3 "2007-02-26T00:35:57Z")
If a) and b) were both true, we’d all be the same sex. The process by which DNA is shuffled up for sex cells is known as meiosis, and while the [Wikipedia article](http://en.wikipedia.org/wiki/Meiosis) is pretty thorough, there are probably better introductions for a non-biologist. ---
**Author:** ![WordMan](https://avatars.discourse-cdn.com/v4/letter/w/e8c25b/32.png) [@WordMan](https://boards.straightdope.com/u/WordMan)\ **Post date:** [February 26, 2007, 12:58am UTC](https://boards.straightdope.com/t/biologist-geneticists-why-arent-sibs-identical/393657/4 "2007-02-26T00:58:25Z")
\*\*zelie \*\*- yep, that is what I am getting at; I know my assumptions are flawed, but I am looking for a layperson’s explanation as to how. \*\*ultrafilter \*\*- who knew you knew genetics the way you know music?! Too cool - lemme read that wiki entry and see if I get it… but I am sure I’d appreciate some scientist Dopers giving me their take… ---
**Author:** ![WordMan](https://avatars.discourse-cdn.com/v4/letter/w/e8c25b/32.png) [@WordMan](https://boards.straightdope.com/u/WordMan)\ **Post date:** [February 26, 2007, 1:01am UTC](https://boards.straightdope.com/t/biologist-geneticists-why-arent-sibs-identical/393657/5 "2007-02-26T01:01:25Z")
Okay - this portion of the Wiki article appears to get at the heart of what I am asking: > [@](#): > > During meiosis, the genome of a diploid germ cell, which is composed of long segments of DNA called chromosomes, undergoes DNA replication followed by two rounds of division, resulting in haploid cells called gametes. Each gamete contains one complete set of chromosomes, or half of the genetic content of the original cell. These resultant haploid cells can fuse with other haploid cells of the opposite gender or mating type during fertilization to create a new diploid cell, or zygote. Thus, the division mechanism of meiosis is a reciprocal process to the joining of two genomes that occurs at fertilization. Because the chromosomes of each parent undergo genetic recombination during meiosis, each gamete, and thus each zygote, will have a unique genetic blueprint encoded in its DNA. In other words, meiosis is the process that produces genetic variation. I notice is mentions “two rounds of division” and specifically references the genetic variation I am asking about - but can someone put this in plainspeak for me? Thanks! ---
**Author:** ![chrisk](https://avatars.discourse-cdn.com/v4/letter/c/6de8d8/32.png) [@chrisk](https://boards.straightdope.com/u/chrisk)\ **Post date:** [February 26, 2007, 1:07am UTC](https://boards.straightdope.com/t/biologist-geneticists-why-arent-sibs-identical/393657/6 "2007-02-26T01:07:55Z")
I think the key point, or a key point, is this: Most chromosomes, in which our DNA is stored, have two versions of every segment of an organism’s genetic code. This is NOT the two halves of the DNA double helix (because from one half you can always work out the other.) They’re two different DNA strands ‘in parallel’ - like the left and right side of the book containing recipes with similar structure. A lot of the time they’ll be exactly the same, that’s why members of a species are mostly put together the same way. But there are a few variations. Eggs and sperm are constructed with only one version of each chromosome - and also with only one allosome - these are the genetic structures that contain sex-linked information, (the X and Y, where XX makes a female and XY a male.) These are not quite like the chromosomes, in that they’re not of the same length, but otherwise function the same way. Half of each of a mother’s chromosomes go into each egg, but it’s not the same halves every time. Same goes for sperm. Since sperms might contain an X allosome or a Y allosome, the result if they combine with an X egg might be a male or a female. Also, there are hundreds of millions of possible combinations from a particular father and mother if you consider all the chromosomes in a human cell. Does that help?? ---
**Author:** ![zelie\_zelerton](https://avatars.discourse-cdn.com/v4/letter/z/59ef9b/32.png) [@zelie\_zelerton](https://boards.straightdope.com/u/zelie_zelerton)\ **Post date:** [February 26, 2007, 1:40am UTC](https://boards.straightdope.com/t/biologist-geneticists-why-arent-sibs-identical/393657/7 "2007-02-26T01:40:55Z")
[QUOTE=WordMan] \*\*zelie \*\*- yep, that is what I am getting at; I know my assumptions are flawed, but I am looking for a layperson’s explanation as to how. [/QUOTE] Oh in which case I can answer after all. 😃 The genetic material provided by each parent is \*not \* the same in every zygote. Hence you don’t get identical ‘twins’ of different ages. Consider it at a very basic level. The male can contribute either an X or a Y chromosome creating either a female or a male child. Non identical. It’s the same for the rest of the DNA. Sorry I don’t know any good links to explain in more depth but I think a highschool level biology/science book would be a useful place to start. Oh and I see \*\*Chrisk \*\* explained it far better than I did. 🙂 ---
