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The Chromosome – Structure and Number


A chromosome is the structure for the efficient packaging of DNA that is found in the cellular nucleus. Humans have 23 chromosome pairs, typically one maternal and one paternal of each.  The chromosome number will vary in some cases of viable aneuploidies.

Each chromosome has 2 “arms”

  • Short arm (petit), notated “p”
  • Long arm, notated “q”
  • Chromosomes are numbered, 1 through 23
    • 1 through 22 are autosomes, and is not related to whether one is female or male
  • Sex chromosomes are notated X and Y
    • Female Chromosome complement: 46,XX
    • Male Chromosome complement: 46,XY
This illustration depicts DNA packed tightly into chromosomes, as well as a DNA molecule unwound to reveal its 3-D structure.

This illustration depicts DNA packed tightly into chromosomes, as well as a DNA molecule unwound to reveal its 3-D structure | credit: NHGRI

When the chromosomal complement is BALANCED (any multiple of 23), it is EUPLOID

  • ‘Balanced’ does not necessarily mean a normal chromosome complement | 69, XXY is euploid, but is considered to be a chromosomal abnormality

When the chromosomal number is NOT BALANCED, it is ANEUPLOID

  • Common chromosomal trisomies and monosomies are examples of aneuploidy
    • Trisomy 21 / Down syndrome (47, XX, +21 [female]; 47, XY, +21 [male])
    • Trisomy 13 and Trisomy 18
    • Klinefelter syndrome (47,XXY)
    • Turner syndrome (45,X)

Besides additional whole or missing chromosomes, chromosomes can have other structural changes, such as deletions (del), duplications (dup), insertions (ins), inversions (inv), many of which are associated with recognizable genetic syndromes, for example:

  • 22q11.2 deletion syndrome
  • Wolf-Hirschhorn syndrome (WHS): results from deletion of a portion of chromosome 4p

Not all structural changes have significance

  • inv(9)(p11q12) and inv(9)(p11q13): common inversions involving chromosome 9 with no clinical sequelae


  • Chromosomal analysis requires only a single cell, at minimum, and can be performed on tissue/tumor, blood or amniotic fluid samples
  • Chromosomal analysis has an important role in prenatal diagnosis, clinical diagnosis in individuals with multiple congenital anomalies and other complex clinical pictures
  • Karyotype is a pictorial arrangement of chromosomes, that allows for careful analysis of gross numerical or structural changes
  • Chromosomal microarrays are being increasingly utilized as a method to analyze chromosomes as they offer the ability to detect submicroscopic abnormalities that may go undetected with conventional karyotyping (see ‘Related ObG Topics’ below)

Learn More – Primary Sources:

GHR: Chromosomes & mtDNA

CDC: Genetic Basics

NIH: NHGRI – Chromosomes

NIH: NHGRI – Chromosome Abnormalities