Extra or Missing Chromosomes

What will be the effect on our body if chromosome number is altered from 46?

-A high school student from Pakistan

June 3, 2004

If a body has too few or too many chromosomes, it usually won't survive to birth. Getting the number of chromosomes right is so complicated that nature often makes mistakes. Believe it or not, around 10% of sperm and 50% of eggs contain abnormal chromosomes and more than half of first trimester spontaneous abortions are due to chromosome abnormalities.

Given these stark numbers, are there any cases where a person can survive with the wrong number of chromosomes? Yes, but there are usually associated health problems.

The only case where a missing chromosome is tolerated is when an X or a Y chromosome is missing. This condition, called Turner syndrome or XO, affects about 1 out of every 2,500 females. XO is one of the most common chromosomal abnormalities.

About 1 in 500 male births have an extra X chromosome (XXY) or an extra Y chromosome (XYY). Extra copies of chromosomes 13, 18, and 21 are all tolerated to some degree. An extra chromosome 13 or 18 usually results in severe and debilitating symptoms. Having an extra chromosome 21 results in Down syndrome. About 1/600 live births have Down syndrome.

Besides having a whole extra or missing chromosome, there are many instances of having a missing or extra bit of chromosome. Often this happens by a piece of a chromosome being moved to another chromosome (called translocation). The severity of symptoms is usually dependent on how much of the chromosome is missing or duplicated.

As you can see, there are numerous instances of fetuses having more or less than the usual 46 chromosomes. Most are miscarried but a small percentage makes it to term. All of this shows how hard it is to get making a person right. The table below lists the conditions described above along with some of the associated symptoms:

+/- Chromosome #

Disorder

Symptoms/ Life expectancy

-X or -Y

Turner Syndrome(XO)

See link below

+X

Klinefelter Syndrome (XXY)

Sterile, normal sexual function. Testosterone therapy often required.

+Y

XYY

Impulse control problems, high distractability

+21

Down Syndrome

See link below

+18

Edward's syndrome, Trisomy 18

20 - 30% die in the first month, 90% die by age one

+13

Patau's Syndrome,

Trisomy 13

Average length of survival is approximately 4 days

By Dr. Barry Starr, Stanford University