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Genetic diagrams

Gregor Mendel (1822-1884) was a Czech monk who used peas in breeding experiments in the 1850s and 1860s. He published his ideas in 1866, just after Darwin published On the Origin of Species in 1859. Darwin knew characteristics could be passed on but never knew about genes or DNA.

Mendel studied the inheritance of different characteristics in pea plants. He found that when he bred red-flowered plants with white-flowered plants, all the offspring produced red flowers. If he bred the red-flowered plants with each other, most of the offspring had red flowers, but some had white. This was because the allele for red flowers is dominant, and the allele for white flowers is recessive.

One of Mendel's observations was that the inheritance of each characteristic is determined by 'units' that are passed on to descendants unchanged. The genetic diagram shows all of the possible alleles for a particular characteristic.

Dominant alleles are capital letters, while the recessive alleles are lower-case letters.

This genetic diagram shows the outcome of Mendel's first cross. All the offspring have red flowers (100%), even though they are heterozygotes and carry the recessive allele for white flowers (Ff).

Three-quarters (75%) of the offspring have red flowers (FF and Ff) and a quarter (25%) have white flowers (ff).

It can be shown as: FF : Ff : ff

1 : 2 : 1

Mendel's work expanded the knowledge of genetic inheritance before DNA had even been discovered.

Mendel's work was not accepted by most scientists when he was alive for 3 main reasons:

• when he presented his work to other scientists he did not communicate it well so they did not really understand it
• it was published in a scientific journal that was not well known so not many people read it
• he could not explain the science behind why characteristics were inherited

The idea that genes were located on chromosomes emerged in the late 19th century when better microscopes and staining techniques allowed the visualization and behaviour of chromosomes during cell division.

In the early 20th century, it was observed that chromosomes and Mendel's 'units' behaved in similar ways. This led to the theory that the 'units', now called genes, were located on chromosomes.

In the mid-20th century two scientists, James Watson and Francis Crick worked out the structure of DNA. By using data from other scientists Rosalind Franklin and Maurice Wilkins, they were able to build a model of DNA. They showed that bases occurred in pairs, and x-ray data showed that there were two chains wound into a double helix. This model was used to work out how genes code for proteins.

Scientists now think that most features are affected by information in many areas of the genome. The science of genomics includes genome sequencing and bioinformatics. Genome sequencing and bioinformatics analyse DNA and the patterns in genome sequences. Genetic testing is helping us to predict who may develop certain diseases.

Many years of work from different scientists' focusing on DNA, chromosomes and genes, has led us to the possibility of treating genetic conditions using gene therapy.