The Chromosomal Theory of Inheritance 1902 – The Birth of Cytogenetics

Chromosomes, DNA molecules wrapped around a protein scaffold, are defined as long, coiled, threadlike structures. Chromosomes can be seen under a microscope during cell division. 

Sperm and egg cells each contain 23 single chromosomes. Human body cells contain 23 pairs, one chromosome in each pair from the mother and one from the father. When an egg is fertilized, the chromosome number rises to 46. 

In 1865, the Austrian monk Gregor Mendel observed that organisms' traits are inherited through units we call genes. However, it was the German biologist Theodor Boveri and the American geneticist and physician Walter Sutton who identified chromosomes as the carriers of this genetic information in 1902.

• Working with sea urchins, Boveri concluded that sperm and egg cells each have half a set of chromosomes. When sperm and egg cells combined to produce sea urchin embryos with abnormal chromosome numbers, however, the embryos developed abnormally. Boveri concluded that different chromosomes affect different aspects of an organism's development. 
• Sutton's work on grasshoppers showed that matching chromosome pairs separate when sex cells form.

Boveri and Sutton not only proposed that chromosomes carry parental genetic information, but also showed that chromosomes remain independent entities even when invisible at different stages of a cell's life. This contradicted the widespread belief that chromosomes simply “dissolved” during division and re-formed in the daughter cells. 

Their work combined cytology (the science of cells) and genetics (the science of heredity) to create cytogenetics as a new discipline.

During the formation of sperm and egg cells, the parents' matching chromosomes exchange small segments in a process called “crossing over”. The resulting chromosomes are therefore not inherited unchanged from either parent. An incorrect chromosome number can cause genetic disorders; for example, people with Down syndrome have 47 chromosomes.

Historical Figures Mentioned in the Article: Theodor Heinrich Boveri (1862–1915), Walter Stanborough Sutton (1877–1916)

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