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
You can actually end up inheriting two of one parent’s genes and none of the other; it’s called “uniparental disomy”.
Particularly in the uterus, it’s in the father’s best interest for his offspring to grow as big as possible. It’s in the mother’s interest to limit this growth, because of the risk to her (and current and future babys’) life to give birth to too big a baby.
As part of this conflict, certain genes are switched off by a process called genomic imprinting, so only the mother’s or father’s copy is active. For example, for insulin-like growth factor 2, only the father’s copy is active, but for its receptor (in mice) only the mother’s copy is active. They basically battle it out and find a balance.
Now, this can be an issue when certain genes are copied twice from one parent but none from the other. If two copies are inherited from one parent but they’re both imprinted to turn off, that gene won’t have any active copies. You see this in Prader-Willi syndrome; in this, certain genes on the long arm of chromosome 15 have two maternal copies, which are both turned off. This presents with intellectual delay, short stature, hyperphagia (excessive appetite) and obesity.
Conversely, what if the long arm of chromosome 15 had two paternal copies? Different genes in this section would have no working copies because of imprinting, and you get a different condition called Angelman syndrome. In this you have developmental delay, excessive happiness, poor coordination etc..
There can also be issues with uniparental disomy (two copies from the same parent) if that parent is a carrier for a recessive condition. In recessive conditions, you need two broken copies of the gene for the condition to appear. Uniparental disomy can therefore cause recessive conditions to appear if the wrong gene is copied twice from the same parent.
To avoid this, we have lots of checks built into cell division when we create our eggs and sperm cells during meiosis.
Siblings look different because when we make our sperm and egg cells, each cell starts with a copy of our mum’s chromosome and a copy of our dad’s. When the sex cells are created, these chromosomes are duplicated before sections of these two chromosomes are swapped around (called crossing over). These then divide twice to form four sex cells, each with a random mixture of maternal and paternal chromosome sections.
This happens differently each time, so your children end up looking different!
There’s also influence from other factors in the womb, in later development etc..
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