Most people who have stopped to think about it has always known that animals and plants inherit their appearance and nature from their parent organisms.
A complication for early scientists was the belief in spontaneous generation, which assumed that parents were not needed to provide inherited information.
In 1831, botanist Robert Brown announced the discovery that each living cell contains a nucleus. Others had seen nuclei, but now they were seen as universal.
Once people realised that we are made of cells, and that cells reproduced by dividing, it became fairly obvious there must be something inheritable in the cell.
In 1879, Walther Flemming discovered a thread-like material in the nucleus of cells. These threads were later named the chromosomes, because they took stains.
In 1883, Edouard van Beneden announced the principles of genetic continuity of chromosomes and reported the occurrence of what we would now call meiosis.
In 1888 Theodor Boveri verified August Weismann's predictions of chromosome reduction (which we call meiosis today) by direct observation in the worm Ascaris.
In 1900, Walter Sutton observed homologous pairs in the chromosomes of a grasshopper, and he reported in 1902 that they separate to opposite ends in meiosis.
In 1903, Walter Sutton and Theodor Boveri independently confirmed that the chromosomes behave in a way that matched what was known of Mendelian inheritance.
By the careful study of genetics, Sutton and Boveri were able to recognise that the only thing in the cell behaving like Mendel's genes was the chromosomes.
Alfred Sturtevant worked in the Columbia 'fly room' and actually constructed the first genetic map of a chromosome in 1913, using linkage data.
In 1927, H. J. Muller used X-rays to cause artificial gene mutations in Drosophila, showing that the mutation rate was 1500 times higher when X-rays were used.
It was not enough to know that the chromosomes carry the genes in some way, because chromosomes are made up of both protein and nucleic acids.
In 1956, Joe-Hin Tjio and Johan Albert Levan revised Walther Flemming's 1898 estimate of the human chromosome count from 24 pairs to 23 pairs.
In 1973, Bruce Ames published details of the 'Ames Test' to identify DNA-damaging chemicals. The test has since become a widely used to screen for carcinogens.
All living things are made up of small units called cells. Each cell usually contains a nucleus with coded information that the cell uses to operate.
Genes were first located as existing on the chromosome by the Sutton-Boveri theory which has stood the tests of time. The genetic code is further proof.
The gender in animals is usually determined by the sex chromosomes, although different groups such as birds and insects do this differently from mammals.
In mammals, normal males have an X-chromosome and a Y-chromosome, females have two X chromosomes. A sperm cell carries an X or a Y, an ovum carries an X only.
The 'plans' for a living thing are found in chemical strings in the nucleus of the cell, and mistakes sometimes get made when these plans are copied.
Each gene in the nucleus codes for a single unique protein. Most proteins work as enzymes that catalyse the conversion of one biochemical into another.
A biochemical pathway involves a large number of conversions, each managed by an enzyme. If any enzyme is changed, the end product may never be produced.