# Genetically Inherited Characteristics: Average, Even Distribution, or Bipolar Distribution? **URL:** \ **Category:** Factual Questions\ **Created:** [April 19, 2018, 1:54pm UTC](https://boards.straightdope.com/t/genetically-inherited-characteristics-average-even-distribution-or-bipolar-distribution/812705 "2018-04-19T13:54:04Z")\ **Posts on this page:** 13\ **Page:** 1
**Author:** ![Fotheringay-Phipps](https://avatars.discourse-cdn.com/v4/letter/f/d9b06d/32.png) [@Fotheringay-Phipps](https://boards.straightdope.com/u/Fotheringay-Phipps)\ **Post date:** [April 19, 2018, 1:54pm UTC](https://boards.straightdope.com/t/genetically-inherited-characteristics-average-even-distribution-or-bipolar-distribution/812705/1 "2018-04-19T13:54:04Z")
My question is about children of parents who are different in some way, and how that works out for the children’s characteristics to the extent that they inherit this genetically from the parents. (Meaning, I’m aware that there will be variation due to environmental factors, random fluctuation, regression to the mean etc. I’m asking about that component which is inherited genetically.) As an illustration of the concept, suppose you were measuring the IQ of 1,000 people each of whom had one parent with an IQ of 100 and one parent with an IQ of 120. Would the IQ of these thousand people be (for the most part) even distributed between about 100 and about 120, be clustered about 110, or have two clusters about 100 and 120. Similar for two parents of different heights (relative to their respective genders), and so on for other characteristics which have genetic components. I imagine it varies by characteristic, e.g. one would expect that hair and eye color to be either/or. But other things are clearly not (e.g. skin color). My own observation - WRT things like body types, intelligence, and personality - is that children tend to take after one parent or the other more than to be a blend of the two parents with regards to individual attributions (though it’s very common for kids to be a blend of both parents in the sense of having the body type of one and the personality of the other, and so on). But I imagine this is something that would have been extensively studied. ---
**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:** [April 19, 2018, 2:05pm UTC](https://boards.straightdope.com/t/genetically-inherited-characteristics-average-even-distribution-or-bipolar-distribution/812705/2 "2018-04-19T14:05:59Z")
Yes, of course it varies from one trait to another. So much so that any general answer to the question is impossible. I gather that the specific trait you’re interested in is intelligence, but any study there is going to be enormously complicated by the fact that we don’t even know how genetic it is, since it’s surely influenced (to some degree) both by genetics and by upbringing. ---
**Author:** ![Fotheringay-Phipps](https://avatars.discourse-cdn.com/v4/letter/f/d9b06d/32.png) [@Fotheringay-Phipps](https://boards.straightdope.com/u/Fotheringay-Phipps)\ **Post date:** [April 19, 2018, 2:10pm UTC](https://boards.straightdope.com/t/genetically-inherited-characteristics-average-even-distribution-or-bipolar-distribution/812705/3 "2018-04-19T14:10:37Z")
> [@Chronos](#): > > I gather that the specific trait you’re interested in is intelligence […] You’re wrong about that. The main things I had in mind were actually body types, intelligence, and personality in no particular order (these are three areas that I’ve made this observation as indicated). I chose IQ for the illustration because it’s measurable and isn’t expected to vary between mother and father, so it was the simplest illustration. ---
**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:** [April 19, 2018, 2:18pm UTC](https://boards.straightdope.com/t/genetically-inherited-characteristics-average-even-distribution-or-bipolar-distribution/812705/4 "2018-04-19T14:18:40Z")
OK, so it’s one of the three you’re interested in. But personality has all of the same issues to an even greater degree. Body type, you might be able to get some real information on, depending on precisely what you mean by that, but even there there are questions of nature vs. nurture (parents who eat too much will probably also feed their children too much, etc.). Oh, and one other possibility that you didn’t mention, but which will be relevant for many traits, is reversion to the mean. That is to say, given a trait which has a measurable value that averages 100 on some scale, and parents who are a 100 and 120 respectively, their offspring are likely to be closer to 100 than they are to 120. ---
**Author:** ![Fotheringay-Phipps](https://avatars.discourse-cdn.com/v4/letter/f/d9b06d/32.png) [@Fotheringay-Phipps](https://boards.straightdope.com/u/Fotheringay-Phipps)\ **Post date:** [April 19, 2018, 2:39pm UTC](https://boards.straightdope.com/t/genetically-inherited-characteristics-average-even-distribution-or-bipolar-distribution/812705/5 "2018-04-19T14:39:41Z")
> [@Chronos](#): > > Oh, and one other possibility that you didn’t mention, but which will be relevant for many traits, is reversion to the mean. You’re wrong about that too. Reread the OP. ---
**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:** [April 19, 2018, 2:43pm UTC](https://boards.straightdope.com/t/genetically-inherited-characteristics-average-even-distribution-or-bipolar-distribution/812705/6 "2018-04-19T14:43:08Z")
No, as in, the mean of the entire population, not just the mean of the two parents. ---
