Chromosomal abnormalities occur in around 1 in 200 births and account for up to half of all miscarriages.

As a cell gets ready to divide, chromosomes are separated into each of the dividing cells (see what is cell division?) through proteins that bind and physically separate and move.

A failure in this chromosome distribution during the process of cell division, in the making of gametes, results in sperm or oocytes lacking, or gaining, a chromosome. At fertilization these will produce cells, and subsequent embryos, containing only a single chromosome or three chromosomes (trisomy) instead of the usual pair.

These numerical chromosomal abnormalities tend to occur more in females during oogenesis (the production of oocytes), and the chance of this occurrence increases with maternal age; it is suspected that the aged molecular apparatus in these cells, involved in the cell division, leads to mistakes in chromosome separation as a cell divides.

Although extra numbers of any chromosome can occur, only extra copies of the X and Y chromosomes (Klinefelter’s, Triple X and XYY syndromes), the 21^st (Down’s syndrome) and, to a smaller extent, the 18^th and 13^th chromosomes, have any compatibility with life. A foetus lacking any chromosome of a pair, apart from the X chromosome (Turner’s syndrome), is unable to survive.

Normal males inherit an X and a Y chromosome while females have two X chromosomes.

Because females have two X chromosomes, they must inherit two copies of every gene on the X chromosome. In contrast males only inherit one X chromosome and one Y chromosome. This might mean that females could produce twice the amount of protein from these genes, on the X chromosome, than males. Therefore, to overcome this, females maintain only a single active X chromosome in each cell, while the second X chromosome becomes inactivated. This inactivated X chromosome is referred to as a Barr body and different cells randomly inactivate either of the two X chromosomes, so it is not the same X chromosome activated in every cell.

But there are around 18 genes on the X chromosome that are also found on the, very gene-sparse, Y chromosome. As there should be no gene imbalance between males and females for these genes, they escape inactivation on the otherwise inactivated X chromosome in females.

The importance of a Y chromosome for in male sex determination was first confirmed when a peculiar cat was discovered… a male calico!

Calico cats are normally females. This is because the gene for fur colour, containing a black and a ginger variant (i.e. allele), is on the X chromosome. Therefore, only a female with two X chromosomes and the two different alleles for the colours can produce the calico colour patterning. The pattern of a calico cat demonstrates the phenomenon of X inactivation pattern; the patches of ginger and black depend on which X chromosome is active i.e. if the X chromosome carrying the black colour gene is inactivated, then the skin here will produce the ginger colour from the active X chromosome. When a rare male calico cat was found (who had an XXY sex chromosome karyotype) it showed that the presence of two X chromosomes did not lead to femaleness, and instead confirmed that it is a copy of a Y chromosome that is needed to determine maleness.

An individual inheriting an extra chromosome 21 (i.e. three copies) has Down syndrome. The extra set of chromosome 21 genes, consequently produce increased levels of certain proteins, leads to increased risks of hearing and vision defects, heart abnormalities and also a range of developmental difficulties involving delayed coordination skills and mental abilities such as speech and short-term memory.

People with Down syndrome can lead independent lives. Chris Burke, best known for his role as Corky Thatcher in the TV series “Life Goes On”, became the first person with Down syndrome to star in a weekly television series.

Sujeet Desai is an accomplished professional musician, playing 6 musical instruments as well as holding a 2nd dan black belt in Tae Kwan Do, being a “Sensei” martial arts teacher, and winning a number a gold and silver medal for swimming at the 1999 Special Olympics.

In march 2011 William Loughnane, 26, from County Clare, become the first person with Down’s Syndrome to pass a driving test in Ireland, needing just five lessons.

Individuals inheriting just a single sex chromosome of a pair, i.e. the X chromosome have Turner syndrome. Having fewer copies of those few genes on the missing X chromosome individuals are usually shorter in stature.

Standing 145 cm tall, the actress Linda Hunt, has been suggested to show typical Turner’s sydrome.

