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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$\begingroup$ DNA does not only code for proteins. Other products (e.g. non-messenger RNAs like microRNAs or ribosomal or transfer RNAs) exist. Also, DNA has further functions aside from protein-coding. Many DNA regions are regulatory, bind transcription factors, affect the topology of chromatin, host modifications, and much more. Lastly, it's important to be aware that hereditary traits can also be non-DNA-based. There is a very active field of research called epigenetics. Worth reading about! $\endgroup$– S PrCommented Jan 25, 2019 at 14:15
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$\begingroup$ "only for protein synthesis". those are bold words for someone built from proteins ;-) $\endgroup$– szulatCommented Jan 25, 2019 at 19:14
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$\begingroup$ Sure, but I am wondering what makes my nose stick out one way more than an other, when the instruction manual only indicates the local proportions of various proteins I am made of. $\endgroup$– James WellCommented Feb 18, 2019 at 12:14
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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 traits, however, are also affected by environmental variation (and other sources 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 sources 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 Khan academy for example. You will want to know the terms translation and transcription.
Now, in reality DNA can affect an individual's phenotype by means other than proteins (or polypeptides to keep it more general) that are synthesized from it. Indeed, many loci (locus=position on a chromosome) affect the expression of other proteins, others are translated into RNA but not transcribed into proteins (e.g. rRNA, tRNA, RNAi, ...) but yet still have an important action on a cell's physiology.
