If a person with lighter skin is to live in areas close to the equator for a long amount of time, their skin would get darker, and hence their phenotype gets 'altered'.

If I'm to look at their gene that codes for skin color, though, I'd still see the one for lighter skin (which shows how it's not inheritable), but that would refer that the phenotype is also for lighter skin (when it's not the case in this situation)

'Phenotype' is defined as the morphological expression of characters on the basis of genetic constitution/genotype. How then, considering my example, is phenotype dependent on genotype?

I'm currently a high school student having mostly introductory/basic knowledge on genetics and heredity. I would appreciate some help in gaining conceptual clarity.

  • $\begingroup$ Even if a person gets a darker skin tone under these conditions, they are still pretty much different from people with a lot of pigmentation. $\endgroup$
    – Chris
    Commented May 21 at 11:48
  • 1
    $\begingroup$ Following what David writes, if your book or something says "phenotype is dependent on genotype", be careful that you don't misinterpret this as "phenotype is entirely dependent on genotype". Entirely, completely, absolutely, whatever synonym you might choose it is not in the statement so do not insert it on your own to make the claim stronger than it is. $\endgroup$
    – Bryan Krause
    Commented May 21 at 23:18
  • $\begingroup$ I thought it useful to provide an answer (more for the general footnote, which is relevant to other questions), but the logical fallacy is anticipated in the comment made by @BryanKrause . $\endgroup$
    – David
    Commented May 22 at 14:32

3 Answers 3


The Logical fallacy

‘Phenotype’ is defined [by the poster] as the morphological expression of characters on the basis of genetic constitution/genotype

There is no reason to accept this particular definition, but I find it reasonable enough. However I maintain that it is a logical fallacy to say — as the poster does — that it implies that if a morphological characteristic of an organism can, in certain circumstances, be caused by non-genetic factors, then it can never be a ‘phenotype’. To allow that implication the definition of phenotype would have to be something like:

A morphological expression of characters arising solely from the genetic constitution of an individual.

which it is not what the definition states.

The term phenotype was coined and is used, solely in the relation to genetics. Hence, if the presence or absence of a particular gene causes a particular morphology, that morphology can be regarded as the phenotype of that gene or allele. This does not exclude the morphology also being the phenotype of another gene/allele, because the term is employed in relation to a specific gene. (Of course, some morphological features have a multigenic cause, and some genes have no observable phenotype, but that is irrelevant to the poster’s proposition.)

Incidentally, Mendel — who never used the term phenotype but a more cumbersome (Czech or German) phrase instead — would have been well aware that the size of peas could be affected by their access to nutrients and the weather. This did not invalidate his using size to access the effects of various breeding crosses and make conclusions about their origin in terms of invisible ‘factors’, only later called ‘genes’.

Footnote: Terminology in the biological sciences

Students from the numerical sciences (especially physics and mathematics) are in the habit of thinking in terms of definitions relating to the laws of the universe, and in this sphere a legalistic dissection of terms is often valid. The biological sciences are experimental sciences in which there are few, if any, universal laws. Ideas about biology develop from a limited number of observations (historically on whole organisms, rather than at the molecular level), and are refined as more observations are made and it becomes possible to provide molecular explanations for them. To communicate ideas it is necessary to make formal proposals and often to devise new terms to express them. Such proposals are not like laws or theorems in the numerical sciences. They frequently require modification as greater complexity is revealed and the anarchy of biology throws up exceptions. Biological scientists do not spend time debating the meaning of terms devised years previously. When new discoveries emerge they modify their picture of science and either modify or add riders to their terminology. They certainly do not spend their time shouting that a particular giant of the past had feet of clay. (Only seldom is there a complete revolution in ideas, or yesterday’s hero declared a charlatan.)

An example of the way genetics adjusts can be seen in Beadle and Tatum’s historic proposition: “one gene—one enzyme”. This was a made in 1941 at a time when the molecular nature of genes was still in debate (many, if not most, still thought that they were protein in nature) and not a single protein had been sequenced. The importance and originality of the proposal was in formulating a molecular relationship between nebulous entities, genes, and known molecules, enzymes.

