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 grouped up into 46 larger structures called chromosomes, and they’re in pairs.
So when you inherited half your genes from your mother you didn’t just get a random selection of 50% of her DNA, you got one copy from each set of chromosomes.
Then you get another copy from your father.
23 pairs, 46 chromosomes.
That ensures that every human baby has the complete set.
Now most of your genes *are* copies, there’s very little genetic variation between individuals. Sure a person in Egypt and a person in Japan can have different colored hair or eyes, but they have the same cellular metabolic pathways.
Most of this stuff is critical to life that can’t be changed, only the superficial details can vary much between individuals or even entire species – a mosquito shares a lot of those cell metabolism genes with you too because they’re so important.
The genes are bundled into packages called Chromosomes. You get 1 set of Chromosomes from each parent, but each (except the Sex Chromosomes in Men) comes in a pair, with 2 copies of each gene, 1 from each parent.
As for why siblings look different, on average, they only share ~~25~~50% of your DNA (well except for the 99.9% you share with ALL of humanity). That makes differences.
Each gene is a recopy for making a protein. If you get two recopies for the same protein that is no issue as the cell will only make as much of the protein as it needs. And if one gene is damaged somehow that does not matter as you have a spare one.
There are some effects you get from this though. For example the gene for the brown colour in your eyes can be missing or weak. If you are missing the gene entirely you end up with blue eyes. But if you have only one gene, and this happens to be a weak variant of the gene, you can end up with green eyes as the blue and brown colours mix. But if you have a strong brown gene or two of them then you will get brown eyes.
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.
There are 23 pairs of students (46 kids) in one classroom A and another 23 pairs in different classroom, B.
you go to the first pair in A and randomly pick one. you go to the first pair in B and randomly pick one. the chosen two form a new pair and go sit in Classroom C. go back to A and B and repeat for the second pair, third pair, fourth etc.
when you are done you will have 23 pairs of students in classroom C, 50% from Classroom A and 50% from Classroom B. but the namelist would be quite unique.
because if you reset (tell everyone to go back) and go make a new Classroom C you will not get the same name list, because there were a sequence of 23 random selections. the unique name list in the new classroom is what makes a new person’s DNA unique even if they came from the same parents.
identical twins are different. once Classroom C is defined, it gets copied directly into Classroom D so the name list and thus DNA is identical.
if you mess up and accidentally pick three people per step or miss a pair, that new class will probably not even start or may start but be cancelled later on (no embryo, or spontaneous abortion). sometimes messing up can still work and you end up with deformities or something like Down Syndrome.
if one of the classrooms has a bad student (gene disease, mutation), there is a chance that he or she can be helped by their new partner and the pair can still work (recessive mutation). but it won’t be great if the new partner also has the same recessive mutation. if he or she is particular bad then no good partner can help and the new pair will be bad regardless (dominant mutation). good or bad is subjective. this explains why mutations sometimes show but sometimes only get carried.
is it possible that you can do Classroom C, reset and make Classroom D with the same name list by chance? it is always possible, except I didn’t mention that each chromosome (pair of students) can have around 1000 genes. so the closer analogy would be that each student in this experiment has 1000 pets with unique nicknames each and you are actually making a 20+ thousand pair pet name list instead of 23 pairs.
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..
The 50% thing is a bit misleading: you have two complete* sets of genetic code, one from each parent. Your parents also have two copies of the code, one from each of *their* parents.
When sperm and egg cells are created, a process called [crossing over](https://www.genome.gov/genetics-glossary/Crossing-Over) occurs: your father’s cells line up both copies of his genetic code and randomly choose chunks from either the code they got from your grandmother or from your grandfather.
It’s a bit like lining up two copies of the Bible, cutting them up at the same random points, and reassembling them by picking each verse from one copy or the other. You might get the first half of Genesis from one copy and the second half from the other, but you won’t have two copies of anything.
Your father then creates sperm sells with this randomized genome, your mother creates egg cells with a different randomized genome selected from her parents’ genes, and those two copies are combined to make your genome.
Your siblings will get a different randomized sample from each parent, so they’ll be a different combination of your shared grandparents’ genetic code.
Your DNA comes in two “sets” (you have two of every chromosome) so you actually have two copies of every gene, with one from your father and one from your mother. The set from your father will be a mishmash of roughly half of his mother’s / half of his father’s DNA, same with your mother. This happens because of the so-called “crossing over” event that happens when sperm/eggs are made. In crossing over, the mother/father set of DNA in the person exchange sections of their chromosomes to make “remixed” chromosomes. One set of these remixed chromosomes are in the sperm, another set in the egg, and that makes the two sets in each person.
This remixing is random, so siblings of the same mother and father are really getting different sections of their grandmothers’ / grandfathers’ DNA.
You get half from each, but you get half of their whole. What pieces will make this half is a bit random.
Example:
Your father is a purple monster. His genes are red and blue.
Your mom is a green monster. Her genes are blue and yellow.
You’ll get one color from each parent. Your options are:
* Purple like dad. Red from dad, blue from mom.
* Orange monster. Red from dad, yellow from mom.
* Blue monster. Blue from both.
* Green monster like mom. Blue from dad, yellow from mom.
Your siblings can look like you, but they are most likely to not.
You inherit 23 chromosomes from your mom, and 23 from your dad. But they also inherited that number from your grandmothers and grandparents. So siblings are different because when your dad gives you 23 chromosomes, it could be 10 from you grandma and 13 from your grandpa, or 22 and 1, they’re randomly separated. So even though both you and your sibling have 23 chromosomes from your dad, it is different combinations of those 23. The same thing happens with your moms chromosomes.
For example, I have brown eyes and my brother blue eyes. I could have my grandpas chromosomes of brown eyes, and my brother got my grandmas, that has blue eyes (it’s more complicated that this).
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