# Why do we have two of some things but only one of others?

**URL:** <https://boards.straightdope.com/t/why-do-we-have-two-of-some-things-but-only-one-of-others/958333>\
**Category:** Factual Questions\
**Created:** [January 25, 2022, 1:27am UTC](https://boards.straightdope.com/t/why-do-we-have-two-of-some-things-but-only-one-of-others/958333 "2022-01-25T01:27:46Z")\
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**Author:** ![Max\_S](https://avatars.discourse-cdn.com/v4/letter/m/46a35a/32.png) [@Max\_S](https://boards.straightdope.com/u/Max_S)\
**Post date:** [January 26, 2022, 9:41pm UTC](https://boards.straightdope.com/t/why-do-we-have-two-of-some-things-but-only-one-of-others/958333/17 "2022-01-26T21:41:47Z")

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As to why we have two of some things, for the immediate pathological “why” (rather than the evolutionary why),

> [@RickJay](#):
>
> a person has two kidneys

It seems that the [FOXD1](https://www.ncbi.nlm.nih.gov/gene/2297) gene “promotes nephron progenitor differentiation”, which I read as meaning it causes a single proto-kidney tissue to split into two distinguishable proto-kidneys. So whatever causes us to have two kidneys, may be contained in that protein.

Here is a study from 2005 where, in their own words,

> **Genetic deletion of** the forkhead transcription factor, **Foxd1, results in** striking renal abnormalities, including the loss of these discrete zones and **pelvic fused kidneys**.

Levinson RS, Batourina E, Choi C, Vorontchikhina M, Kitajewski J, Mendelsohn CL. Foxd1-dependent signals control cellularity in the renal capsule, a structure required for normal renal development. Development. 2005 Feb;132(3):529-39. doi: [10.1242/dev.01604](https://pubmed.ncbi.nlm.nih.gov/15634693/). Epub 2005 Jan 5. PMID: 15634693.

~Max

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