Eli5: What are recessive genes? Why do we always get passed down genes that don’t show?
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FYI I’m a curious/nosey reader not an expert:
Recessive genes typically, almost always require that they are received from both parents in order for the trait they carry to show. Let’s use eye color for an example: you get a big B, from mom for brown, and a little b, from dad for blue. The big B is dominant over the little b so you get brown eyes but….you still get both genes from both parents. This means you are heterozygous for brow eyes(Bb). Homozygous would be BB (Brown eyes) or bb (blue eyes). Heterozygous can be Bb or bB. Gametes (egg and sperm) from parents are randomized disco combinations, literally life’s box of chocolates. So If you are heterosexual and you also meet a brown eyed partner that it heterozygous (Bb) for eye color there is a 1 in 4 chance you can have a child with blue eyes. We carry many recessive genes and lots of “junk” dna. If the coding poses no threat then why not pass it on a reduce enenergy on trying to weed it out? On the other hand our knowledge of genetics is always growing and genetics are always changing so we don’t fully understand the purpose of non-expressed dna and this is just a simple example; there can be other genes that override gene expression of certain traits. Life is a lovely misunderstood puzzle.
In the most basic form, you get 2 versions of each gene called alleles. one from each parent. some of the alleles are recessive, meaning you need one from each parent for it to show, some are dominant which means you only need one.
take eye colour for example. the allele for brown eyes is dominant while the one for blue eyes is recessive. If someone has 2 brown eye alleles, they will have brown eyes, if they have 2 blue eye alleles they will have blue eyes, but if they have one of both, they will have brown eyes as brown is dominant. If you have a recessive allele but don’t show it, your called a “carrier” of that gene.
when you have kids, you only pass on one of these alleles. if someone with 2 brown alleles has a kid with someone with 2 blue alleles, the kid will always get one of both and have brown eyes.
however if 2 people with one brown and one blue allele have a kid, theres a 25% chance the kid will get have 2 brown, 50% chance they’ll have one brown, one blue, and a 25% chance they’ll get 2 blue snd have blue eyes. This means 2 people with brown eyes can have a kid with blue eyes if they both had the recessive gene.
This example was with eye colour, but it also goes for genetic diseases. Even if you, and your partner don’t have any genetic diseases, you can still pass them on to yours kids if you were both carriers
edit to say im not an expert, this is just what i remember from school
So I’m going to simplify it even more than what some other people have said
Take a piece of paper and put one drop of light blue paint and one drop of dark brown paint next to each other. This is your dad and mom’s eye color.
You get half your genes from your mom and half from your dad. Your dad’s eyes are blue so all he can give you is a drop of blue. Your mom’s eyes are brown and she gives you a drop of brown. So put a drop of blue paint on the paper and then put a drop of brown paint on top of it. Which one is visible? The brown, because the brown is the “stronger” (i.e. dominant) color. But the blue is still there underneath the brown, right? That’s the recessive gene.
Now you have a child with a person who has blue eyes. But because of your dad you have a non visible blue gene to give. Your partner has ONLY blue genes to give, because clearly if they had a brown gene, they’d have brown eyes, right?
So you have Brown/blue and blue/blue. That combination can produce one either blue/blue or Brown/blue.
That’s the way recessive/dominant genes work.
Recessive genes are genes we carry that do not have a phenotype, meaning whatever they code for is not actively displayed in the creature, unless you carry both alleles of it (alleles are one half of a gene pair).
For instance, if you carry two alleles for the recessive gene for blue eyes, your eyes will be blue.
But if you carry one Brown allele and one Blue allele, you eyes will be brown. This is because blue is recessive, weak, while brown is dominant, strong. The dominant gene will always be expressed.
The only way to have blue eyes therefore is to inherit a recessive blue allele from each parent.
It looks something like this:
*Father_____B________b*______________________________________________________________________
**Mother**____________________________________________________________________________________
**B**__________BB________Bb___________________________________________________________________
**b**__________Bb________ bb___________________________________________________________________
In this table, you can see both the mother and father have both B and b alleles, so they themselves both have brown eyes.
In terms of their children, there are 3 possibilities:
1. 25% chance of BB child – BROWN EYES
2. 50% chance of Bb child – BROWN EYES
3. 25% chance of bb child – BLUE EYES
Two blue eyed parents can NEVER have a brown eyed child, because they carry only the recessive gene, otherwise their eyes would be brown!
Hope this helps!
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