Chromosome Variant Factsheet

At the XXY Project we are anti anything that depicts Chromosome Variations of any description as being abnormal because what they’re describing is not the natural variation these differences are but rather something that’s broken and something that needs to be fixed. Medical Science, Clinicians and the like are steadfastly stuck on the notion that anything which deviates from the binary sexes (XX Female & XY Male) is abnormal and so only speak in those terms when describing XXY which then lends itself to additional pathological terms such as Klinefelter’s Syndrome where expectant mothers, those who are aware of the fetuses sex are told the child will have a syndrome which in most instances (87%) would see the fetus aborted. As a community, we owe it to ourselves to change the language used when describing us, doing so would allow for greater overall awareness and the realisation of how being XXY and or Variant is really not as bad as what it is otherwise made out to be.


Chromosome variations can be numerical or structural. A numerical variant means an individual is either missing one of the chromosomes from a pair or has more than two chromosomes instead of a pair. A structural variant means the chromosome’s structure has been altered in one of several ways.


What Are Chromosomes?

Chromosomes are the structures that hold genes. Genes are the individual instructions that tell our bodies how to develop and function; they govern physical and medical characteristics, such as hair colour, blood type and susceptibility to disease.

Many chromosomes have two segments, called “arms,” separated by a pinched region known as the centromere. The shorter arm is called the “p” arm. The longer arm is called the “q” arm.


Where Are Chromosomes Found In The Body?

The body is made up of individual units called cells. Your body has many different kinds of cells, such as skin cells, liver cells and blood cells. In the centre of most cells is a structure called the nucleus. This is where chromosomes are located.


How Many Chromosomes Do Humans Have?

The typical number of chromosomes in a human cell is 46: 23 pairs, holding an estimated total of 20,000 to 25,000 genes. One set of 23 chromosomes is inherited from the biological mother (from the egg), and the other set is inherited from the biological father (from the sperm).

Of the 23 pairs of chromosomes, the first 22 pairs are called “autosomes.” The final pair is called the “sex chromosomes.” Sex chromosomes determine an individual’s sex: females have two X chromosomes (XX), and males have an X and a Y chromosome (XY). The mother and father each contribute one set of 22 autosomes and one sex chromosome.


How Do Scientists Study Chromosomes?

For a century, scientists studied chromosomes by looking at them under a microscope. In order for chromosomes to be seen this way, they need to be stained. Once stained, the chromosomes look like strings with light and dark “bands,” and their picture can be taken. A picture, or chromosome map, of all 46 chromosomes is called a karyotype. The karyotype can help identify variations in the structure or the number of chromosomes.

To help identify chromosomes, the pairs have been numbered from 1 to 22, with the 23rd pair labelled “X” and “Y.” In addition, the bands that appear after staining are numbered; the higher the number, the farther that area is from the centromere.

In the past decade, newer techniques have been developed that allow scientists and doctors to screen for chromosomal variations without using a microscope. These newer methods compare the patient’s DNA to a normal DNA sample. The comparison can be used to find chromosomal variations where the two samples differ.

One such method is called noninvasive prenatal testing. This is a test to screen a pregnancy to determine whether a baby has an increased chance of having specific chromosome differences. The test examines the baby’s DNA in the mother’s blood.


What Are Chromosome Variations?

There are many types of chromosome variations. However, they can be organised into two basic groups: numerical variations and structural variations.

  • Numerical Variations: When an individual is missing one of the chromosomes from a pair, the condition is called monosomy. When an individual has more than two chromosomes instead of a pair, the condition is called trisomy.

An example of a difference caused by numerical variations is Klinefelter’s Syndrome also known as 47XXY, where sex along with the severity of symptoms varies amongst individuals, typical symptoms include sterility, dysfunctional gonads, learning difficulties and poor muscle tone (hypotonia) in infancy.  An example of monosomy, in which an individual lacks a chromosome, is Turner syndrome. In Turner syndrome, a female is born with only one sex chromosome, an X, and is usually shorter than average and unable to have children, among other difficulties.

  • Structural Variations: A chromosome’s structure can be altered in several ways.
    • Deletions: A portion of the chromosome is missing or deleted.
    • Duplications: A portion of the chromosome is duplicated, resulting in extra genetic material.
    • Translocations: A portion of one chromosome is transferred to another chromosome. There are two main types of translocation. In a reciprocal translocation, segments from two different chromosomes have been exchanged. In a Robertsonian translocation, an entire chromosome has attached to another at the centromere.
    • Inversions: A portion of the chromosome has broken off, turned upside down, and reattached. As a result, the genetic material is inverted.
    • Rings: A portion of a chromosome has broken off and formed a circle or ring. This can happen with or without loss of genetic material.

Most chromosome variations occur as an accident in the egg or sperm. In these cases, the variation is present in every cell of the body. Some variations, however, happen after conception; then some cells have the variant and some do not.

Chromosome variations can be inherited from a parent (such as a translocation) or be “de novo” (new to the individual). This is why, when a child is found to have a variation, chromosome studies are often performed on the parents.


How Do Chromosome Variations Happen?

Chromosome variations usually occur when there is an error in cell division. There are two kinds of cell division, mitosis and meiosis.

  • Mitosis results in two cells that are duplicates of the original cell. One cell with 46 chromosomes divides and becomes two cells with 46 chromosomes each. This kind of cell division occurs throughout the body, except in the reproductive organs. This is the way most of the cells that make up our body are made and replaced.
  • Meiosis results in cells with half the number of chromosomes, 23, instead of the normal 46. This is the type of cell division that occurs in the reproductive organs, resulting in the eggs and sperm.

In both processes, the correct number of chromosomes is supposed to end up in the resulting cells. However, errors in cell division can result in cells with too few or too many copies of a chromosome. Errors can also occur when the chromosomes are being duplicated.

Other factors that can increase the risk of chromosome variations are:

  • Maternal Age: Women are born with all the eggs they will ever have. Some researchers believe that errors can crop up in the eggs’ genetic material as they age. Older women are at higher risk of giving birth to babies with chromosome variations than younger women. Because men produce new sperm throughout their lives, paternal age does not increase the risk of chromosome variations.
  • Environment: Although there is no conclusive evidence that specific environmental factors cause chromosome variations, it is still possible that the environment may play a role in the occurrence of genetic errors.

Source: National Human Genome Research Insitute 

 

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