**Author:** ![Qadgop\_the\_Mercotan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/qadgop_the_mercotan/32/83_2.png) [@Qadgop\_the\_Mercotan](https://boards.straightdope.com/u/Qadgop_the_Mercotan)\ **Post date:** [February 26, 2007, 1:44am UTC](https://boards.straightdope.com/t/biologist-geneticists-why-arent-sibs-identical/393657/8 "2007-02-26T01:44:59Z")
Well, the human genone may contain as many as 100,000 gene pairs, with up to 25,000 of them responsible for coding proteins directly. Each pair is made up of two separate genes, which are not all going to be exactly the same as their partner gene, even if they both code for the same thing, such as brown eyes. Thru the miracle of meiosis, the pairs are split, at random, and shipped out, in a sperm or ovum, awaiting their destiny. No way each split is going to be identical, with 100,000 pairs to be divided. I believe the odds of achieving that are 2 to the power of 100,000 (tho I may be wrong on that particular calculation. It’s still long odds). ---
**Author:** ![Polycarp](https://avatars.discourse-cdn.com/v4/letter/p/82dd89/32.png) [@Polycarp](https://boards.straightdope.com/u/Polycarp)\ **Post date:** [February 26, 2007, 1:47am UTC](https://boards.straightdope.com/t/biologist-geneticists-why-arent-sibs-identical/393657/9 "2007-02-26T01:47:28Z")
Each parent has two genes, which may or may not be identical, at each gene site on each chromosome. During meiosis, one of those two enters the “germ cell” (ovum or sperm) for each site. While there is a certain amount of groups of genes sticking together, it’s easy to see that a massive number of possible combinations are possible from any two parents. Quick example: Father: Bb eyecolor (brown with blue gene recessive), AO blood (type A with O recessive), RR (Rh positive, homozygous), WW (homozygous for ear-wiggling muscles being functional – and I don’t _know_ this is a real characteristic, but it’s typical), MM (homozygous for no sickle-cell gene), Kk (some other characteristic, maybe hirsuteness)… Mother: BB (homozygous for brown eyes), OO (homozygous for type O blood), Rr (heterozygous for Rh factor), Ww, MM, Kk… Kids: BB or Bb, AO or OO, RR or Rr, WW or Ww, MM, and KK, Kk, or kk, depending on which gene they inherited from each parent. Extend this for thousands upon thousands of gene pairs, and you see the unlikelihood of identicalness unless both parents are homozygous at every site (improbable to a very high degree). ---
**Author:** ![kimera](https://avatars.discourse-cdn.com/v4/letter/k/f0a364/32.png) [@kimera](https://boards.straightdope.com/u/kimera)\ **Post date:** [February 26, 2007, 3:40am UTC](https://boards.straightdope.com/t/biologist-geneticists-why-arent-sibs-identical/393657/10 "2007-02-26T03:40:15Z")
Here’s how I explain it to little kids. Let’s day you have a cell that has a diploid number of 4. This means that 2 chromosomes came from mommy and 2 came from daddy. The haploid number for this cell is 2 (haploid is half diploid). m1, m2, d1, d2 For each chromosome of mommy’s there is a corresponding chromosome of daddy’s. This means that they are friends. When meiosis occurs, the friends come together. m1d1, m2d2. and in their excitement double (invite their identical twins) m1m1d1d1, m2m2d2d2 All 4 new friends hang out together in what is called a tetrad. Since the haploid number is 2, there are two groups of friends. The 4 of them sit together and exchange information back and forth. This is called crossing over. (with kids I say that they are exchanging articles of clothing so they swap a shirt for a shirt. or a pair of pants for pants) After they finish sharing information, they split up into teams to play a game. The teams are picked randomly, in a process called independent assortment. m1m1 and d2d2 can be on a team together. Or m1m1 and m2m2 can be on a team together. They just make sure (or try to anyway) to keep the haploid number so that there are 2 groups. These teams are still too big for the game they are trying to play so they break in half again. This time, the twins break up. That means that m1m1 split up with a m1 in each resulting team. There are 2 people remaining on each team for 4 teams total. Each team has a 1 person and a 2 person. These teams are what are called gamates which are sperm and eggs. An organism can produce 2^n combinations, where n represents the haploid number. Our number is 2^23. This doesn’t even consider crossing over. You are very unique indeed. ---