**Author:** ![Fotheringay-Phipps](https://avatars.discourse-cdn.com/v4/letter/f/d9b06d/32.png) [@Fotheringay-Phipps](https://boards.straightdope.com/u/Fotheringay-Phipps)\ **Post date:** [April 19, 2018, 2:48pm UTC](https://boards.straightdope.com/t/genetically-inherited-characteristics-average-even-distribution-or-bipolar-distribution/812705/7 "2018-04-19T14:48:28Z")
That’s exactly what “regression to the mean” is all about, and what I specifically acknowledged in the OP ---
**Author:** ![Grim\_Render](https://avatars.discourse-cdn.com/v4/letter/g/a87d85/32.png) [@Grim\_Render](https://boards.straightdope.com/u/Grim_Render)\ **Post date:** [April 19, 2018, 3:49pm UTC](https://boards.straightdope.com/t/genetically-inherited-characteristics-average-even-distribution-or-bipolar-distribution/812705/8 "2018-04-19T15:49:45Z")
The answer is very large. You are correct that the answer is different for most characteristics. You are best off getting a textbook, or several of them for anything resembling a compete answer. These are some terms describing ways in which genetic traits are inherited: Autosomal dominant: A trait that is expressed if an individual has only one copy of a gene. Autosomal recessive: A trait that requires two genes to be expressed (Homozygoteous trait). Example: Cystic Fibrosis. Pleiotropic: A gene that affects several traits, possibly otherwise unrelated traits. Example: Marfan syndrome. Genomic imprinting: A gene that expresses differently depending on weather it is inherited from a female or male parent. Antagonistic Pleiotophy: A gene that affects traits that compete with each other. Epigenetic: Gene expressions that are affected by conditions and stresses experienced by a parent of grandparent (Most often by not always female-line) Example: Height. Epistasis: When a trait is affected by multiple genes that are not simply additive. Polygenic: Trait controlled by two or more genes which are additive. Example: Height, skin colour. Mitochondria: Organelle in the cell that has its own DNA and is not part of the nucleus. Thus only inherited from one female parent. Expressivity: Degree to which a trait differs among individuals with the same genes for it. Example: Marfan syndrome, where mutation position affect expression. There are many more. They are not exclusive, you can have an autosomal dominant which is antagonistically pleiotropic and can be affected epigenetically. Expression can also be affected by environment, such as when growth is stunted due to poor nutrition. The gene pool can also contain different genes for the same location which manifest differently. ---
**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:** [April 19, 2018, 4:50pm UTC](https://boards.straightdope.com/t/genetically-inherited-characteristics-average-even-distribution-or-bipolar-distribution/812705/9 "2018-04-19T16:50:52Z")
Don’t forget sex-linked traits, like colorblindness and hemophilia. ---
**Author:** ![RayMan](https://avatars.discourse-cdn.com/v4/letter/r/e495f1/32.png) [@RayMan](https://boards.straightdope.com/u/RayMan)\ **Post date:** [April 19, 2018, 5:09pm UTC](https://boards.straightdope.com/t/genetically-inherited-characteristics-average-even-distribution-or-bipolar-distribution/812705/10 "2018-04-19T17:09:18Z")
> [@Fotheringay-Phipps](#): > > As an illustration of the concept, suppose you were measuring the IQ of 1,000 people each of whom had one parent with an IQ of 100 and one parent with an IQ of 120. Would the IQ of these thousand people be (for the most part) even distributed between about 100 and about 120, be clustered about 110, or have two clusters about 100 and 120. It’s hard to tell what you mean from the title of your thread. I suspect by “even distribution” and “bipolar distribution” you mean to say “uniform distribution” and “bimodal distribution”, respectively. Sent from my SM-G930V using Tapatalk ---
**Author:** ![Fotheringay-Phipps](https://avatars.discourse-cdn.com/v4/letter/f/d9b06d/32.png) [@Fotheringay-Phipps](https://boards.straightdope.com/u/Fotheringay-Phipps)\ **Post date:** [April 19, 2018, 5:12pm UTC](https://boards.straightdope.com/t/genetically-inherited-characteristics-average-even-distribution-or-bipolar-distribution/812705/11 "2018-04-19T17:12:01Z")
> [@RayMan](#): > > It’s hard to tell what you mean from the title of your thread. I suspect by “even distribution” and “bipolar distribution” you mean to say “uniform distribution” and “bimodal distribution”, respectively. Very likely, and thanks for the correction. 🙂 If a mod wants to change the title, that would be fine. ---
**Author:** ![Smeghead](https://avatars.discourse-cdn.com/v4/letter/s/f1d935/32.png) [@Smeghead](https://boards.straightdope.com/u/Smeghead)\ **Post date:** [April 20, 2018, 4:47am UTC](https://boards.straightdope.com/t/genetically-inherited-characteristics-average-even-distribution-or-bipolar-distribution/812705/12 "2018-04-20T04:47:43Z")
It’s biology. If you can imagine it, it’s happening somewhere. ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [April 20, 2018, 1:11pm UTC](https://boards.straightdope.com/t/genetically-inherited-characteristics-average-even-distribution-or-bipolar-distribution/812705/13 "2018-04-20T13:11:12Z")
Forgetting (mostly) the genetics of it, if you had two similar distributions with peaks at 100 and 120 on a scale of 1 to whatever that were sort of bell-shaped and you took an average of those same-sized distributions the peak of that curve would be slightly under 110. The lower limit of 1 skews things in that direction. Among breeding humans and IQ scores, the lower limit isn’t 1 as hardly anyone with an IQ of 20, say, is going to be reproducing. So the peak of the mix is even further under 110.