... I don't get why a gene is subject to evolution whether it is masked or not. Because it shows and causes behavioural changes? Unless I'm missing something here...
Perhaps mutation is the better word here, since mutation can happen at all levels, randomly. Our bodies' genes mutate repeatedly - and natrually, throughout our entre lives as well, in fact, hehe.
But yes, seems some people need a guide to genetics and Punett squares again - here's a quickie.
This should be like, Biology and Genetics 101. You probably know this stuff, but if you don't, for the sake of us all, read.
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First, let's go over meiosis. It's the reproduction of sex cells. Basically, it divides your 46 chromosomes (23 pairs) that compromise your DNA, into four 23 chromosome cells, with characteristics divided up randomly. Say a part of your DNA DNA is AA,Bb,Cc,dd,Ee,Ff - then it can be AA,bb,CC,dd,EE,Ff or AA,BB,cc,dd,ee,FF etc. etc. And then, two these 23 chromosome cells, one from mother and another from father, come together, and forms a complete 46 (23-pair) cell. Each chromosome aligns with its counterpart from one's other parent and pairs up, thus 23 pairs.
And thus, you have a 50% chance of passing on any particular gene to your offspring. Take the X and Y chromosome for example. The male will make 2 X chromosome and 2 Y chromosome cells. The female can only give X chromosomes. Sooo, who's fault is it that you're not having sons? Ding, ding, ding, It's the Guy's fault. Jerks.
In genetics, traits are caused by a specific set of genes - like there's a set of gene for your hair, your eye colour, so on. (Lies, they can be affected by multiple alleles, but let's keep it simple. I'll be simplifying other things along the way as well.) There are also multiple kinds of traits available - for example, not everyone in the world has pink hair or anything. Some have Black, some have Blonde. And for each characteristic, you get a set of genes from your father and from your mother.
So what happens if your father and mother has different hair colour? What determines your hair colour? It obviously doesn't mix them together, but there are traits that are Dominant and Recessive - represented by a CAPITAL and lowercase letter respectively. Dominant traits can "mask" its corresponding recessive traits - meaning only the dominant traits will show up, and despite having that recessive traits within your genes, it won't actually do anything. Say your father is pure blonde and mother is pure black. Since black is dominant and blonde is recessive, you'll get black hair - despite having blonde genes. But, if you were to knock up a spouse who also had the same genes, black dominant and a blonde recessive, you have a chance of giving birth to a blonde child. Best shown by Punnett square.
Generation 1 - Let B = black hair, b = blonde hair
...B B
b|Bb Bb
b|Bb Bb
Your parents are pure black and pure blonde (homozygous can also refer to 'pure'). So, they can only give one specific gene, B or b. However, because B is dominant and b is recessive, all of their offspring - you and your siblings will have black hair. Let's take up you and your spouse then.
Generation 2- Let B = black, b = blonde
...B b
B|BB Bb
b|Bb bb
Despite the fact that both of you are black, you will give one pure black, two 'heterozygous' black (meaning they also have the recessive trait), and one blonde. This is why blondes still exist in the world. Even though Dominant traits mask them, they do not affect recessive traits being passed on at all - meaning it still can equally be passed down. And despite the fact that you don't know about some rare disease of doom, and nobody had it for generations - it can still have been passed on down to you. And if your spouse also has that super rare disease of doom gene... Well, 25% your offspring will be born with the imbecile trait. (And no, genius traits defy the laws of genetics so they will never be passed down. Don't ask me why, they just do.)
So what does this have to do with making sweet, sweet love to your sister/brother/father/mother etc?
Well, basically this has been explained above already. Most genetic diseases are recessive. Therefore, as long as you get someone with a different gene for that set of evil genes, it'll almost never show. So, then what happens if you mate your sister? Well, you're both your parents offspring, so that likelihood of you both having that super evil gene is much higher than it would be going off with a random guy on the street. Meaning, you now have 25% chance of your kid being an Imbecile.
This is why inbreeding is dangerous. It can reveal numerous masked recessive disease traits that's been hidden down your family line, and it increases the chance of breeding Imbeciles, Slows, Hunchbacks and of course, Inbreds, exponentially. This is no problem if your family has none of such traits but... Yeah, good luck with that.
Basically, the Spanish Hapsburgs went and tried that. Looks like they had some bad genes in them after all.