“An extra chromosome in our genome means a person or fetus has one additional chromosome with a pair besides 23 pairs and causes serious health issues.”
‘Chromosome’ is a common term in recent times, we know, it is the unit of inheritance or a place where the DNA is located. Scientifically, a network of histone proteins and DNA is our chromosome having two arms known as p and q arms, a centromere and telomeres, is known as a chromosome.
Genes- a chain of DNA, are located on arms while the portion of telomere and centromere is made up of repetitive DNAs. Any sudden alteration in chromosomes causes serious problems for a person or baby.
There are 46 chromosomes present in pairs in humans means a total of 23 pairs. The event known as non-disjunction during the cell division, especially in meiosis 2 causes an imbalance in chromosome number.
When the normal chromosome number changes, it is known as aneuploidy (scientifically). Here in the present piece of the article, I am explaining the topic “extra chromosome” and try to answer questions related to it. I will also explain some of the common extra chromosomal disorders.
Read more: What is Chromosomal Translocation?- Definition, Mechanism and Types.
What does it mean to have an extra chromosome?
When you see your chromosome report, probably the present question arises in your mind, in fact, it strikes in everyone’s mind who is not from a biology background.
The chromosome test or karyotyping technique, a type of cytogenetic technique, commonly employed to study chromosomes or we can say to find out an extra chromosome.
As we said, there are a total of 46 chromosomes in 23 pairs that are present in our genome. The genome is the total DNA content of all chromosomes.
When nondisjunction occurs during cell division, chromosomes distributed abnormally in cells cause an imbalance in chromosome number. An extra chromosome that occurs with a pair is known as “trisomy” of that particular pair which means a person or fetus may face health issues.
When an extra chromosome occurs with different pairs it causes variable phenotypic effects or adverse conditions. Here is an example to make you understand,
In a normal diploid cell, two homologous 21 number of chromosomes are present which causes normal development of a fetus, but when an extra chromosome 21 is present with the pair, known as trisomy 21 or three 21 chromosomes it results in Down syndrome, a serious mental and cognitive problem.
Here are some more examples of trisomies or an extra chromosome:
| No | Chromosome | Normal (2n) | Extra chromosome | Condition |
| 1 | 13 | Two chromosomes 13 | Three chromosomes 13 | Patau syndrome |
| 2 | 18 | Two chromosomes 18 | Three chromosomes 18 | Edward syndrome |
| 3 | 21 | Two chromosomes 21 | Three chromosomes 21 | Down syndrome |
| 4 | 9 | Two chromosomes 9 | Three chromosomes 9 | Trisomy 9 |
| 5 | 8 | Two chromosomes 8 | Three chromosomes 8 | Warkany syndrome 2 |
| 6 | 22 | Two chromosomes 22 | Three chromosomes 22 | Trisomy 22 |
| 7 | XXX | Two X chromosomes | Three X chromosomes | Triple X syndrome |
| 8 | XXY | One X and Y chromosome in male | Two X with a Y chromosome in male. | Klinefelter syndrome |
Trisomy 13, 18 and 21 are the most common extra chromosomal disorders present in us.
How extra chromosomes occur?
Keep in mind one thing, Extra chromosome, or extra chromosomal condition doesn’t occur suddenly or due to bad habits, probably.
Though it is not common, it occurs naturally without any known reason. Henceforth, no habits, lifestyle, food or other factors are responsible for it. It occurs randomly.
Let’s understand how it happens:
Germ cells- egg and sperm fertilize to form the embryo. Both are haploid cells and each have a single pair of chromosomes. I.e 23 and 23 in egg and sperm, respectively.
They fertilize and make a diploid embryo having 23 pairs (46) chromosomes, each pair from either parent. The event trisomy occurs right before it when individual germ cells do meiosis.
We know meiosis and mitosis right. Miosis governs the cell division of germ cells. When meiosis progresses, due to unknown reasons, chromosomes can’t segregate correctly. This means some germs cells have 22 chromosomes while some have 24 instead of 23.
If we take a condition of an extra chromosome with a germ cell (24 chromosomes) when it is fertilized with another cell, instead of normal 46 chromosomes an additional chromosome inherited in the fetus.
This condition having 47 chromosomes is defined as trisomy or extra chromosome. (This is a simple and non-technical explanation of the mechanism of how trisomy occurs).
Related article: 10 best karyotyping activities and assignments.
Why does an extra chromosome cause problems?
Now this question is very interesting and important too. To go one step ahead as a scientist or students of science we need to understand why it causes problems.
Genes are located on chromosomes, have a definite function and definite numbers. Genes make protein via translation, various proteins, participate in different metabolic and developmental activities, that’s why genes are important.
Two copies of genes are located on two different homologous chromosomes (one on which chromosome)- no reason to worry. An extra chromosome means the third copy of the gene or gene sets located on that chromosome- big reason to worry!
For example, 337 genes are present on chromosome 18, which means, 337 on one chromosome 18 and 337 on another. In the case of trisomy 18, these numbers of chromosomes triplicated.
Resultantly, the gene expression got imbalanced. Gene expression is an important phenomenon that regulates or controls how and in which amount the mRNA is formed.
