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It's my understanding that DNA codes only for protein synthesis. Does that mean that hereditary traits, like the shape of our nose, are determined only by the proportions in which various proteins are synthesised in various parts of our body ?

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Does that mean that hereditary traits, like the shape of our nose, are determined only by the proportions in which various proteins are synthesised in various parts of our body ?

Yes, more or less!

A trait that is called heritable is a trait which variance in the population is in part at least explained by genetic variance. Most of these heritable trait however, are also affected by environmental variaion (and other source of variance). For more information, please have a look at the post Why is a heritability coefficient not an index of how “genetic” something is?

So when you say

[..] are determined only by the proportions in which various proteins [..]

The term "only" is a bit misleading. Other source of variance exist that will affect the variance for this trait. However, it is a pretty good approximation to say that the heritable information is conveyed via the transmission of DNA and the phenotypic expression of this DNA is mediated via proteins.

We refer to the DNA -> RNA -> protein having some phenotypic impact relationship by the term central dogma of molecular biology. You might want to read more about it or maybe have a look at an online course such as this one from Kahn academy for example. You will want to know the terms translation and transcription.

Now, in reality DNA can affect an individuals phenotype by other means than by the proteins (or polypeptides to keep it more general) that are synthesize from it. Indeed, many loci (locus=position on a chromosome) affect the expression of other, others are translated into RNA but not transcribed into proteins (e.g. rRNA, tRNA, RNAi, ...) but yet still have an important action on a cells physiology.

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