Crash Course in Genetics: Inheritance

Why are you the way you are? You might have your father’s eyes, hair and personality, but be the height and build of your mother, or something a little bit different from either. It comes down largely to genetic inheritance.*

You may recall from some science class somewhere that all your cells have two copies of each chromosome (section of DNA) and that you get one copy from each of your parents. So you end up with some of your mother’s traits and some of your father’s, meaning that you end up a bit different from both of them. However, the steps involved are a little more complicated.


Genes and Alleles

An allele is a variation of a gene (if you don’t know what a gene is, check out this previous post). For a simple example, let’s pretend that your height is controlled by just one single gene (it’s far more complicated in real life, with many genes interacting). In this case you could get the “tall version” of the gene (tall allele) or the “short version” of the same gene (short allele). Then if you got two ‘tall alleles’ from your parents, then you’d be tall and if you got both ‘short alleles’ then you’d be short.

But then, what happens if you get one tall allele and one short allele?

Do you end up just kinda medium height?

Well, no, but it needs some explaining. Before Gregor Mendel did his work on genetics, this was a commonly held view. If you had a short parent and a tall parent, then they would mix and come out with medium height children. Similarly if you had a plant with white flowers and one with red, then the offspring plants would have pink flowers. However, this is not the case, as Mendel showed when he bred many pea plants of different heights, flower and seed colours, flower and seed types and always produced more pea plants that had one of the types from its ancestor plants rather than a ‘mixed version’.

It turns out that you can (generally) only express one allele even though you’ve inherited more than one. So how is it decided which one is expressed? In the simple cases, one allele is dominant and one is recessive. For example, perhaps our tall allele is dominant and short is recessive. This means that whenever you inherit a tall allele, it is always expressed. So if you inherit tall alleles from both parents, you’ll definitely be tall. If you inherit one tall allele and one short allele, the tall is dominant, so you’ll be tall. The only way that you can possibly be short is if you get short alleles from both your parents (shown in diagram below).

Punnet square
A diagram (called a Punnet square) for being short or tall dependent on the combination you inherit from your parents.


Every time that a new child/offspring is created, different alleles are inherited from each parent so that the child has a different combination of each. Additionally, with traits that are influenced by many different genes interacting, there will be even more different allele copies to interact in different ways in the new offspring.



*Note that there is still a significant nature vs. nurture debate; however, it is reasonable to conclude that genetics lay a foundation or limits to how far nurture can act upon the individual.

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