# How do sperm and eggs create ‘random’ changes? **URL:** \ **Category:** Factual Questions\ **Created:** [April 27, 2007, 1:53pm UTC](https://boards.straightdope.com/t/how-do-sperm-and-eggs-create-random-changes/401860 "2007-04-27T13:53:26Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![FlippyFly](https://avatars.discourse-cdn.com/v4/letter/f/e95f7d/32.png) [@FlippyFly](https://boards.straightdope.com/u/FlippyFly)\ **Post date:** [April 27, 2007, 1:53pm UTC](https://boards.straightdope.com/t/how-do-sperm-and-eggs-create-random-changes/401860/1 "2007-04-27T13:53:26Z")
How do sperm and eggs create ‘random’ changes? To my knowledge, most cells create exact copies of themselves, and if they don’t, they either die or become cancerous. How then does the body create millions and millions of sperm with not a few but many random changes in genes that effect height, weight, hair color, preference for twinkies; all of the things that make me different from my brother? By what mechanism does the DNA change the parts for these things and not the parts that tell my cells how to make a liver or a red blood cell? ---
**Author:** ![wevets](https://avatars.discourse-cdn.com/v4/letter/w/e9a140/32.png) [@wevets](https://boards.straightdope.com/u/wevets)\ **Post date:** [April 27, 2007, 2:18pm UTC](https://boards.straightdope.com/t/how-do-sperm-and-eggs-create-random-changes/401860/2 "2007-04-27T14:18:00Z")
Cells don’t always create exact copies of themselves. Although copying DNA is very accurate (only a few mistakes per million base pairs), it does introduce random mutations. On top of that, meiosis - which creates the germ cells, sperm and eggs - creates opportunities for crossing over and exchange of genetic material between chromosomes. This allows DNA to be in new combinations that did not exist in the parent cells. Another source of genetic change in the process is the allocation of chromosomes. Since each parent has two copies of all 23 chromosomes in their body, and the question becomes which of the two copies of the chromosome does each sperm or egg get? The number of possible combinations is very high. Many genetic effects are not caused by a single gene (in which a base pair mutation would be just about the only way to change the gene). Instead, they are often the result of the action of many genes (for example, if I recall correctly, five to seven different genes play a role in the determination of skin color). So the combination of genes becomes very important, and can be altered compared to a sibling by the recombination of partts of chromosomes and the assortment of chromosomes in the gametes, and the expression of genes is also a potential source of difference between siblings. There might also be significant differences between you and your brother’s blood or liver cells (are you the same blood type, for example? You don’t have to be), but they are not going to be as easily noticed as the difference in hair, skin, and other external features. ---
**Author:** ![groman](https://avatars.discourse-cdn.com/v4/letter/g/73ab20/32.png) [@groman](https://boards.straightdope.com/u/groman)\ **Post date:** [April 27, 2007, 3:25pm UTC](https://boards.straightdope.com/t/how-do-sperm-and-eggs-create-random-changes/401860/3 "2007-04-27T15:25:56Z")
All the random changes between you and your brother would be mutations. The non-random changes you mention are achieved through you getting 50% of your mom’s DNA and 50% of your dad’s DNA – your brother get’s a different 50% than you do. The way I understand it every human haploid cell (sperm or egg) carries a unique half of the parent DNA. ---
**Author:** ![Jake4](https://avatars.discourse-cdn.com/v4/letter/j/bc8723/32.png) [@Jake4](https://boards.straightdope.com/u/Jake4)\ **Post date:** [April 28, 2007, 1:07am UTC](https://boards.straightdope.com/t/how-do-sperm-and-eggs-create-random-changes/401860/4 "2007-04-28T01:07:22Z")
[QUOTE=groman] All the random changes between you and your brother would be mutations. The non-random changes you mention are achieved through you getting 50% of your mom’s DNA and 50% of your dad’s DNA – your brother get’s a different 50% than you do. The way I understand it every human haploid cell (sperm or egg) carries a unique half of the parent DNA. [/QUOTE] Really?? My bio-fu is very weak, but I thought that each haploid had a full complement of DNA. Then, if multiple combinations of sperm and egg are made (as in fraternal twins), during the combination each set of the two haploids will take a different 50% each time, resulting in the fraternal twins having different characteristics. ---
**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:** [April 28, 2007, 1:25am UTC](https://boards.straightdope.com/t/how-do-sperm-and-eggs-create-random-changes/401860/5 "2007-04-28T01:25:43Z")
I’m not sure I understand the question. Are you asking about mutations (actually changes in the base pairs) or just shuffling the base pairs around? If the latter, then the answer is [chromosomal crossover](http://en.wikipedia.org/wiki/Chromosomal_crossover). > [@](#): > > Chromosomal crossover refers to the process by which two chromosomes, paired up during prophase 1 of meiosis, exchange some portion of their DNA. Crossing over is specifically initiated in zygotene, before the synaptonemal complex develops, and is not completed until near the end of prophase 1. Crossover usually occurs when matching regions on matching chromosomes break and then reconnect to the other chromosome. The result of this process is an exchange of genes, called genetic recombination. ---
**Author:** ![Nametag](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/nametag/32/406_2.png) [@Nametag](https://boards.straightdope.com/u/Nametag)\ **Post date:** [April 28, 2007, 3:52am UTC](https://boards.straightdope.com/t/how-do-sperm-and-eggs-create-random-changes/401860/6 "2007-04-28T03:52:58Z")
Oy, we’re getting far afield. The OP’s question can be more clearly phrased thus: “how does reproduction create diversity?” The answer: **wevets** had it – “Another source of genetic change in the process is the allocation of chromosomes. Since each parent has two copies of all 23 chromosomes in their body, and the question becomes which of the two copies of the chromosome does each sperm or egg get? The number of possible combinations is very high.” IIRC, that’s 2[sup]23[/sup] possible combinations, making identical gametes less likely than you’d think – about 1 in 8.4 million. There are about half a billion sperm in a single ejaculation, but ova are produced in smaller quantities – about 400 in a woman’s lifetime, many more in some other species, but never, I think, approaching much likelihood of producing identical ova. Add in the mutations and such, and you’ve got some diversity goin’ on. But not all species use sexual reproduction. This produces identical offspring, and the only source of gene mixing is lateral gene transfer, such as bacteria do with plasmids. However, reproducing asexually is usually faster and more fecund than sexually, and the number of generations leads to a mutation rate that keeps the evolution rolling along.