Cytogenetics
 
Cytogenetics is the study of genetics by visualization of chromosomes and chromosomal aberrations, usually through light microscopy. There is a distinguished history of chromosomal visualization for analytical purposes. Longley defined the morphology of maize chromosomes , which opened the door for subsequent work on maize cytogenetics by McClintock (2). Staining of prophase and early metaphase maize chromosomes with acetocarmine allowed McClintock to identify 10 distinct linkage groups. In turn this laid the foundation for other maize cytogeneticists to identify chromosomal translocations and inversions visually. For example, it was possible to demonstrate that there is a correspondence between marker genes and cytologically defined chromosomal domains in crosses between heterozygous strains (3). Cytogenetics in Drosophila melanogaster received an enormous stimulus from the systematic study of the polytene chromosomes in salivary glands (4). These chromosomes have visibly distinct banding patterns under a phase microscope and after staining. Each of the four Drosophila melanogaster chromosomes have easily differentiated band patterns. The thousands of bands served as markers that allowed cytogeneticists to discern small deletions, duplications, translocations, and transpositions. Cytological experiments in Drosophila also allowed the discovery of the position effect, the phenomenon in which the position of a gene relative to heterochromatin influences the efficiency with which it is expressed (5).
 Improvements in cytogenetics eventually allowed visualizing metaphase chromosomes in somatic cells in culture. Only in 1956 was it established by these methodologies that human cells contain 46 chromosomes (6). Likewise cytogenetic approaches allowed the discovery that children with Down's syndrome are trisomic for chromosome 21. Medical cytogenetics has since been advanced by chromosome stains, such as Giemsa, that enable separating individual chromosomes in sufficient numbers for highly focused analysis of their biochemistry and genetics.
References
1. A. E. Longley (1952) Bot. Rev. 18, 399–412. 
2. B. McClintock (1929) Science 69, 629–632. 
3. H. B. Creighton and B. McClintock (1931) Proc. Natl. Acad. Sci. USA 17, 485–491.
4. E. Heitz and H. Bauer (1933) Z. Zellforsch Mikrosc. Anat. 17, 67–81. 
5. A. H. Sturtevant (1925) Genetics 10, 117–147. 
6. J. H. Tijo and A. Lean (1956) Hereditas 42, 1–6.