eli5: if you inherit 50% of your genes from your father and 50% from your mother, what stops you from getting two of the same gene, and missing one? also why do siblings look different?
In: Biology
Your genes are bound up in 23 chromosome pairs, with each containing one chromosome from each of your parents.
When your body produces eggs/sperm it splits those pairs up so each egg/sperm only has a single copy of each chromosome, albeit with some shuffling of genes between chromosomes – so the chromosomes you pass on aren’t exactly the same as the ones you inherited.
These then combine with the counterpart pairs in the sperm/egg from your partner. Thus offspring get two sets of genes, each containing one half of the 23 chromosome pairs.
You **can** get situations where an offspring gets two chromosomes from one parent, or no chromosomes from the other. These cases are called “chromosomal abnormalities”. In many instances these genetic mixups result in a non-viable fetus that will often self abort fairly quickly.
In some cases the genetic mixup isn’t lethal but is still often detrimental – for example Downs Syndrome occurs when the offspring gets an extra chromosome 21 from one parent, which results in a number of fairly serious physical and cognitive health problems. Offspring can also inherit a missing chromosome – for example it’s possible for people to be born with only a single X chromosome and no Y chromosome, which again results in significant health issues. It’s not, however, possible for a human to be born with only a Y chromosome – the fertilized egg is flatly non viable and self aborts almost immediately.
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