Chromosomes are tiny structures found within your cells. They contain the DNA information and instructions that define who you are – what you look like, how your body works, and even what genetic diseases you might have.
Humans have 46 chromosomes. But chromosomes come in pairs, so we typically think of them as 23 pairs of chromosomes. The first 22 chromosome pairs (called autosomes) are numbered 1 through 22. We’ll primarily focus on these autosomal chromosomes. The 23rd pair are called the sex chromosomes – men have an X and a Y sex chromosome and women have two X chromosomes.
One autosomal chromosome from each pair comes from your mother and the other comes from your father. This means you get half of your DNA from your mother and half from your father. Each chromosome they pass on to you is a combination of their own pair of chromosomes which they got from their parents (your grandparents).
Parent to child chromosome inheritance
The image above depicts how one pair of chromosomes may be passed from your parents to you. The colors don’t mean anything special – they simply depict the individual chromosomes and chromosome sections.
You’ll notice that the chromosome passed to you from each parent may not be an exact 50/50 combination of their own chromosomes. This means that you might have a bigger portion of one of their chromosomes than the other – you might be more related to one of your grandparents than another on that chromosome. In fact, you might have an exact copy of one of your parent’s chromosomes, and thus you’ll get no portion of their other chromosome.
Grandparent to child chromosome inheritance
If you add in another generation, things get a bit more complex. This depicts just one chromosome pair. Remember that you have 22 pairs that will be various combinations of your grandparent’s chromosome pairs. In this example, one of the mother’s chromosomes (the one she got from her father) was passed on directly to the child. This child will not match his maternal grandmother on this chromosome. I’m not sure how often this non-recombination occurs, but of my 44 autosomes, 6 were not recombined from my parents to me. While lop-sided chromosomes or non-recombination may occur on a particular chromosome, across all 46 chromosomes, things tend to average out – you’ll get around 25% of your DNA from each of your grandparents.
For each generation you go into the past, you will get less and less of that ancestor’s DNA. The chromosome segments they pass on will become smaller or lost due to recombination. This is why autosomal DNA analysis is usually only useful to at most 6 or 7 generations back – you have so little DNA from very distant ancestors that it becomes difficult to analyze it reliably.