Crash Course in Genetics: Genotype vs Phenotype

This post could be subtitled:

Your genes vs your actual characteristics

Wait. Isn’t that confusing? Aren’t your genes what defines your characteristics? That’s true to some extent, and there’s the nature vs nurture argument that was briefly discussed last week. But what I actually intend to write about this week is the fact that you inherit two alleles for each characteristic, one from each of your parents. This means that you have more genetic variety than is actually expressed.

Using last week’s example about height, you had different combinations of tall (T) and short (S), either TT, TS, ST or SS. The particular combination that you get is your genotype: the genes you possess. However, if you had TT, TS or ST, you would be tall in all three instances, so your phenotype (how you actually appear) is tall. The only way to get the ‘short’ phenotype would be to inherit the SS genotype. So for this example, there were four possible genotypes (TT, TS, ST, SS) and only possible two phenotypes (tall or short).

So as a TT, TS or ST, you would look identical in all three cases and there is absolutely no way to tell which genotype combination you have.

So why does your genotype matter if you can’t actually see any difference?

The key point is that if you reproduce, you can pass only one copy, either a T or an S, onto your offspring. If your genotype is TT, then you will pass on a T, and if it is SS, then you will pass on an S. However, if your genotype is TS or ST, you can pass on either a T or an S. This means that you could have two parents who were both tall (tall phenotype), but both had the genotype TS and both passed on the S, so that the child has the genotype SS and therefore the short phenotype.

So if phenotype was the only thing that mattered, then two tall parents should have a tall child. Except this example shows that two tall parents can have a short child which we can only determine based on the genotype of the parents.

In this way, the population actually generally has a greater genetic diversity than is often observed and recessive traits can sometimes spring up in a child seemingly out of nowhere. Unless, of course, you understand the difference between an individual’s genotype and their phenotype.



Earlier posts in this series if you haven’t already read them are:

What is a gene?

The crisis in genetics



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