Chromosome

A chromosome is a microscopic and threadlike part of a cell that carries within it hereditary information in the form of genes. With every species having a characteristic number of chromosomes, with species that reproduce asexually, the chromosome number remains the same in all the cells of that organism. By contrast, in organisms that reproduce sexually, the number of chromosomes in the body cells is diploid, that is, chromosomes are found in pairs. During fertilization, two gametes combine to produce a zygote, a single cell, which has a diploid set of chromosomes. In addition, with organisms that have separate sexes, there are two major types of chromosomes: sex chromosomes and autosomes. The former are involved in determining whether the organism is male or female, and the latter deal with the inheritance of other characteristics.

The first identification of chromosomes was in plants and was made by the Swiss botanist Karl Willem von Nägeli (1817–1891) in 1842. Next, the Belgian scientist Edouard van Beneden (1846–1910) identified chromosomes in ascaris worms. Chromosomes were described in detail by the German anatomist Walther Flemming (1843–1905) and the name was coined by another German anatomist, Heinrich Wilhelm Gottfried von Waldeyer Hartz (1836–1921). The British biologist and geneticist Cyril D. Darlington (1903–1981) discovered the chromosomal crossover and its importance in evolution.