The Number and Morphology of Human Chromosomes

The number and morphology of human chromosomes

Human chromosome analysis is the study of the genetics of chromosomes in the field of cytogenetics.(Cytogenetics)It is called.
It is a compound of Cytology and Genetics.

Chromosome Genetics Area

What is Human Chromosome Analysis?

The field of chromosome genetics is called cytogenetics. It is a compound of cytology and genetics.

Mendel's "law of heredity" for peas (plants) was rediscovered in 1900, and later the regular mode of inheritance of the "unit of heredity" from parent to child was found to be closely related to chromosomes in meiosis. Later, Morgan clarified the relationship between genes and chromosomes by means of Drosophila (living creatures).

It was not long ago that the number and morphology of human chromosomes could be observed and analyzed in detail.

Thio and Levant revealed in 1956 using fetal cell cultures that the number of human chromosomes is 46 (2n=46) (sex chromosomes are XY-type for males and XX-type for females).

Subsequently, techniques for cell culture and chromosome preparation were improved, and it became possible to identify chromosome aberrations related to various congenital abnormalities and hematopoietic diseases. The accuracy of chromosome analysis also progressed dramatically, and a classification and nomenclature of human chromosomes was established.

Since 1960, the standard convention by chromosomes has been revised many times.

Around 1970, the chromosome segregation method made it possible to identify all human chromosomes and to accurately identify structural abnormalities using bands and symbols.


Analysis of chromosomal structural abnormalities

In the 1980s, highly accurate staining methods were introduced and detailed analysis of chromosome structural abnormalities was conducted.

Around 1990, molecular genetic methods were introduced into the field of cytogenetics, and chromosomes could also be analyzed at the DNA level. Around this time, the Human Genome Project was launched to sequence the entire genome and elucidate genetic information and its institutional mechanisms.

In the 2000s, based on the human genome analysis and the database of sequence information of all DNA, a high-precision analysis method of chromosomal DNA using microarrays was developed, and partial deletions and duplications on chromosomal DNA and even the number of base pairs in DNA could be confirmed consecutively.

Map of Human Chromosomes

In 1990, the aforementioned Human Genome Project was initiated at the international level. The goal was to determine the bases of the entire genome and to elucidate the whole picture of genetic information and its institutional mechanisms.

Sequencing of small fragmented DNA clones progressed at an accelerated pace and whole genome elucidation was reported in 2003.

Human genome analysis estimates the number of human genes at 22,000-25,000.

Mammals, including mice, have about the same genome size and number of genes as humans. The human genome is 3,100 Mb (3.1 billion bp).

The average size of the gene region, which includes exons and introns, is estimated to be 33 kb, so genes occupy only less than 30% of the human genome (3,100 Mb). The majority of the genome is non-genetic region (see above).

Location and Number of Genes

Genes are not evenly distributed on the DNA molecule. This is due to the fact that DNA is folded in many layers in a regular fashion to form chromosomes. Certain genes reside in fixed positions on the chromosomes.

This band of chromosomes produces areas of high and low gene density along the longitudinal axis. The uneven distribution of genes across the genome can be seen in the chromosomes. This represents the number of genes on the chromosome.


The number of genes has nothing to do with chromosome size. For example, even a small chromosome, such as autosomal chromosome 19, has a very large number of genes compared to other chromosomes of similar size.

In addition, chromosomes 13, 18, and 21, which are fertile trisomies, have fewer genes than the other chromosomes.

Trisomy is possible because the number of genes is small to begin with. If other chromosomes are trisomic, growth stops in utero and a miscarriage occurs.