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I'm not a biologist but, I'm trying to enter molecular biology from a perspective. I have noticed that concept of genotype is quite well defined while phenotype is not. If I understand well, gene expression builds up enzymes that ultimately control all chemistry inside the cell, and which are sensible to the environment. Then, how correct/incorrect would it be to state that "phenotype is the enzyme composition within the cell"? Is it at least a good approximate idea?

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    $\begingroup$ Phenotype is considered the physical/observable representation of the genotype. $\endgroup$ – Macedon93 Aug 24 '15 at 19:45
  • $\begingroup$ Where is your quote from? The entire enzyme (protein) composition of the cell is correctly referred to as the proteome. As not all proteins play a role in individual traits, then no, that is a bad approximation of the idea. $\endgroup$ – AMR Aug 24 '15 at 20:23
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    $\begingroup$ Have you read this? If not, please do. If so, what specific questions do you have about it? $\endgroup$ – MattDMo Aug 25 '15 at 1:04
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Genotype:

Simply genotype is the inherited genetic information an individual has, and is conceptually quite easy to grasp. For example, if height is determined by gene $A$ and there are two alleles (variants of the gene, $A_1$ and $A_2$), then the three possible genotypes at that locus are $A_1A_1$, $A_1A_2$, and $A_2A_2$.

Phenotype:

The definition of a phenotype is a little more difficult to grasp, in part because it's so broad. As a quantitative geneticist I would say its any characteristic of an individual that can be measured. There's an enormous variance in the phenotypes that are used, they can be characteristics that are physiological, morphological, behavioural, life-history, even fitness itself. Personally I've used phenotypes from gene expression, which is one of the "lowest" (most close to genotype on the bridge between genotype and phenotype) phenotypic levels, through to lifespan, rates of ageing, and lifetime offspring production. Phenotypes are determined by genotype and/or environment (all non-genetic effects). For example, height may be affected by several genes, but it is also affected by nutrition during development and growth (among other things). As an evolutionary biologist, I feel the need to point out that selection operates on the phenotype, it is because genotype often has some effect on the phenotype that we see evolution by inheritance of genetic information.

Correctness of your statement:

"Phenotype is the enzyme composition within the cell"

Would be sort of incorrect. It certainly wouldn't be appropriately defining phenotype as a general thing. It would be saying that phenotype means enzyme composition within cells, and the appropriate sentence depends on what you are trying to say. The following would say that, in the discussed case, the enzyme composition of the cell is what you consider to be the phenotype:

"In this study the enzyme composition within the cell is the phenotype."

Whereas, the following defined the concept of phenotype:

"Phenotypes are characteristics which could be, in principle, measured, such as the enzyme composition within a cell."

The problem of your statement is it makes it sound like phenotype, as a general thing, is enzyme composition within cells. It's kind of a problem of while enzyme composition of a cell can be a phenotype, not all phenotypes are enzyme compositions of cells. Similarly to all humans are mammals but not all mammals are humans.

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Phenotype is the physical result that one sees that is the outcome of genetic makeup. Genotype is the code that is written in the DNA (or RNA if you are a retrovirus). Enzyme composition in the cell, hair color, proclivity towards certain diseases; pretty much everything is a product of genoptype. There are some other factors in the environment that affect gene expression, but you are very much the product of your genotype.

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    $\begingroup$ Your answer lacks references and completely ignores environmental effects and gene-environment interactions, which are large components of phenotype. $\endgroup$ – fileunderwater Aug 26 '15 at 10:19
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    $\begingroup$ I also believe this answer can be improved if it includes external factors. $\endgroup$ – HelloWorld Jan 14 '16 at 0:33

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