When Chromosomes Don't Splice
Sadly, the world isn’t perfect. Even meiosis doesn’t always occur without problems. Every once and a while, when everything seems to be going perfectly, something unexpected happens. Keep in mind that the number of chromosomes in a diploid cell has to be divided in half in a haploid cell. One of the most important steps in this process is the segregation of homologous chromosomes in separate cell during the first meiotic division. Sometimes a pair of chromosomes doesn’t split and end up in the same gamete.
Eventually, two of the four cell that are produced during meiosis will miss a chromosome and the genes on that chromosome. Most of the times these cells perish. Each of the remaining cells will have an extra chromosome, and thus some extra genetic material. But isn’t that good? Doesn’t this mean more genetic variation, which is a good thing? Well, no.
Sometimes these ‘stuffed’ cells die and that’s that. However, occasionally they survive and become a sperm or egg cell. Things really get tragic when an abnormal cell and a normal one merge. When this happens, the resulting zygote will have three specimens of the same chromosome instead of two. The term denoting this is trisomy.
And herein lies the problem: all cells that are formed through mitosis to ‘build’ the new individual will be trisomous, and cell don’t really know how to handle that. To be viable, a female individual needs two X chromosomes. When there are two active X chromosomes in a female individual with three X chromosomes, this individual would die. So, in each cell one of the sex chromosomes is randomly turned off and converted in an unexpressed Barr body.
- Example: Down syndrome
One of the possible consequences of an extra chromosome is Down syndrome, a condition that often goes together with slow mental development and premature aging. Scientists have discovered which chromosome is connected with the syndrome, and that is chromosome 21. When an egg cell with two chromosomes 21 is fertilized by a normal sperm cell, which has only one sample of this chromosome, the descendant will have 47 chromosomes (24 + 23) and subsequently has Down syndrome.
Most people already know that the risk of genetic abnormalities increases with the age of the mother. But do you know why? In women, meiosis already starts in the fetal stage. When the first meiotic division is completed, these cells remain in the ovaries, waiting for puberty, when one of them undergoes the second meiotic division each month and prepares for fertilization. When a cell had to wait 40 or 45 years for its turn, which means that a lot of time has passed in which chromosomal problems might have occurred, which leads to an increasing chance of genetic abnormalities in the fetus.