# Genetics question **URL:** \ **Category:** Factual Questions\ **Created:** [January 14, 2017, 4:47pm UTC](https://boards.straightdope.com/t/genetics-question/777121 "2017-01-14T16:47:35Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![davidmich](https://avatars.discourse-cdn.com/v4/letter/d/e56c9b/32.png) [@davidmich](https://boards.straightdope.com/u/davidmich)\ **Post date:** [January 14, 2017, 4:47pm UTC](https://boards.straightdope.com/t/genetics-question/777121/1 "2017-01-14T16:47:35Z")
Hi I found this exchange between two posters on a blog. I’m just calling them A and B for simplicity. I think both answers are flawed or incomplete. I’m trying to get a better answer. I look forward to your feedback. A: Ok let me put it simply as this: 1 chromosome in your body is comprised of 2 chromatids (1 chromatid from your mom and 1 chromatid from your dad) Since you get one allele from each parent you therefore have 2 alleles for each gene on 1 chromosome. B: I don’t think that’s accurate… 1 chromosome is made of 2 sister chromatids, but both of these chromatids are identical (hence why crossing over in Prophase I occurs between non-sister chromatids). I think wikipedia is right, we all have 1 copy of a gene and 2 alleles. There should only be one allele per chromosome. In this way, since we all have 2 chromosomes (one from dad, one from mom) per gene we should have 2 alleles per gene. The two alleles should be on separate homologous chromosomes… I think B is correct and I actually made a flash card of it to help those that are more visual learners. ---
**Author:** ![Darren\_Garrison](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darren_garrison/32/92_2.png) [@Darren\_Garrison](https://boards.straightdope.com/u/Darren_Garrison)\ **Post date:** [January 14, 2017, 5:10pm UTC](https://boards.straightdope.com/t/genetics-question/777121/2 "2017-01-14T17:10:45Z")
Both are wrong. “Sister chromatids” are what you call the two ([half](https://en.wikipedia.org/wiki/Semiconservative_replication)) new copies of chromosomes during the process of copying but before they separate. It is a moment in time. [Look at the wiki](https://en.wikipedia.org/wiki/Sister_chromatids). ---
**Author:** ![Darren\_Garrison](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darren_garrison/32/92_2.png) [@Darren\_Garrison](https://boards.straightdope.com/u/Darren_Garrison)\ **Post date:** [January 14, 2017, 5:19pm UTC](https://boards.straightdope.com/t/genetics-question/777121/3 "2017-01-14T17:19:11Z")
I see that [this](https://forums.studentdoctor.net/threads/a-chromosome-has-two-alleles.848982/) is the thread. I find it disturbing that the official ADA practice test got the answer wrong. ---
**Author:** ![davidmich](https://avatars.discourse-cdn.com/v4/letter/d/e56c9b/32.png) [@davidmich](https://boards.straightdope.com/u/davidmich)\ **Post date:** [January 15, 2017, 5:09am UTC](https://boards.straightdope.com/t/genetics-question/777121/4 "2017-01-15T05:09:42Z")
> [@Darren\_Garrison](#): > > I see that [this](https://forums.studentdoctor.net/threads/a-chromosome-has-two-alleles.848982/) is the thread. I find it disturbing that the official ADA practice test got the answer wrong. Thanks Darren Garrison. My feeling was correct. I expected a higher standard from the ADA. ---
**Author:** ![Terminus\_Est](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/terminus_est/32/3087_2.png) [@Terminus\_Est](https://boards.straightdope.com/u/Terminus_Est)\ **Post date:** [January 15, 2017, 9:05am UTC](https://boards.straightdope.com/t/genetics-question/777121/5 "2017-01-15T09:05:10Z")
Good God, that was some serious misinformation on that page. At least dentists don’t have to know much about genetics to be able to fix your teeth.