By Deronte' Smith
Updated Aug 30, 2022
Advances in genetics now enable clinicians to diagnose, predict, and treat a wide range of hereditary conditions. Central to this capability is the karyotype—a systematic representation of an individual’s chromosomes. By accurately constructing a karyotype, researchers can uncover genetic anomalies that underlie many disorders.
Begin by aligning each pair of homologous chromosomes in descending order of size. Use the centromere position and banding pattern as guiding criteria to ensure each chromosome is correctly matched.
Write the total number of chromosomes present in the specimen. For humans, this number is 46, unless abnormalities exist. Non‑human species such as dogs or pigs have distinct counts that must be noted.
Follow the total count with a comma, then denote the sex chromosomes: XX for females and XY for males. For example, 46,XX identifies a female; 46,XY identifies a male.
When an extra or missing chromosome is present, append a + or – sign followed by the chromosome number, separated by a comma after the sex chromosomes. For instance, a male with an extra chromosome 19 would be written as 47,XY,+19.
If a chromosome is fully added or lost, update the initial total accordingly (e.g., 47 instead of 46). Partial deletions or duplications do not alter the first number.
Use the abbreviation del to signify a missing portion of a chromosome. Place it in a third set of parentheses, immediately after the sex chromosomes, and enclose the affected chromosome number.
Immediately after the first parentheses, add another set of parentheses containing either p (short arm) or q (long arm). This indicates which part of the chromosome is missing.
In the second parentheses, insert the breakpoint number. For example, a female lacking the long arm of chromosome 5 is represented as 46,XX,del(5)(q16). Here, “del” indicates a deletion, “(5)” identifies the chromosome, “q” marks the long arm, and “16” denotes the breakpoint.
Other abnormal karyotypes may follow different conventions. Always refer to the latest cytogenetic guidelines when documenting anomalies.
