Your guide to health and prenatal genetics
Mother hold baby

Assess your personalised prenatal risks, choose screening tests, and find nearby maternity clinics

What is a chromosome?

In humans, chromosomes are found in the cells nucleus and are made of DNA and proteins; they carry genetic information. Chromosomes are visible only during cell divisions, when each chromosome has a constriction point called the centromere, which divides the chromosome into two sections (“arms”) – short arm (“p”) and long arm (“q”).

Normally, a human person has 46 chromosomes (or 23 pairs), where half are inherited from a mother and the other half from a father. Therefore, humans have diploid genome (see “genome”) which contains 22 pairs of so called autosomes and one pair of sex chromosomes. The autosome pairs are labelled with numbers (1-22 in humans) roughly in order of their sizes, while sex chromosomes are labelled with their letters– X and Y. A set of “XX” defines genetic normal female and “XY” – normal genetic male. A set of all chromosomes is called karyotype, and normal female karyotype is defined as 46, XX, and normal male – 46, XY.

The chromosomal disorders – structural (see “microdeletions”) or number abnormalities (see “trisomy”) – usually is the cause of various congenital disorders with mental retardation and other physical disabilities. Most of them occur naturally and can be suspected by prenatal screening, including modern non-invasive prenatal testing (see “NIPT”), followed by diagnostic confirmation with invasive testing (see “invasive prenatal diagnostic testing”). A woman and her partner then can make the most appropriate reproductive choices for them, including termination related to serious health problems of the fetus.