- An introduction to genetic mutations
- Mutagens and carcinogens
- The effects of mutations
- Impact of mutations on translation into amino acids
- Mutation as a source of variation
- Aneuploidy & chromosomal rearrangements
- Genetic variation in prokaryotes
- Evolution of viruses
Aneuploidy and nondisjunction. Down syndrome and related disorders. Chromosomal rearrangements.
Some things just work well in pairs. Everyday examples include shoes, gloves, and the earbuds on a music player. If you're missing one member of a pair, it's likely to be a nuisance, and might even be a serious problem (for instance, if you're already late for school!).
Pairs are important in genetics, too. Most of your cells contain chromosomes, rod-like structures made of DNA and protein, that come in matched pairs. These chromosomes carry tens of thousands of genes, which tell your body how to develop and which keep it functioning from moment to moment during your lifetime.
If a chromosome pair loses or gains a member, or even part of a member, the delicate balance of the human body may be disrupted. In this article, we’ll examine how changes in chromosome number and structure come about, and how they can affect human health.
Aneuploidy: Extra or missing chromosomes
Changes in a cell's genetic material are called mutations. In one form of mutation, cells may end up with an extra or missing chromosome.
Each species has a characteristic chromosome number, such as chromosomes for a typical human body cell. In organisms with two full chromosomes sets, such as humans, this number is given the name . When an organism or cell contains chromosomes (or some other multiple of ), it is said to be euploid, meaning that it contains chromosomes correctly organized into complete sets (eu- = good).
If a cell is missing one or more chromosomes, it is said to be aneuploid (an- = not, "not good"). For instance, human somatic cells with chromosome numbers of or are aneuploid. Similarly, a normal human egg or sperm has just one set of chromosomes (). An egg or sperm with or chromosomes is considered to be aneuploid.
Two common types of aneuploidy have their own special names:
- Monosomy is when an organism has only one copy of a chromosome that should be present in two copies .
- Trisomy is when an organism has a third copy of a chromosome that should be present in two copies .
Aneuploidy also includes cases where a cell has larger numbers of extra or missing chromosomes, as in , etc. However, if there is an entire extra or missing chromosome set (e.g., ), this is not formally considered to be aneuploidy, even though it may still be bad for the cell or organism. Organisms with more than two complete sets of chromosomes are said to be polyploid.
Nondisjunction of chromosomes
Disorders of chromosome number are caused by nondisjunction, which occurs when pairs of homologous chromosomes or sister chromatids fail to separate during meiosis I or II (or during mitosis).
Meiosis I. The diagram below shows how nondisjunction can take place during meiosis I if homologous chromosomes don't separate, and how this can lead to the production of aneuploid gametes (eggs or sperm):
Meiosis II. Nondisjunction can also happen in meiosis II, with sister chromatids (instead of homologous chromosomes) failing to separate. Again, some gametes contain extra or missing chromosomes:
Mitosis. Nondisjunction can also happen during mitosis. In humans, chromosome changes due to nondisjunction during mitosis in body cells will not be passed on to children (because these cells don't make sperm and eggs). But mitotic nondisjunction can cause other problems: cancer cells often have abnormal chromosome numbers.
When an aneuploid sperm or egg combines with a normal sperm or egg in fertilization, it makes a zygote that is also aneuploid. For instance, if a sperm cell with one extra chromosome () combines with a normal egg cell (), the resulting zygote, or one-celled embryo, will have a chromosome number of .
Genetic disorders caused by aneuploidy
Human embryos that are missing a copy of any autosome (non-sex chromosome) fail to develop to birth. In other words, human autosomal monosomies are always lethal. That's because the embryos have too low a "dosage" of the proteins and other gene products that are encoded by genes on the missing chromosome.
Most autosomal trisomies also prevent an embryo from developing to birth. However, an extra copy of some of the smaller chromosomes (13, 15, 18, 21, or 22) can allow the affected individual to survive for a short period past birth, or, in some cases, for many years. When an extra chromosome is present, it can cause problems in development due to an imbalance between the gene products from the duplicated chromosome and those from other chromosomes.
The most common trisomy among embryos that survive to birth is Down syndrome, or trisomy 21. People with this inherited disorder have short stature and digits, facial distinctions including a broad skull and large tongue, and developmental delays. Here is a karyotype, or image of the chromosomes, from a person with Down syndrome, showing the characteristic three copies of chromosome 21:
About in every newborns is born with Down syndrome. However, the likelihood that a pregnancy will result in an embryo with Down syndrome goes up with a woman's age, particularly above years. This is probably because of more frequent nondisjunction in the developing eggs of older women.
Human genetic disorders can also be caused by aneuploidies involving sex chromosomes. These aneuploidies are better-tolerated than autosomal ones because human cells have the ability to shut down extra X chromosomes in a process called X-inactivation. You can learn more in the article on X chromosome inactivation.
In another class of large-scale mutations, big chunks of chromosomes (but not entire chromosomes) are affected. Such changes are called chromosomal rearrangements. They include:
- A duplication, where part of a chromosome is copied.
- A deletion, where part of a chromosome is removed.
- An inversion, where chromosomal region is flipped around so that it points in the opposite direction.
