Understanding of genetics including the work of Mendel
Genetic diagrams
In the mid-19th century Gregor Mendel (1822-1884) 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. This went against the prediction that the colours would blend to produce pink flowers. If he bred these plants with each other, most of the offspring had red flowers, but some had white. This was because the Different forms of the same gene. for red flowers is An allele that always expresses itself whether it is partnered by a recessive allele or by another like itself. and the allele for white flowers is Describes the variant of a gene for a particular characteristic which is masked or suppressed in the presence of the dominant variant. A recessive gene will remain dormant unless it is paired with another recessive gene..
This was the basis of our understanding of inheritance. At this point we did not know what a The basic unit of genetic material inherited from our parents. A gene is a section of DNA which controls part of a cell's chemistry - particularly protein production. was. One of Mendel's observations was that the inheritance of each characteristic is determined by 'units' that are passed on to descendants unchanged. We now know these as genes.
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).
The genetic diagram below shows Mendel's second cross.
Three-quarters (75%) of the offspring have red flowers (FF and Ff) and a quarter (25%) have white flowers (ff).
The ratio of the different phenotypes can be shown as:
Red : white = 3 : 1
The ratio of the different genotypes can be shown as:
FF : Ff : ff = 1 : 2 : 1
Mendel's work expanded the knowledge of genetic inheritance before Deoxyribonucleic acid. The material inside the nucleus of cells, carrying the genetic information of a living being. had even been discovered.
Mendel's work was not accepted by most scientists when he was alive for three 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 The structure made of DNA that codes for all the characteristics of an organism. emerged in the late 19th century when better microscopes and staining techniques allowed the visualisation 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 strands coiled into a double helix. This model was used to work out how genes code for proteins.
