Medical Genetics - V. M. Zaporozhan 2005

Introduction to Medical Genetics
Cytological Foundations of Heredity
Human Karyotype

The karyotype is the diploid set of Chromosomes of a given Organism species, characterized by a constant number, size, and shape of chromosomes. The human karyotype contains 46 chromosomes, or 23 pairs: 22 pairs are identical in males and females and are called autosomes, while one pair consists of sex chromosomes (females have two identical XX chromosomes, whereas males have different ones, XY). Paired chromosomes are termed homologous. They have the same length and shape, undergo conjugation during Meiosis, and carry allelic genes.

The first international Classification of Human chromosomes was adopted in 1960 at a genetic conference in Denver, USA, and was named the «Denver System» (Standardized Nomenclature for Human Chromosomes). Most of its provisions remain in effect today. The fundamental principles of the classification were developed by the English cytogeneticist Patau. In his classification, he accounted for the length and shape of chromosomes depending on THE POSITION OF the primary constriction, as well as the presence of secondary constrictions and satellites. Later, the classification was updated at the Paris Conferences due to the discovery of differential chromosome banding (1971) and The Development of techniques for studying prometaphase chromosomes (1981).

Modern classification of human chromosomes is based on the current international cytogenetic nomenclature (ISCN — International System for Human Cytogenetic Nomenclature, 1995). It incorporates the findings obtained through Molecular Cytogenetic Methods.

According to the ISCN, all pairs of autosomes are numbered with Arabic numerals from 1 to 22 in descending order of their length and are divided into 7 groups based on chromosomal length and shape (Fig. 1.13). The groups are designated by uppercase English letters from A to G. These groups are distinctly distinguishable from one another. The sex chromosomes are designated by the Latin letters X and Y and are placed at the end.

Group A (pairs 1–3) comprises the longest chromosomes; pairs 1 and 3 are metacentric, and pair 2 is submetacentric. Their absolute length ranges from 11 to 8.3 µm.

Group B (pairs 4–5) consists of long submetacentric chromosomes that cannot be distinguished from one another without differential banding. Their absolute length is 7.7 µm.

Group C (pairs 6–12) includes medium-sized submetacentric chromosomes with an absolute length ranging from 7.2 to 5.8 µm. Under standard (routine) staining, the X chromosome cannot be distinguished from other chromosomes in this group; its size is similar to that of the 6th and 7th chromosome pairs.

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Fig. 1.13. Normal male karyotype. Trypsin-based G-banding.

Group D (pairs 13–15) represents medium-sized acrocentric chromosomes that differ markedly in shape from all other human chromosomes. All three pairs feature a secondary constriction and satellites on their short arms. The length of the proximal Regions of the short arms varies; satellites may be absent or, conversely, quite large, sometimes displaying bright fluorescence while other times lacking it entirely. Their absolute length is 4.2 µm.

Group E (pairs 16–18) comprises relatively short submetacentric chromosomes with an absolute length of 3.6–3.2 µm.

Group F (pairs 19–20) consists of small metacentric chromosomes. In preparations with routine staining, they appear identical, but they are clearly distinguishable upon differential banding. Their absolute length is 2.9 µm.

Group G (pairs 21–22) includes the smallest acrocentric chromosomes, which bear a satellite on their short arm. The Variability of their short arms is as pronounced as that in Group D chromosomes. Their absolute length is 2.3 µm.

The Y chromosome is a small acrocentric chromosome, 2.8 µm in length. It is usually (though not always) larger than the Group G chromosomes. The chromatids of its long arm typically run parallel to each other, which distinguishes it from Group G chromosomes, whose long-arm chromatids diverge at a wide angle. Occasionally, it exhibits a secondary constriction in the long arm.

The X chromosome is submetacentric and 6.8 µm long. Structurally, it resembles the chromosomes of Group C, from which it can be differentiated by banding patterns.