- A translocation, where a piece of one chromosome gets attached to another chromosome. A reciprocal translocation involves two chromosomes swapping segments; a non-reciprocal translocation means that a chunk of one chromosome moves to another.
In some cases, a chromosomal rearrangement causes symptoms similar to the loss or gain of an entire chromosome. For instance, Down syndrome is usually caused by a third copy of chromosome 21, but it can also occur when a large piece of chromosome 21 moves to another chromosome (and is passed on to offspring along with a regular chromosome 21). In other cases, rearrangements cause unique disorders, ones that are not associated with aneuploidy.
Want to join the conversation?
- And what about a cell/organism containing 2n - 2 chromosomes, supposing these two missing ones are paired up? Is this cell/organism considered aneuploid or euploid?(8 votes)
- It is anueploid. The number of chromosomes for a species is fixed. For example, if a human cell had 44 chromosomes instead of 46, it is anueploidic in nature; nullisomic in fact (2n-2). Just because 44 is an even number doesn't mean it is euploidic! Hope this helped :-)(22 votes)
- How is chromosomal "rearrangement" different from "crossover"?(8 votes)
- Crossovers (recombination events) occur between homologous chromosomes (actually sister chromatids). Meaning, recombination occurs between chr13 sister chromatid from Mom crossing over with sister chromatid of chr13 from Dad.
Duplications and inversions can happen on a single chromosome. So, you can have a region of, let’s say, chr22 duplicated. Or that region might get inverted. Translocations can involve a region of (for example) chr13 swapping places with a region of chr22.
I think the key is that crossovers are typically between homologous chromosomes whereas rearrangements are a broader category where they CAN be between homologs but there are also non-homologous chromosomal rearrangements.(13 votes)
- If there was an instance of a gamete with -1 chromosome and a gamete with +1 chromosome that joined together, would that individual be considered "normal"?(5 votes)
- That’s a really good question. I think you’re right! Though the odds of that happening are extremely rare. Cool thought experiment though!(3 votes)
- If there's a diploid (2n) cell that went through the cell cycle but somehow all of the chromosomes stuck together and went to one daughter cell while the other daughter cell had no chromosomes, is the daughter cell with the chromosomes considered tetraploid (4n) at that point because there are now 4 chromosomes per homologous pair, or would it actually be considered 2n+2n? For instance, if n=12, then it would be 2n+24?(4 votes)
- The likelihood of that actually happening is very rare. But that does not mean it's impossible, there's never a 0% (or 100%) of anything in science (in most cases).
If that was to happen it would be called tetraploid a form of polyploidy. It's not likely to happen but it has happened a species of frogs (from the genus Neobatrachus) has been found where they actually contain 4n instead of the usual 2n. The probability of one of these mutations to occur is low, and then to have this happen [at least] twice and to find each other to mate was probably close to 0%.(4 votes)
- does nondisjunction automatically lead to one monosomy and one trisomy?(4 votes)
- Mitotic nondisjunction can occur with the inactivation of either topoisomerase II, condensin, or separate. This will result in 2 diploid daughter cells, one with 2n+1 and the other with 2n-1.
If nondisjunction occurs during meiosis I, it is the result of the failure of the tetrads to separate during anaphase I. At the end of meiosis I, there will be 2 haploid daughter cells, one with n+1 and the other with n-1. Both of these daughter cells will then go on to divide once more in meiosis 2, producing 4 daughter cells, 2 with n+1 and 2 with n-1.
Nondisjunction in meiosis II results from the failure of the sister chromatids to separate during anaphase II. Since meiosis I proceeded without error, 2 of the 4 daughter cells will have the normal haploid number. The other 2 daughter cells will be aneuploid, one with n+1 and the other with n-1.
If meiotic nondisjunction, then yes. The result is monosomy and trisomy.
- Curious to know if there is any evolutionary effect on how human (and other eukaryotic organisms) chromosomes are ordered. For example, is there something evolutionarily special or significant about the genes encoded on chromosome 1 versus the genes encoded on chromosome 22?(4 votes)
- Is translocation essentially formed from the process of crossing over?(3 votes)
- Good question!
Translocations can be the result of crossing over between sequences that are similar but located on different chromosomes.
One source of these events are the repetitive elements§ that make up most of the genome in many species including humans.
Another way that translocations can happen is if the DNA is broken in multiple places — e.g. by exposure to radiation.
In some cases the DNA will heal, but with the "wrong" parts of chromosomes stuck together.
§Note: A major component of the repetitive DNA comes from the many different families of transposons — pieces of DNA that can copy themselves to new places within the genome.(2 votes)
- Is the annotation of the daughter cells for the nonjunction in mitosis diagram wrong? I think the upper one is 2n-1 and the bottom one is 2n+1(3 votes)
- What happens if nondisjunction occurs during mitosis? I assume the cell would likely undergo apoptosis, but I'm not sure...(2 votes)
- In many cases, polyploidy takes place and one cell ends up with zero chromosomes, while the other one has doubled garniture.
It leads to apoptosis of thew one cell without chromosomes.(3 votes)
- I think the diagram for nondisjunction in meiosis I is incorrect. It looks like sister chromatids failing to separate during mitosis.(2 votes)
- Yes, it is correct. Maybe it looks like that but it is correct. Look at it again. :)(3 votes)