In high-school I learned that your DNA separates into your fathers contribution and your mothers contribution and these get doubled and then doubled again. This creates four sperm in males and two in females, only two because the egg keeps the entire cell with the cytoplasm and all the mitochondria, so that when it joins with a sperm cell it becomes a whole cell. The sperm is not a viable cell on its own and thus dies shortly after it is created. Interestingly enough, it is because of the manner of these differences that have allowed us smart humans to track the history and diversity of humanity, because only the egg has the mitochondria and mitochondria is a separate species that lives within all of our cells. The mitochondria all come from our mothers and from their mothers and their mothers until we shared the same mothers. And since the mitochondria DNA mutates ate a consistent rate we can track the divergences across humanity to determine who is related to whom and how far back in history we can co before the populations converge. And the answer is that humanity had a common source about one hundred thousand years ago. But this pales when compared to the awesomeness of Sexual Recombination.
So it is true that half your DNA comes from you mother and half from your father, but it is also true that one quarter of your DNA comes from each of your grandparents and one eighth of your DNA is from you great grandparents, one sixteenth from their parents and so on. So it is not as clear cut as high-school claimed, but we knew that we just did not get to see behind the curtain to see the wizard working.
When normal DNA splits to create a new cell, to build up your body or to replace damaged cells, your DNA copies itself completely and the two sets split up the cytoplasm and the mitochondria and then e cell walls pinch and there are two cells instead of one. It gets more complicated than that, the DNA splits the two copies are split apart and the nucleus splits apart separates so the cell can divide evenly. In sex cells they do this too, but then the magic happens.
So for the people that don't know what is in every single cell in their body, each cell has a blob in the center called the nucleus. In the nucleus is a copy of all the chromosomes in your body, how many chromosomes does not really matter because it does not have an affect on the creature that has them, some creatures have more than humans and som have less. I think the fruit fly has eight chromosomes and humans have twenty-three, but a fruit fly has more genes than us. We have one set of chromosomes from each of our parents and each set of chromosomes has one copy of our genes.
The genes are obviously in pairs and not every copy of the gene is identical; a lot of them are different and that is why everyone looks different. For any particular gene, there may be many different versions of that gene, like for eye colour. Most people's eyes are brown, but some are light brown and others dark brown, some are nearly black. Some eyes are blue, light blue through dark blue, then there is green and hazel and there are variations in those colours too. These are controlled by genes and the variations within a family unit, or the lack of variation is an example of the interplay between the differences. My father has hazel eyes, my mother has hazel eyes, my sister has hazel eyes and I have blue eyes. Both my parent have one copy of the gene for blue eyes and a copy for hazel eyes and since the hazel copy is more powerful than the blue, it is dominant. So every pair of genes a person has falls into a double dominant or double recessive or a dominant and recessive pair. My parents have a dominant/recessive eye trait, I have a double recessive blue eyes and my sister has either double dominant or dominant/recessive eye colour trait. And there is this type of relationship for many genes in our bodies.
So, you receive one set of genes from each of your parents and these genes are combined in you to make you unique to your parents and all your siblings, unless you have an identical twin.
Your father gives you a set of genes that I will call Yellow. Your mother gives you a set of genes that I will call Blue. Some of your Blue genes are dominant over your Yellow genes and vise-versa. Your genes are Yellow and Blue. When it comes to sexual recombination, your genes get copied and split as the above mentions. Then your chromosomes get sticky and stick together, your chromosomes look like a line that splits down the center, lengthwise, one side yellow and one side blue. The chromosomes are stuck where some of the genes begin and end, not all the genes just some of them randomly, by chance. Where the genes get sticky, the break, either before or after the gene. The broken chromosomes then switch and heal the break. And it happens all over each of the chromosome pairs at the same time. When the process is finished you have two chromosomes that are a checkerboard of blue and yellow with the opposite is similar, yellow where blue is in the other and blue where yellow is in the other. They, the two chromosomes split and become sex cells, two sperm cells or one egg and one dead egg, because the dead egg does not have the cytoplasm an egg needs.
So in recap, your mother gives you one blue chromosome and your father gives you a yellow chromosome, so that you can have two of each set of genes. Your sex cells are green, a mix of all the yellow and blue genes you have.
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