After I created an understanding of DNA, translation, and transcription in the first step of my IRaD project, I had another question: how do people end up so differently after going through the same genetic copying process? DNA strands make up chromosomes that define who we are as human beings. Every person has 23 chromosomes, each with two copies—one from each parent—so how could one small mistake make such a big difference?
In order to answer my question, I looked at the genetic makeup of my immediate family’s eye color. I used a family graph of my own family to understand how genes and chromosomes are passed on from parent to child. In my family, my mom has brown eyes, my dad has blue eyes, I have brown eyes, and my sister has blue eyes. Our family inheritance of eye color looks like this:
My dad is homozygous and my mom is heterozygous and their children, my sister and I, are one of each. Based on my parents’ eye color, there are four possibilities for their children to be: Bb, bb, Bb, bb. My parents’ children have a fifty-fifty chance of having either brown or blue eyes7.
Genetics are very important to the understanding of Down’s syndrome because Down’s syndrome is a genetic mutation that is passed down from parent to child. It is caused by trisomy 21, when a child is born with three copies of chromosome 21. This can happen two ways. The first, and more common, is when one of the parent’s cells splits incorrectly and gives two chromosome 21s instead of the normal one. When this happens, then the child receives two chromosome 21s from one parent and one from the other creating a trisomy of chromosome 21. Given that this does not occur during the other parent’s cell during meiosis, the child will be born with Down’s syndrome.
The second and much less common way for a child to have Down’s syndrome is when there is a translocation of chromosomes 14 and 211. The rare translocation is when there is a chromosome 21 attached to the 14th chromosome of a parent that is then passed down through their cell and is the copy of chromosome 14 that is given to the child during meiosis3.
My knowledge of genetics, transcription, and translation provide a foundation for my understanding of Down’s syndrome. When there is an extra copy of chromosome 21, extra proteins for the third copy are made through transcription and translation. Having too much of most proteins interferes with normal functioning. In some cases, it can make something work extra which can possibly be extra helpful, but in other cases it can be extremely harmful and even cause a protein to just not work.
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