Karyotype description method

What is karyotype?

Among genetic tests, chromosome test results are written according to an international convention called the International System for Human Cytogenetic Nomenclature (ISCN), in which the shape, size, and number of chromosomes are described as "karyotypes. In humans, normal chromosomes consist of 22 pairs of 44 autosomes and one pair of sex chromosomes, for a total of 23 pairs of 46 chromosomes.

General principles stated

First, the total number of chromosomes including sex chromosomes is written, followed by a comma and then the sex chromosome configuration. The female chromosomes are represented as XX and the male chromosomes as XY.

• Normal women: 46, XX
• Normal male: 46, XY

For cases with chromosomal abnormalities, sex chromosome abnormalities should be written first, followed by autosomal abnormalities in descending order of chromosome number.

Numerical and structural anomalies

Abnormalities in number are indicated by writing + or - in front of the chromosomes that are increasing or decreasing in number.

For structural aberrations, the aberration string is shown below, followed by the aberrant chromosome number in the first bracket and the site of the chromosome band in the second bracket. If there are multiple chromosome numbers or chromosome bands, separate them with a semicolon (;).

Symbols used to denote karyotypes (main ones)

• add: Additionaml material of unknown origin.
• cen: Centromere
• c: Constitutional anomaly
• del: Deletion
• der: Derivative chromosome
• fra: Fragile site
• ins: Insertion
• inv: Inversion
• mar: Marker chromosome
• mat: Maternal origin
• p: Short arm of chromosome
• pat: Paternal origin.
• q: Long arm of chromosome
• r: Ring chromosome
• rec: recombinant chromosomes
• t: Translocation 
• ter: Terminal of chromosome

Examples of numerical anomalies

• Trisomy of chromosome 21 (Down syndrome): 47, XX, +21
• Triploid: 69, XXY

Examples of structural anomalies

• Inversion of chromosome 14 q11 to q32 (chronic lymphocytic leukemia): 46, XY, inv(14)(q11q32)

Simplified and detailed formulas

For structural anomalies, in addition to the above simplified formula, a detailed formula is sometimes used when there are complex structural anomalies.

A semicolon (;) indicates a break point in the chromosome, two colons (::) indicate cleavage and rejoining, and → indicates the extent of the fragment. Chromosome ends are denoted by ter, short arm ends by pter, long arm terminals by qter, and centromeres are denoted by cen. The description of chromosomes with structural aberrations begins at the end of the short arm (pter) and ends at the end of the long arm (qter).

• Deletion of the q13-q33 region on the long arm of chromosome 5
  • Simplified formula: 46, XX, del(5)(q13q33)
  • Detailed expression: 46, XX, del(5)(pter→q13::q33→qter)

Mosaic and Chimera

Mosaicism is the possession of two or more genetic traits within a single individual due to mutation or chromosomal recombination, while chimaera is the possession of genetic traits from different individuals. The karyotype description is written with a slash (/) and the two karyotypes that the individual has before and after the slash (/). Mosaic and chimaera are also distinguished by writing the strings mos and chi before the karyotype.

• Mixed normal and 21 trisomy traits (Down syndrome) : mos 46, XX/47, XX, +21

derived chromosome

Inversions and deletions of one chromosome, deletions of both arms, or unbalanced translocation products of multiple chromosomes are called derived chromosomes (der: Derivative chromosome).

• Derived chromosome 1 resulting from translocation of 1p32 and 3q21 and 1q25 and 11q13: 47,XY,(1;11)(q25q13) + der(1)t(1;3)(p32;q21)

recombinant chromosome

Recombinant chromosomes (rec: recombinant chromosomes) are produced by meiotic transfer of carriers of structurally abnormal chromosomes such as inversions and insertions.

• 46 ,XX ,inv(2)(p21q31) Recombinants produced in meiosis in inverted carrier females: 46, XX, rec(2)dup(2p)inv(2)(p21q31)mat 

Ambiguous chromosome bands

Vague identification

Cases with ambiguous numerical or structural anomalies are represented by (?) to denote the case.

• Probably a case of chromosome 21 loss: 45, XX, -?21 

Breakpoint or chromosome number ambiguous 

Cases with ambiguous cleavage points or chromosome numbers are represented by using ( ~ ).

• Deletion in any of bands 21-24 of chromosome 1 long arm: 46, XY, del(1)(q21-24)

If there are two possibilities 

Cases with two possible outcomes are represented by (or).

• Add fragment on p13 or q13 of chromosome 19: 46, XX, add(19)(p13 or q13) 

Incomplete karyotype (inc)   

Cases with incomplete analysis due to poor quality chromosome specimens are marked with inc at the end in the karyotype.

• Cases with a deletion in the long arm of chromosome 1 but with other abnormalities (inc, [ ]: number of cells analyzed): 46, XX, del(1)(q21), inc[4]. 

normal mutation

This is a finding that generally does not present with symptoms and is not thought to affect pregnancy or offspring.

Variation in length, number, and position

Variations in the length of the heterochromatin portion (h), satellite stalk (stk), and satellite (s) are represented by writing the chromosome and arm site followed by a string of h, stk, or s followed by an increase in length (+) or decrease in length (-). Changes in number and position are represented by writing the chromosomes and long and short arms followed by the string of h, stk, or s.

• Increased length of heterochromatin in the long arm of chromosome 16: 16qh+
• Decreased length of heterochromatin in the Y chromosome long arm: Yqh-

vulnerable site

It is the location of a gene that is prone to chromosomal structural abnormalities and is represented by fra (Fragile site).

• Two fragile sites on chromosome 10 (q22.1, q25.2): fra(10)(q22.1), fra(10)(q25.2) 

Number Anomalies

Sex chromosome abnormalities

• Klinefelter's syndrome: 47, XXY
• X monosomy: 45, X

autosomal abnormality

• 13 and 21 trisomies: 48, XX, +13, +21
• 22 monosomy: 45,XX, -22

Uniparental disomy (upd)  

A case in which two chromosomes or chromosome regions both originate from one parent.

• Maternal uniparental dysomy with chromosome 15: 46, XY, upd(15)mat