Understanding of genetics including the work of Mendel
Mendel's observations of petal colour
In the mid-19th century a monk called Gregor Mendel (1822-1884) studied the inheritance of different characteristics in pea plants. He also carried out work with flowers. Mendel discovered that when he bred red-flowered plants with white-flowered plants, all of the offspring had red flowers. If he bred these red flowering plants with each other, most of the offspring had red flowers, but some had white flowers.
We now know that the reason behind this observation is because the allele 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.. One of Mendel's ideas was that the inheritance of each characteristic is determined by 'units' that are passed on to descendants unchanged. We now know these 'units' as genes.
The genetic diagram below shows all of the possible alleles for a particular characteristic. Dominant alleles are capital letters, while the recessive alleles are lower-case letters.
The genetic diagram shows the outcome of Mendel's first cross of red and white flowers. All of the offspring have red flowers (100%), even though they are heterozygous and carry the recessive allele for white flowers (Ff).
When Mendel crossed two of these offspring together, he obtained the results shown in the genetic diagram below.
Three-quarters (75%) of the offspring have red flowers (FF and Ff) and a quarter (25%) have white flowers (ff).
It can be shown in a ratio as:
Red flowers : white flowers: 3:1
and also,
FF : Ff : ff
1 : 2 : 1
The development of our understanding of genetics
Mendel's work expanded our knowledge of genetic inheritance before DNA had even been discovered.
Mendel's work, however, 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 chromosomes emerged in the late 19th century when better microscopes and staining techniques allowed scientists to visualise the 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 two scientists called Rosalind Franklin and Maurice Wilkins, Watson and Crick 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.
In the early 21st century, the entire human genome was sequenced. Scientists are now working out the functions of our different genes.
Many years of work from different scientists focusing on DNA, chromosomes and genes have led us to the possibility of treating genetic conditions by changing our genes. This is called gene therapy.
