I wanted to talk about the biology behind traits getting passed down to children, maybe kindle some Holla Forumsack interest in biology because it's pretty cool stuff. A while back I posted about some computer simulations I made of foreign genes getting introduced into a population's gene pool. There was a lot of interest so I thought I'd expand it into a full-blown thread.
Some background. DNA, deoxyribonucleic acid, is the molecule that encodes the genome–genetic information that is passed down from parent to offspring. DNA consists of two very long chains built up of a sequence of four bases (adenine, thymine, cytosine, and guanine, or A, T, C, and G). The chains are wound around each other in a double helix held together by weak attraction between the bases pic related. The bases are complementary in the sense that an "A" base on one chain pairs with a "T" on the other, and only a "T". The other two letters "C" and "G" pair in this way also. This is why we consider base-pairs to be the fundamental unit of the genome. Genes are sections of the genome that code for individual proteins, and they're typically thousands of base-pairs long. Only a small fraction of the genome actually codes for proteins.
Each cell in a human's body has its DNA organised into 23 pairs of chromosomes. They are essentially a method of tightly packing the DNA into a smaller volume, preventing it from getting tangled up as it floats within the cell cytoplasm. A chromosome is hundreds of millions of base-pairs long, IIRC. The DNA doesn't exist in this state all the time, because it needs to be loose in order for proteins to be made. Anyway, within a pair of chromosomes, the DNA is very similar between the paired elements, so overall it's like having two copies of the genome. The copies aren't exact, so in some genes they can be the exact same sequence (called being homozygous for that gene) and other genes can have sequences that differ slightly (called being heterozygous for that gene), usually in one particular basepair. A small change, but in many cases it's enough to alter the functionality of a protein. Variants of a gene are called alleles.
When people have kids, half of their genomes are passed down to produce the genome of the kid. The DNA from a single parent gets passed down as one element of each pair of chromosomes, so there are 23 halves, and when sperm and egg meet these halves are united into the child's set of chromosomes. So, a person has a maternal chromosome and a paternal chromosome. The process of selecting which half (within each parent) is fairly complicated. The germ cells that become sperm or eggs undergo a type of division called meiosis, which is a two-stage process. In the first stage, the DNA is packed into chromosomes, and is duplicated, so there are four copies of the genome in the germ cell. This is the stage at which genetic recombination happens, shuffling the parent's DNA within the pairs of chromosomes. The effect of this is that a parent doesn't pass down an entire maternal chromosome or paternal chromosome, they may pass down a new chromosome which is a mix of both. The first stage concludes with a round of cell division, so the germ cell becomes two more cells with the entire genome of the parent except a little shuffled from the recombination. The second stage of meiosis just splits the DNA up again so that it's half of the genome, so there are four cells where each cell contains one of each pair of chromosomes. These are the cells that become sperm and egg.