Basic Biology & Chromosome Conditions

Basic Biology

Your body is made up of trillions of cells. Within each cell is your genetic material, or DNA. DNA is the instruction manual that tells your body how to grow and develop.

DNA is packaged into structures called chromosomes. You inherit your chromosomes from your parents.

Humans typically have 46 chromosomes, occurring in 23 pairs. Females typically have two X chromosomes and males have one X and one Y chromosome.



Sometimes, an error can occur that leaves a sperm or egg cell with a missing or extra chromosome.

If that happens, the resulting pregnancy may also have a missing or extra chromosome. Generally chromosome conditions are not inherited.


Chromosome Conditions

Trisomy 21

One genetic condition, trisomy 21 refers to an extra copy of chromosome 21. Trisomy 21 causes Down’s syndrome.

You can learn more about Down’s syndrome on the NHS website.1

Trisomies 13 and 18

Trisomies 13 (Pataus’ syndrome) and 18 (Edwards’ syndrome) are rare conditions caused by an extra copy of chromosome 13 or chromosome 18 respectively. These are often tested for during pregnancy.

These conditions are more severe than Down syndrome and often the baby does not survive.2-3 Visit the NHS website for further information on Trisomy 13 and Trisomy 18.


Trisomy 13


Trisomy 18

Other chromosome conditions

Chromosomes that determine whether we are male or female are labeled “X” and “Y” and are also called ‘sex chromosomes.’ Some people have a missing or an extra sex chromosome.

The effects of these conditions are typically less severe than the trisomies already discussed but may include learning disabilities and sometimes infertility, or the inability to have children.4-5

The chance of these conditions

The chance of having a baby with a chromosome condition can be influenced by the age of the mother and, in some cases, family history.

As women get older, this chance may increase.

Regardless of a woman’s age, there is always some chance for every pregnancy to be affected by a chromosome condition.

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