Of course, we now know that not all proteins are enzymes, and some genes encode RNA (the structure and functions of which were unknown in 1941). Moreover with structural determination it became evident that some enzymes and proteins have more than one polypeptide chain, encoded by distinct genes (two genes—one enzyme). More recently, the discovery of alternative splicing means that in certain cases the situation is one genes—many enzymes.

Does this in any way diminish the contribution Beadle and Tatum made with their one gene—one enzyme hypothesis? I, for one, would wish to have half their insight.

  • $\begingroup$ Perhaps at some point we should turn the second half of your answer here into a canonical one for misunderstood definitions in biology. I think this comes up a lot, for example just recently biology.stackexchange.com/q/114739/27148 is effectively the same issue. $\endgroup$
    – Bryan Krause
    Commented May 22 at 15:13
  • $\begingroup$ @BryanKrause — That would be fine with me. $\endgroup$
    – David
    Commented May 23 at 16:34
  • $\begingroup$ @David Thank you for the insight you provided with the footnote. I am, obviously, new to this field and realise my mistake. I've been rather accustomed to physics and chemistry, and hence had such a doubt. $\endgroup$
    – Mel
    Commented May 23 at 18:30

Yes, phenotype maps only noisily onto genotype. I recommend looking into the concept of phenotypic plasticity.

For example, temperature or food availability has regularly been used to alter phenotypes in clonal (~same genotype) populations of organisms, such as flies, plants, or Daphnia.

In one dramatic example, high population densities of tadpoles leads some of them to adopt cannibalistic behavior and morphological changes for preying on their siblings.

That said, genotype can itself control the way in which phenotype will change. This is the idea underlying the "reaction norm" first described by Richard Woltereck in 1909. He famously wrote "Reaction norm = Genotype", to describe that what is inherited in the genotype is not a deterministic program for creating phenotype, but rather a set of possibilities for what your phenotype can be.

This is not dissimilar to what MakM says, but the idea of plasticity is a core concept in biology that should be underlined as a common theme.

Interested to see what David writes.


Skin color is a polygenic trait. This means that it is controlled by multiple genes. (This can make it a little more complicated than some of the traits you learn about in your introductory classes).

If I understand correctly, you are asking about a single person that temporarily lives in another region of the world. During that time, their skin color darkens but their DNA stays the same. Now, I am no expert, but I am pretty sure that if they moved back, their skin color would eventually lighten again. This is due to skin tanning which is caused by melanocytes present in your skin. They produce a dark pigmentation in the skin to help protect against UV damage. (This is why increased sun exposure increases the risk of skin cancer. The UV rays damage your DNA and to prevent this, your body builds up melanin)

The way that I understand it is this: The genetics of skin tanning are always present in your DNA. If you have albinism and do not possess the ability to produce much melanin, regardless of where you live, you will still have the same (or near same) skin tone. The genes for the response to UV damage are always present in your DNA, but they are expressed only at certain times (after sun exposure).

Thingnes J, Oyehaug L, Hovig E, Omholt SW. The mathematics of tanning. BMC Syst Biol. 2009 Jun 9;3:60. doi: 10.1186/1752-0509-3-60. PMID: 19505344; PMCID: PMC2714304.

Visconti, A., Duffy, D.L., Liu, F. et al. Genome-wide association study in 176,678 Europeans reveals genetic loci for tanning response to sun exposure. Nat Commun 9, 1684 (2018). https://doi.org/10.1038/s41467-018-04086-y

Lucock MD. The evolution of human skin pigmentation: A changing medley of vitamins, genetic variability, and UV radiation during human expansion. Am J Biol Anthropol. 2023 Feb;180(2):252-271. doi: 10.1002/ajpa.24564. Epub 2022 Jun 25. PMID: 36790744; PMCID: PMC10083917.

  • $\begingroup$ We were given only definitions related to polygenic traits and non-mendelism under classical genetics but I did not think of it this way. I think the only way to confirm that the phenotype is altered is crossing with another individual and studying the progeny, right? $\endgroup$
    – Mel
    Commented May 23 at 18:36

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