Why do some traits, like skin tone, seem to be a mix of both parents, where others like eye & hair colour are from a single parent?
In: Biology
Some traits are determined by a single or very few genes. Others are determined by multiple genes. Multiple gene traits allow for mixing from both parents. Singles are one or the other.
All of your traits come from both of your parents.
Explained simply, on a monogenic inheritance model, you have a gene which tells you what eye colour you have. This gene has two possible alleles (variants), one blue and one brown and each person always has two alleles. Now, the brown allele is dominant (this means you only require one brown allele to have brown eyes, no matter what the other allele is). The blue allele is recessive (this means you need two blue alleles to have blue eyes).
So, imagine your mum has blue eyes – this means she has two recessive alleles (aa).
Your dad has brown eyes – this means he either had two dominant alleles (AA) or one dominant and one recessive (Aa).
You, as their child, inherit one allele from your mum and one from your dad. In my example, you always get the recessive (blue) one from mum, but you can get either the recessive or the dominant one from dad.
So if your mum is aa and your dad is AA, you will always end up with brown eyes, because there is only one combination of alleles you can get (Aa). If your dad is Aa, though, you can end up with either blue or brown eyes (it’s fifty fifty in this case, actually).
The monogenic model is simplified (though it does work sometimes, see Gregor Mendel and his experiments) and in reality many traits require the cooperation of more genes. The inheritance model is then more complicated.
So genes are what makes something heritable, you get one set from each parent and for some characteristics there is a single dominant gene and a single recessive one meaning you express only one gene from one parent.
However for a lot of characteristics there are many different genes all contributing to that one characteristic, some may act against each other and some may act with each other, and some may be ‘codominant’, meaning both sets from each parent are expressed.
So for skin tone, there are many genes contributing and some are codominant, meaning there can be a variety of skin tones even if your parents have vastly different or very similar tones. (of course not including tan and other environmental factors)
For eyes, there are actually so many genes that it is possible for anyone to have any colour eyes, regardless of their parents eye colour (though the probability obviously is very low for rarer colours like green, grey and Amber)
Hope this answers your question, please feel free to further query in the comments or by message
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