Genetically why do some traits get merged and others don't?

Some genetic traits are dominant. Brown eyes are dominant, blue eyes are recessive.

I think the blending of skin colour is the more dominant reflection of mixed race children. To have a child of mixed race be lacking in melanin or very dark with it would be less chance.

Genetics is a crap shoot. You never really know how the babies will turn out, but there are certain things that can be predicted almost perfectly in some cases, like eye colour, and certainly blood type. If pappa’s mommy and daddy and pappa all have brown eyes, and mommy’s mommy and daddy was one blue and one brown, but mommy’s eyes were brown, there is no chance little Junior is going to have blue eyes because the brown eye’d pappa side is going to over run the one blue eyed grandparent.

Here’s a picture showing how blood types are inherited: http://en.wikipedia.org/wiki/File:Codominant.jpg

Not only how we look, but also the inherited diseases we get….. like Cystic fibrosis. CF is recessive, so both parents have to be carriers for them to have a child with the disease. Same with sickle-cell anaemia, a disease we have malaria to thank for, it would seem.

Haemophilia is also another one, but it happens to be sex-linked, meaning it is more likely to occur in one sex, in this case males more than females. This is because females have two X chromosomes while males have only one, so the defective gene is guaranteed to manifest in any male who carries it. Because females have two X chromosomes and haemophilia is rare, the chance of a female having two defective copies of the gene is very low, so females are almost exclusively asymptomatic carriers of the disorder.

In my family, we have a genetic mutation called polydactyly. In our family it represents in girl babies as extra toes and in boy babies as extra fingers. I had an extra toe on each foot (non functioning) and my son was born with an extra finger, (also non functioning).

Here’s a good quick and dirty Wiki page: http://en.wikipedia.org/wiki/Autosomal_dominant

Here is what you were talking about with skin colour: http://en.wikipedia.org/wiki/Human_skin_color

First, when a black and white couple have children the hair can turn out many different colors or textures depending on the parents hair. Same with color of their skin.

My girlfriend is the darkest in her family, she is a great grandchild of a slave owner; all other family members are black. Her Uncle lived as a white doctor. Her mom used to send her down the street for a neighbor to do her hair, because she did not know what to do with it. Her siblings had hair more like a white person. In America many many of our black citizens have some white in them and the recessive traits sometimes show up in their children even when they themselves are considered black.

@cazzie You are incorrect about the eye color. The brown eyed person can be recessive blue. So that parent has a 50–50 chance of throwing off the blue gene to an offspring. If both parents are brown-eyed recessive blue, the kid has a good shot of being blue eyed.

And genes can be similar to the grandparents a lot. I’ve seen a couple of white and black mixing up [with Asian grand grandparents and had the kid to look like half Asian and they all were “wtf!!” until someone told them they have Asian grand grandparents. Then they chilled out but it’s really weird. To see a white man marrying a black woman and have an Asian kid :P

Some phenotypes are controlled by a single gene while other phenotypes are controlled by several genes working together. Those that are controlled by a single gene it is easy to see what’s going on and how the genotype relates to the phenotype. Once you have even 3 genes working together to control the expression of something, say skin colour for example, it gets way too complicated to see what’s going as there are just so many variables.

JLeslie… that is why I went into the grandparents. Two brown eyed parents having a brown eyed child having a grandchild.

@cazzie But, both brown eyed grandparents can be recessive blue and have a brown eyed child recessive blue. Or, did I get confused?

That goes too far back.

@cazzie You brought up the grandparents.

@JLeslie to answer your question. Yes. and Yes.

@JLeslie They may throw a slightly lighter eyed child, like hazel, but it is extremely unlikely that they will have a blue or light eyed child. If I were the ‘brown+brown=brown’ father and my brown eyed wife had a blue eyed child, I’d secretly run a paternity test. (all children’s true eye colour doesn’t come out for years, though.)

@cazzie I think you are thinking of two blue eyed people having a brown eyed child?

@JLeslie no, I am not. Blue is recessive.

@cazzie I’m not sure why we are disagreeing? A brown eyed person can be recessive blue, we agree on that right? I would agree statistically if blue had not shown up in many generations probably the recessive trait does not exist in the particular people we are speaking of, but it is not impossible. My maternal grandfather had brown eyes, his entire family who I know had brown eyes. Both his children were blue eyed, he was obviously recessive blue.

I am trying to account for the parents’ parents’ eye colour in the example for the very reason that I’m trying to demonstrate that if only ONE of the child’s parents has a recessive blue, the child won’t have blue eyes.

If BOTH parents had recessive blue, that is a completely different story. My example is for only one parent to be recessive blue. See where I was going with it now?

@cazzie Yes, ok, if one parent is recessive blue than the child won’t have blue eyes, there we agree.

Some traits are determined by a single gene, others by several genes combined (height, skin colour, etcetera). Plus there is the whole recessive/dominant business.

Yes, some traits are determined by a single gene, and others are determined by a group. Also…if a gene or group proves to be successful, it will of course be passed on.

@Blackberry Also…if a gene or group proves to be successful, it will of course be passed on.

Actually, That’s not how it works. The genetic combination’s success determines how much time the organism survives and thus how many chances of reproduction it gets, therefore the creatures with more favourable genetic combinations will produce more offspring on average, making the gene dominant over time.

@Thammuz Oops, that is correct. Sorry!

Sometimes it can be a big surprise. My first husband had blue eyes. His family was so proud of the often repeated tale that they were 100% descended from the original inhabitants of Central America, that his blue eyes were a shock.

His mother and her family actually had to leave the small town she was born in because of her tainted blood.