Many people, scientists and non-scientists alike, object to what they perceive as genetic determinism. This is often a reaction to geneticists apparently over-reaching and claiming that some trait or condition is “caused by” a single gene. A common rejoinder is that any biological process obviously involves many hundreds of gene products, interacting with each other in complex ways and so it is nonsense to say that the trait is determined by a single gene. That is absolutely true, if you are using the word “gene” purely in the molecular biology sense – as a piece of DNA that encodes a particular product (usually a protein). But geneticists also use it in the original sense, as a unit of heredity – a genetic variant or mutation that can be passed on across generations and that influences some phenotype.
Genetics is not about how a characteristic arises, it is about how variation in that characteristic arises. For example, when you are describing someone, you might say: “She’s got blue eyes”, but you probably wouldn’t say: “She’s got two eyes”. Both characteristics are determined by the genetic program, but only one is affected by genetic variation. Eye colour is therefore a trait, because it varies across the population and that variation is due to genetic differences. Having blue eyes, insofar as it necessarily involves having eyes in the first place, is obviously not caused by a single piece of DNA – it takes thousands of gene products to build eyes, blue or otherwise. But having eyes that are blue, as opposed to brown, can be due to a single genetic variation.
As it happens, though, eye colour is also a good example of genetic interactions. Because, while it’s true that a single mutation can explain the difference in eye colour between some people, it’s also true that many people carry more than one such mutation in any of several different genes. The ultimate colour that emerges is thus often determined by interactions amongst multiple genetic variants.
- Kevin Mitchell enlightening post, No gene is an island
Genetics is not about how a characteristic arises, it is about how variation in that characteristic arises. For example, when you are describing someone, you might say: “She’s got blue eyes”, but you probably wouldn’t say: “She’s got two eyes”. Both characteristics are determined by the genetic program, but only one is affected by genetic variation. Eye colour is therefore a trait, because it varies across the population and that variation is due to genetic differences. Having blue eyes, insofar as it necessarily involves having eyes in the first place, is obviously not caused by a single piece of DNA – it takes thousands of gene products to build eyes, blue or otherwise. But having eyes that are blue, as opposed to brown, can be due to a single genetic variation.
As it happens, though, eye colour is also a good example of genetic interactions. Because, while it’s true that a single mutation can explain the difference in eye colour between some people, it’s also true that many people carry more than one such mutation in any of several different genes. The ultimate colour that emerges is thus often determined by interactions amongst multiple genetic variants.
- Kevin Mitchell enlightening post, No gene is an island
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