Last week, I explained that most individuals have 22 pairs of autosomal chromosomes and 1 pair of sex chromosomes (for a total of 46 chromosomes all together). But a subset of people has chromosomal abnormalities in the form of an extra chromosome or a missing chromosome. Most chromosome abnormalities are fatal and result in early miscarriages, but some abnormalities are survivable. One of the most common survivable chromosome abnormalities is Down syndrome, in which there are three copies of chromosome 21 instead of two (known as a trisomy). The extra chromosome copy causes physical, mental, and developmental abnormalities.
There are also several possible sex chromosome abnormalities including Turner syndrome (only one X chromosome), Klinefelter syndrome (XXY), and Triple X syndrome (XXX). Chromosomal abnormalities result from errors in the cellular division process that forms gametes. The age of the mother is considered a risk factor for these abnormalities. As proteins involved in cellular division degrade, the likelihood of chromosome abnormalities occurring in the mature egg cells increases. As a result, doctors often recommend prenatal genetic screening for pregnant women over the age of 35.
Assuming you are a human, most cells in your body should contain 46 chromosomes (23 from your mother and 23 from your father). These cells are called diploid (a word meaning a cell that has two of each chromosome—the maternal copy and the paternal copy). Almost every cell in your body is diploid, and diploid cells use a process called mitosis to reproduce copies of themselves. Mitosis is the process that turns an embryo into a full-fledged human, but it doesn’t stop there. Mitosis is happening all over your body, constantly, as your cells and tissues replicate and rejuvenate themselves.
For the most part, the cell itself can divide by simply growing in size and splitting itself in half. The difficult part of mitosis is making sure each cell ends up with a complete diploid set of chromosomes. To do this, the cell starts by replicating each of the 46 chromosomes as faithfully as possible. The copied chromosomes line up along the equator of the cell, and each copy gets pulled to one side or the other. With the copied chromosomes separated, the cell can split into two new diploid “daughter” cells (mitosis always forms daughter cells—I suspect simply because “son cells” doesn’t have the same ring to it).
While most cells in your body are diploid and undergo mitosis, a subset of your cells, the gametes, only have one set of 23 chromosomes (they are called haploid). Gametes form through a different process called meiosis that separates out the maternal and paternal copies of each chromosome randomly. Meiosis starts the same way as mitosis, with all 46 chromosomes being copied. The chromosomes line up at the equator, but rather than the two copies separating, the maternal and paternal chromosomes separate.
Before they separate, the maternal and paternal chromosomes swap some genes in a random manner (a process called recombination). Recombination allows for independent assortment of the genes into different gametes (which is why you can inherit a random set of traits from your mother or father). The resulting daughter cells are haploid with each of the 23 chromosomes having an identical copy. These cells then undergo mitosis to form four haploid “granddaughter” cells with a random assortment of your genes.
When gametes fuse, the 23 chromosomes from the mother and the 23 chromosomes from the father combine to form a new diploid assortment of genes and chromosomes. The random assortment of genes inherited from each parent enables genetic diversity to flourish from generation to generation.
Chromosome abnormalities arise because of errors during meiosis that forms gametes. When the chromosomes fail to separate during either phase of meiosis (known as nondisjunction), it results in gametes that have either extra or missing chromosomes. Nondisjunction occurs because the enzymes and proteins that facilitate separation of the chromosomes during meiosis are broken down or inactivated (nondisjunction can also occur during mitosis—potentially resulting in cancer).
Some chemicals are linked with an increased risk of nondisjunction including alcohol, cigarette smoke, and certain insecticides. Additionally, the age of the mother is considered a risk factor. Since females are born with all of their eggs pre-made, they are more likely to have eggs that are damaged or slightly degraded as they age. Mothers over 35 are typically encouraged to undergo prenatal screening for Down syndrome and other chromosomal abnormalities. And doctors encourage women to freeze their eggs early on if they are considering having children “later in life” (35 doesn’t seem like later in life to me, but biology apparently has its own ideas).
Many chromosomal abnormalities are fatal and result in miscarriage early on. All autosomal chromosome monosomies (where there is only one copy of a chromosome) are fatal. But a few autosomal trisomies (where there is an extra chromosome copy—three in total) are survivable when the chromosome is relatively small. Down syndrome is the most common autosomal trisomy (it’s an extra copy of chromosome 21) affecting about 1 in 700 babies each year in the US. Individuals with Down syndrome may have intellectual disabilities, cognitive delays, heart defects, and an increased risk of developing certain medical conditions like celiac disease or even leukemia.
In the sex chromosomes, monosomies (like Turner syndrome) and trisomies (like Klinefelter syndrome) are less likely to be fatal or debilitating. Turner syndrome, a monosomy where an individual has only one X chromosome, results in females that are shorter than average height and infertile. A sex chromosome monosomy is viable because typically one of the X chromosomes in XX individuals is turned off to keep the dosage equal to that of XY individuals. Klinefelter syndrome, an XXY trisomy, results in males with less testosterone, delayed or incomplete puberty, and infertility. The symptoms of Klinefelter syndrome can be so mild though that up to 75% of affected individuals may remain undiagnosed. Other sex chromosome abnormalities such as Triple X syndrome and XYY are even more mild, resulting mostly in changes in average height. These abnormalities are likely to be undiagnosed unless the individual undergoes specific genetic screening or genome sequencing.
Comment below or email me at contact@anyonecanscience.com to let me know what you think about this week’s blog post and tell me what sorts of topics you want me to cover in the future. And subscribe below for weekly science posts sent straight to your email!
