Human Chromosomes Introduction

Human Chromosome Nomenclature

• Human chromosome nomenclature is based on the results of several international conferences (Denver 1960, London 1963, Chicago 1966, Paris 1971, Paris 1975, Stockholm 1977, Paris 1980, Memphis 1994, Vancouver 2004).
• The current nomenclature incorporates and supersedes all previous ISCN recommendations.

Chromosome Number and Morphology

• Autosomes are numbered from 1 to 22 in decreasing order of length.
• Sex chromosomes are referred to as X and Y.
• Chromosomes can be arranged into seven groups (A-G) based on size and centromere position.
• Group A: Large metacentric chromosomes (1-3).
• Group B: Large submetacentric chromosomes (4-5).
• Group C: Medium-sized metacentric or submetacentric chromosomes (6-12, X).
• Group D: Medium-sized acrocentric chromosomes with satellites (13-15).
• Group E: Relatively short metacentric or submetacentric chromosomes (16-18).
• Group F: Short metacentric chromosomes (19-20).
• Group G: Short acrocentric chromosomes with satellites (21-22, Y).

Non-Banding Techniques

• Parameters used to describe non-banded chromosomes:
  • Length of each chromosome as a percentage of the total length of a normal haploid set.
  • Arm ratio of the chromosomes.
  • Centromeric index.

Banding Techniques

• A band is defined as a part of a chromosome that is clearly distinguishable from its adjacent segments by appearing darker or lighter with one or more banding techniques.
• Q-staining methods produce a fluorescent banding pattern.
• G-staining methods produce an almost identical pattern of dark and light bands.
• R-banding techniques give patterns that are opposite in staining intensity to G-staining methods.
• Two principle groups of banding techniques:
  • Those resulting in bands distributed along the length of the whole chromosome (e.g. G-, Q, and R-bands).
  • Those that stain specific chromosome structures and hence give rise to a restricted number of bands (e.g. C-bands, T-bands, and NORS).

Chromosome Band Nomenclature

• Each chromosome is considered to consist of a continuous series of bands, with no unbanded areas.
• Bands are allocated to various regions along the chromosome arms, and the regions are delimited by specific landmarks.
• Landmarks include the ends of the chromosome arms, the centromere, and certain bands.
• Regions are numbered from the centromere outward.

X- and Y-Chromatin

• Inactive X chromosomes and the heterochromatic segment on the long arm of the Y chromosome appear as distinctive structures in interphase nuclei.
• The terms X-chromatin (Barr body, sex chromatin, X-body) and Y-chromatin (Y-body) should be used to describe these structures.

Karyotype and Idiogram

• The term karyogram should be applied to a systematized array of the chromosomes prepared by drawing, digitized imaging, or photography.
• Karyotype is the use of nomenclature to describe the normal or abnormal, constitutional or acquired, chromosomal complement of an individual, or cell line.
• The term idiogram should be reserved for the diagrammatic representation of a karyotype.