# Biology/Evolution Question **URL:** \ **Category:** Factual Questions\ **Created:** [January 3, 2004, 10:53pm UTC](https://boards.straightdope.com/t/biology-evolution-question/222536 "2004-01-03T22:53:36Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![Mnem](https://avatars.discourse-cdn.com/v4/letter/m/7bcc69/32.png) [@Mnem](https://boards.straightdope.com/u/Mnem)\ **Post date:** [January 3, 2004, 10:53pm UTC](https://boards.straightdope.com/t/biology-evolution-question/222536/1 "2004-01-03T22:53:36Z")
I was wondering if it is possible for recessive traits such as blue eyes and blonde hair to, over time, completely disappear and be replaced with dominant traits, like brown eyes. Today, it is very easy for different populations to mix, so is it not possible for these recessive traits to completely disappear and be replaced with the more dominant traits over many, many years? And if it is not possible, how do recessive traits keep on surviving? Any help to my question is appreciated. ---
**Author:** ![scm1001](https://avatars.discourse-cdn.com/v4/letter/s/c4cdca/32.png) [@scm1001](https://boards.straightdope.com/u/scm1001)\ **Post date:** [January 3, 2004, 11:04pm UTC](https://boards.straightdope.com/t/biology-evolution-question/222536/2 "2004-01-03T23:04:13Z")
recessive only means that it does not appear in the phenotype (what you look or behave like) when mixed with a dominant, not that it is not useful or convey some evolutionary advantage. They can die out if the trait is no longer useful, but so can dominant genes ---
**Author:** ![Mnem](https://avatars.discourse-cdn.com/v4/letter/m/7bcc69/32.png) [@Mnem](https://boards.straightdope.com/u/Mnem)\ **Post date:** [January 3, 2004, 11:08pm UTC](https://boards.straightdope.com/t/biology-evolution-question/222536/3 "2004-01-03T23:08:54Z")
What if it is still useful though, is it possible for it to die out simply because it is being replaced with a dominant gene? ---
**Author:** ![Joe\_Random](https://avatars.discourse-cdn.com/v4/letter/j/e8c25b/32.png) [@Joe\_Random](https://boards.straightdope.com/u/Joe_Random)\ **Post date:** [January 3, 2004, 11:20pm UTC](https://boards.straightdope.com/t/biology-evolution-question/222536/4 "2004-01-03T23:20:40Z")
> [@](#): > > \*Originally posted by Mnem \* > \*\*is it possible for it to die out simply because it is being replaced with a dominant gene? \*\* If it’s being replaced (i.e. selected against) then yes. However, most recessive traits are not being replaced with dominant genes. For instance, people with brown eyes may still have the blue eye gene. It is not replaced; Its effects are merely suppressed. ---
**Author:** ![Mnem](https://avatars.discourse-cdn.com/v4/letter/m/7bcc69/32.png) [@Mnem](https://boards.straightdope.com/u/Mnem)\ **Post date:** [January 3, 2004, 11:29pm UTC](https://boards.straightdope.com/t/biology-evolution-question/222536/5 "2004-01-03T23:29:54Z")
Thank you scm1001 and Joe Random, you’ve been a great help. So in the future is it at all possible for, let’s say, blue eyes to completely disappear? ---
**Author:** ![Ilsa\_Lund](https://avatars.discourse-cdn.com/v4/letter/i/a6a055/32.png) [@Ilsa\_Lund](https://boards.straightdope.com/u/Ilsa_Lund)\ **Post date:** [January 3, 2004, 11:31pm UTC](https://boards.straightdope.com/t/biology-evolution-question/222536/6 "2004-01-03T23:31:03Z")
In short, no. Even if S (selection coefficient) equaled 1 (completely lethal), heterozygosity would preserve the gene. ---
**Author:** ![Joe\_Random](https://avatars.discourse-cdn.com/v4/letter/j/e8c25b/32.png) [@Joe\_Random](https://boards.straightdope.com/u/Joe_Random)\ **Post date:** [January 3, 2004, 11:35pm UTC](https://boards.straightdope.com/t/biology-evolution-question/222536/7 "2004-01-03T23:35:29Z")
> [@](#): > > \*Originally posted by Mnem \* > \*\*So in the future is it at all possible for, let’s say, blue eyes to completely disappear? \*\* There is only one way for that to happen, and that is for people with the “blue eye” gene to stop passing that gene to their children. This could be accomplished if having blue eyes became an evolutionary disadvantage (or if having non-blue eyes became an advantage). It could also happen if humanity becomes both proficient at genetic engineering, and has the desire to eradicate blue eyes. In other words, it’s highly unlikely to happen in the foreseeable future. ---
**Author:** ![Ilsa\_Lund](https://avatars.discourse-cdn.com/v4/letter/i/a6a055/32.png) [@Ilsa\_Lund](https://boards.straightdope.com/u/Ilsa_Lund)\ **Post date:** [January 3, 2004, 11:38pm UTC](https://boards.straightdope.com/t/biology-evolution-question/222536/8 "2004-01-03T23:38:55Z")
> [@](#): > > This could be accomplished if having blue eyes became an evolutionary disadvantage (or if having non-blue eyes became an advantage). No. See my previous post. ---
**Author:** ![echoreply](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/echoreply/32/3641_2.png) [@echoreply](https://boards.straightdope.com/u/echoreply)\ **Post date:** [January 3, 2004, 11:51pm UTC](https://boards.straightdope.com/t/biology-evolution-question/222536/9 "2004-01-03T23:51:12Z")
It is very unlikely that a recessive trait can be stamped out, because many people won’t even know they have it. (Yes, with modern technology it may be possible to make these determinations, but I’m thinking back to the early 20th century eugenics stuff.) Even if almost everybody has brown eyes, every once in a while two of them will have a child with blue eyes, because they are both carriers of the blue eye gene and might not even know it. It is much easier to stamp out a dominant trait, because it is easy to tell who has the gene. If brown eyed people were not allowed to have children (either through natural selection, or by sterilizing them at birth, etc.) then brown eyes could be wiped out in one generation, if selection was perfect (no brown eyed people modify their phenotype by wearing blue contact lenses and get to have kids). ---
**Author:** ![Mnem](https://avatars.discourse-cdn.com/v4/letter/m/7bcc69/32.png) [@Mnem](https://boards.straightdope.com/u/Mnem)\ **Post date:** [January 4, 2004, 12:08am UTC](https://boards.straightdope.com/t/biology-evolution-question/222536/10 "2004-01-04T00:08:57Z")
That makes sense. Thank you all for your insightful posts. 🙂