Also suggested is Missy Marlowe, a 1988 Olympic gymnast and 1987 Pan-American Games gold medalist, who is now spokesperson for the Turner Syndrome Society.

The inheritance of multiple X chromosomes results in a triple-X female. Having more gene copies, especially those coding for bone growth, leads to individuals being taller than average with unusually long legs and slender torsos, but otherwise appear normal and are fertile.

One lady with this anomaly was Ewa Klobukowska, a 1964 Olympic sprint bronze medallist who was the first woman to fail the sex chromosome test during the 1967 European Cup. “One chromosome too many to be declared a woman for the purposes of athletic competition” was the statement, and she was obliged to return all her medals. She gave birth to a baby the following year.

Inheritance of an extra Y chromosome, known as XYY syndrome, can lead to higher levels of testosterone. This results, in sometimes severe, facial acne during adolescence and increased height. Though generally such people appear entirely normal and often completely unaware of their condition. Occurring in around 1 in 1,000 males, a small percentage are infertile, due to the production of increased amounts of certain hormones causing in inadequate sperm production.

Stefan Kiszko, who was jailed for the murder of 11-year-old Lesley Molseed in Manchester in 1975, had XYY syndrome. He spent 16 years in prison before he was released, after evidence showed that his semen samples contained no sperm, in contrast to the semen found on Lesley Molseed’s clothes, which did contain sperm. The semen sample from the crime scene was subsequently used to provide the DNA evidence which helped to convict the real murderer who is currently serving a life sentence for the crime.

However, it is still often suggested that the high testosterone levels of XYY men can make them more prone to violence and criminal activity. Much of this stems from some studies between 1965 and 1968 although many later studies cast serious doubt on any direct and simple linkage. Around this time was the murder trial for Richard Speck who raped and brutally murdered 8 nurses in a Chicago dormitory. His attorney argued that his acne and aggressive behaviour was proof of XYY and therefore he could not be considered legally responsible for his uncontrollable urges. Two separate chromosome tests that showed him to be normal XY were overlooked in the confusion of courtroom drama, and decades later many books still associate Speck with the XYY condition. Nevertheless he was sentenced to life in prison where he eventually died.

The idea of a link between XYY and criminality was used as the basis of the television fiction series “The XYY man” about an individual with XYY, and thereby a natural criminal.

The “Alien 3” movie also used this theme, being set in an off world penal colony for XYYs.

Individuals who inherit more than one X chromosome but still possess a Y chromosome are male known as Klinefelter syndrome (XXY).

This is one of the most common chromosomal abnormalities (1 in 1,000) births leading to increased height some feminine body contours little facial and body hair and lower testosterone levels. Though most are fertile and have normal mental abilities. It should also be noted that people with Klinefelter syndrome are not more inclined to be homosexual, although a small number (such as those listed) may be transgendered.

The artist Lili Elbe was thought to have had this disorder. He married Gerda Wegener, one of the most influential Art Deco artists of the early twentieth century, and cross-dressed for her when she needed a female model. He actually became Gerda’s favourite model and the 1920’s small breasted feminine ideal may have been influenced by Lili’s figure. Lili then underwent sex reassignment surgery and it was under one of these surgical procedures that she died in 1931. This story is now the focus of a major movie where Lili is played by Nicole Kidman.

The Dutch actress Véronique Françoise Caroline Renard was diagnosed with Klinefelter’s syndrome and in 1982, at the age of 17, transitioned from male to female. She added her third name after one of her friend Caroline Cossey, who was also diagnosed with the condition.

Some people have inherited more than two X chromosomes along with the Y chromosome resulting in karyotypes such as XXXY, which are still considered forms of Klinefelter syndrome. This is the chromosomal arrangement of Caroline Cossey, also known as Tula, who appeared in the James Bond movie “The Spy who Loved Me”. Though raised as a boy, she opted to live as a girl at a young age, undergoing sex-assignment surgery and subsequently becoming a well known model and actress.