**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:** [February 26, 2007, 3:51am UTC](https://boards.straightdope.com/t/biologist-geneticists-why-arent-sibs-identical/393657/11 "2007-02-26T03:51:29Z")
What hasn’t been mentioned yet I think (unless I overlooked it somewhere in someone’s post) is that not only does each parent have two different copies of each chromosome which are independently shuffled into sperm and eggs, the genetic material on each chromosome gets shuffled with that of the matching one through a phenomenon called crossing over. The two chromosomes line up during meiosis, then reciprocal breaks take place, and material that was originally on one chromosome gets swapped with that on the other. Imagine that the father has two paired chromosomes, with a different allele for each gene on each one: These are the two chromosomes before crossing over: ABCDEFGHIJ abcdefghij These are the two chromosomes after crossing over: abCFefgHIJ ABcfEFGhij The same thing happens with each chromosome from the mother. The Y chromosome is the only one that does not undergo crossing over, because it has no reciprocal pair. (X-chromosomes can cross over in females.) Since this happens independently for each sperm and egg that are created, a nearly infinite number of different combinations are possible. ---
**Author:** ![Kimstu](https://avatars.discourse-cdn.com/v4/letter/k/ecd19e/32.png) [@Kimstu](https://boards.straightdope.com/u/Kimstu)\ **Post date:** [February 26, 2007, 3:57am UTC](https://boards.straightdope.com/t/biologist-geneticists-why-arent-sibs-identical/393657/12 "2007-02-26T03:57:54Z")
> [@QtM](#): > > No way each split is going to be identical, with 100,000 pairs to be divided. I believe the odds of achieving that are 2 to the power of 100,000 (tho I may be wrong on that particular calculation. It’s still long odds). Hmmm. This tells us why non-twin siblings are generally **not** genetically identical, and we wouldn’t expect them to be. But have any sibling pairs beaten those odds? Is there any recorded instance of non-twin siblings who **did** turn out to be genetically identical or nearly so? ---
**Author:** ![nivlac](https://avatars.discourse-cdn.com/v4/letter/n/3bc359/32.png) [@nivlac](https://boards.straightdope.com/u/nivlac)\ **Post date:** [February 26, 2007, 5:14am UTC](https://boards.straightdope.com/t/biologist-geneticists-why-arent-sibs-identical/393657/13 "2007-02-26T05:14:07Z")
Good question, but one that has been answered on these boards before. Tried to locate the link, but couldn’t. Anyhow, this answer in [howstuffworks](http://science.howstuffworks.com/question92.htm) gives a pretty good explanation. ---
**Author:** ![WordMan](https://avatars.discourse-cdn.com/v4/letter/w/e8c25b/32.png) [@WordMan](https://boards.straightdope.com/u/WordMan)\ **Post date:** [February 26, 2007, 2:08pm UTC](https://boards.straightdope.com/t/biologist-geneticists-why-arent-sibs-identical/393657/14 "2007-02-26T14:08:46Z")
All good and helpful - thank you, science Dopers, for fighting my ignorance. I think, for me, the key concepts that are opening my eyes are the concepts of “crossing over” (thanks, \*\*Kimera \*\*and **Colibri** ) and the “random split” that can happen during meiosis (thanks, **QtM** ). I can see where those forces introduce much randomness into progeny. Fascinating. ---
**Author:** ![Hypno-Toad](https://avatars.discourse-cdn.com/v4/letter/h/b2d939/32.png) [@Hypno-Toad](https://boards.straightdope.com/u/Hypno-Toad)\ **Post date:** [February 26, 2007, 3:13pm UTC](https://boards.straightdope.com/t/biologist-geneticists-why-arent-sibs-identical/393657/15 "2007-02-26T15:13:50Z")
[This](http://boards.straightdope.com/sdmb/showthread.php?t=404405) is the question I asked about identical sperm. ---
**Author:** ![WordMan](https://avatars.discourse-cdn.com/v4/letter/w/e8c25b/32.png) [@WordMan](https://boards.straightdope.com/u/WordMan)\ **Post date:** [February 26, 2007, 4:20pm UTC](https://boards.straightdope.com/t/biologist-geneticists-why-arent-sibs-identical/393657/16 "2007-02-26T16:20:02Z")
[QUOTE=Hypno-Toad] [This](http://boards.straightdope.com/sdmb/showthread.php?t=404405) is the question I asked about identical sperm. [/QUOTE] Ah - also very helpful. Thanks! ---