When the number of mRNAs gets changed, the amount of protein formed from it will also change. This causes serious problems associated with genes located on this chromosome.
What are some extra chromosome disorders?
Extra chromosomes may occur with any pair or in any person, it’s random, although some are so common such as trisomy 21, trisomy 18, trisomy 13, trisomy X and XXY.
Here in the present section we have explained some of the common types of extra chromosomal disorders.
List of missing chromosome disorders
Trisomy 21- extra chromosome 21:
Trisomy 21 also known as Down syndrome is one of the most common numerical chromosomal abnormalities having 3 different chromosomes 21 instead of 2.
Occurred by the event known as non-disjunction, trisomy 21 causes serious mental retardation and cognitive failure by birth.
Flattened face, small head, short and thick neck, unusual ear shape, poor muscle tone, mental and cognitive problems are common symptoms of the present condition.
Down syndrome can’t be cured and occur by birth. Read more: Karyotype of Down Syndrome (Trisomy 21)- Explained.
Trisomy 18- extra chromosome 18:
Another common trisomy known as Edward syndrome occurs by the non-disjunction and results in triplication of chromosome 18.
Common symptoms of present condition are prominent back head and small head, small mouth and jaws, small sternum, feeding problems, heart defect and clenched feets.
Edward syndrome can’t be cured and occur by birth as well (a type of congenital chromosome defect). Read more: A karyotype of Edwards Syndrome- Explained.
Trisomy 13- extra chromosome 13:
The less common trisomy 13 also known as Patau syndrome occurs by the same reason explained above- the event of nondisjunction and abnormal distribution of chromosomes during meiosis.
It has an extra chromosome 13 with the pair and present 1 in 15,000 livebirth (review).
Some of the common symptoms of the present condition are; underdeveloped nostrils, eye problems, low birth weight, small and sloppy head and forehead, respectively, low-set and unusual ears, Hernia and cleft palates.
Patau syndrome is a congenital, incurable birth defect.
Trisomy X- extra chromosome X:
Extra X chromosomes cause two different conditions in males and females in females it is triple X while in male it causes Klinefelter syndrome or XXY.
Trisomy X:
The present condition is also referred to as 47, XXX, triple X syndrome or superfemale. It only occurs in female individuals and has no severe abnormal phenotypes.
The reason for the triple X syndrome is the same as explained above.
Females having triple X chromosomes are taller than normal ones and have normal fertility.
Sometimes weak muscle tones, kidney difficulties and learning problems are observed but are mild.
The reason why abnormal health issues are lesser, associated with the present condition is the mechanism known as X chromosome inactivation that inactive X chromosomes express abnormally.
47, XXY:
47, XXY is also known as Klinefelter syndrome only occurs congenitally in male individuals and occurs by abnormal chromosomal distribution. The condition is also referred to as sex chromosome abnormality.
Some of the common symptoms of present condition are; Taller than average, broad girl like hips and longer legs, less facial and body hair, gynecomastia, small male genitalia.
Note that the 47, XXY is not a type of trisome but occurs by the presence of one extra X chromosome in males results in 47 chromosomes.
Extra Y chromosome:
Extra Y chromosome isn’t possible in females, because the Y chromosome is only present in males only, however in males as well the presence of Y chromosome can’t produce substantial adverse health conditions.
The condition is known as XYY or Jacob’s syndrome. Note that only YY chromosome isn’t possible in nature, there are important genes present on X chromosomes required for normal development in both males and females.
The XYY or extra Y chromosome known as Disomy of Y have some mild to fewer health conditions which aren’t so noticable.
If you want to learn more on the present topic you can read our article: Are YY chromosomes possible?
Do people with extra chromosome abnormalities such as the triple X syndrome die early?
No. In any of the extrachromosomal disorders, early death isn’t reported. This means people with trisomy 21, trisomy X, trisomy 18 can live a long and normal life.
Detection of extra chromosome:
One of the classic, most reliable, affordable and trusted methods for encountering extra chromosomes is karyotyping.
Samples like whole blood, amniotic fluids or chorionic villi are processed, cultured using different culture media under aseptic conditions for 72 hours.
Metaphase chromosomes are stained and banded using the giemsa and observed through microscopy.
Experts observe various well separated metaphase plates, capture images and prepare karyogram or karyotype to interpret results.
They cut and stick chromosomes with a pair, this can be done computationally as well. Any extra chromosome if present can be indicated in the result.
The whole test is known as chromosome test, karyotype test or genetic test (broadly).
For congenital or prenatal studies amniotic fluid or chorionic villi samples are isolated and used for karyotyping.
Other techniques like FISH, spectral karyotyping and Microarray are also used for detecting extra chromosomal conditions.
Related article: Interpreting human karyotype report.
Conclusion:
If your baby has an extra chromosome condition listed above, you have to worry about it, those are serious health conditions which a patient has to bear through their life.
Some have serious mental problems while others are developmental or reproductive. Take advice from a genetic counselor or your doctor for further information and management.
Remember none of the parents, not their bad habits or lifestyle is responsible for conditions like extra chromosomes therefore do not blame each other. Try to support each other, instead.