**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:** [February 26, 2007, 4:56pm UTC](https://boards.straightdope.com/t/biologist-geneticists-why-arent-sibs-identical/393657/17 "2007-02-26T16:56:19Z")
[QUOTE=Qadgop the Mercotan] Well, the human genone may contain as many as 100,000 gene pairs, with up to 25,000 of them responsible for coding proteins directly. Each pair is made up of two separate genes, which are not all going to be exactly the same as their partner gene, even if they both code for the same thing, such as brown eyes. Thru the miracle of meiosis, the pairs are split, at random, and shipped out, in a sperm or ovum, awaiting their destiny. No way each split is going to be identical, with 100,000 pairs to be divided. I believe the odds of achieving that are 2 to the power of 100,000 (tho I may be wrong on that particular calculation. It’s still long odds). [/QUOTE] Not quite “at random”. It’s not like the alleles all split up and get shuffled, like individual cards in a deck. The chromosomes literally cross over at certain points, and whole chunks get shuffled-- more like “cutting” a deck of cards. So the closer two alleles are located physically on the chromosome, the more likely they will stay together during the shuffling. I’m sure that it wasn’t an accident that in **Colibri** ’s example the letters are not randomly mixed, but mixed in groups. The wikipedia article on [chromosomal crossover](http://en.wikipedia.org/wiki/Chromosomal_crossover) has a good explanation. ---
**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:** [February 26, 2007, 5:54pm UTC](https://boards.straightdope.com/t/biologist-geneticists-why-arent-sibs-identical/393657/18 "2007-02-26T17:54:50Z")
[QUOTE=John Mace] I’m sure that it wasn’t an accident that in **Colibri** ’s example the letters are not randomly mixed, but mixed in groups. [/QUOTE] Yes, that’s exactly what I was trying to indicate in the example. On re-reading the thread, I see **kimera** also mentioned crossing-over. It should perhaps be mentioned that although independent segregation of chromosomes in meiosis and crossing over will shuffle the parental genes, this won’t make any difference if the alleles for a single gene are the same on both chromosomes. If both parents have blue eyes, then both have the allele for blue eyes on both chromosomes, and all offspring will have blue eyes. Many genes are essentially monotypic, and only have one allele that occurs at any significant frequency. In a single population, especially one that is inbred, many individuals will have genes that are identical by descent. Therefore the actual number of potential combinations will be far less than if there really were four different parental alleles available at a single genetic locus. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [February 26, 2007, 6:30pm UTC](https://boards.straightdope.com/t/biologist-geneticists-why-arent-sibs-identical/393657/19 "2007-02-26T18:30:27Z")
> [@](#): > > (haploid is half diploid) Bit of a hijack, but this terminology has always bothered me. Haploid should logically be one quarter of diploid. To be consistent, we should have called those chromosone counts “monoploid” and “diploid”. Or possibly “haploid” and “monoploid”, but that would leave us without a good term for triploids and the like (such as many domesticated plants). On the subject of chromosones crossing over, how common is it? Does the typical human have a chromosone which is the result of a crossover between the parents’ chromosones? Does each chromosone in an individual have a crossover from the previous generation? Ten thousand crossovers? What’s the rate? I usually neglect crossovers in discussions of genetics, for the sake of simplicity, and 2[sup]23[/sup] is a plenty big enough number. But is it a good approximation? ---
**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:** [February 26, 2007, 6:44pm UTC](https://boards.straightdope.com/t/biologist-geneticists-why-arent-sibs-identical/393657/20 "2007-02-26T18:44:48Z")
[QUOTE=Chronos] On the subject of chromosones crossing over, how common is it? Does the typical human have a chromosone which is the result of a crossover between the parents’ chromosones? Does each chromosone in an individual have a crossover from the previous generation? Ten thousand crossovers? What’s the rate? I usually neglect crossovers in discussions of genetics, for the sake of simplicity, and 2[sup]23[/sup] is a plenty big enough number. But is it a good approximation? [/QUOTE] Each chromosome normally has several crossovers during meiosis (but nowhere near 10,000). Offhand, I would say it is generally greater than 1 but less than 10. Almost all chromosomes, except the Y, will have crossovers from previous generations. See the article on recombination frequency [here.](http://en.wikipedia.org/wiki/Recombination_frequency)