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. If he bred these 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 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 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.
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.
Family trees
Pedigree analysis
Doctors can use a pedigree analysis chart to show how genetic disorders are inherited in a family. They can use this to work out the probability (chance) that someone in a family will inherit a condition. A pedigree analysis is usually undertaken if families are referred to a genetic counsellor following the birth of an affected child.
The pedigree analysis diagram is used to show the relationship within an extended family. Males are indicated by the square shape and females are represented by circles. Affected individuals are red and unaffected are blue. Horizontal lines between males and females show that they have produced children.
This analysis shows both male and female are affected, and every generation has affected individuals. There is one family group that has no affected parents or children, but the remaining two families have one affected parent and affected children too.
Sex determination
Inheritance of biological sex
Human body cells have 23 pairs of chromosomes in the nucleus. Twenty two pairs are known as autosomes, and control characteristics, but one pair carries genes that determine sex – whether offspring are male or female:
- males have two different sex chromosomes, X Y
- females have two X chromosomes, XX
Chromosomes from a male
These diagrams are known as human karyotypes, and show all the chromosomes aligned in pairs. The blue box shows the two sex chromosomes – these are different sizes, therefore an X (larger chromosome) and a Y (smaller one).
Chromosomes from a female
The red box shows the two sex chromosomes – these are the same size, both two X larger chromosomes.
Genetic diagram
A genetic diagram, like a Punnett square, shows how chromosomes may combine in zygotes. The diagram below shows how biological sex is inherited.
The two possible combinations are:
- an X chromosome from the mother and an X chromosome from the father – producing a girl (female phenotype from the XX genotype)
- an X chromosome from the mother and a Y chromosome from the father – producing a boy (male phenotype from the XY phenotype)
The ratio of female to male offspring is 1:1 – on average, half of the offspring will be girls and half will be boys. This can also be converted into a probability of 50% (XX) and 50% (XY).
Limits of genetic testing
Genetic tests are not available for every possible inherited disorder, and are not completely reliable. They may produce false positive or false negative results, which can have serious consequences for the parents involved.
False positives
A false positive is a genetic test that wrongly detected a certain allele or faulty chromosome. The individual could believe that they have inherited a genetic condition, when they have not.
False negatives
A false negative is a genetic test that has failed to detect a certain or faulty chromosome. The parents may be given incorrect results. These results can have an impact on the lives of individuals, such as planning the level of care needed for children with inherited disorders.
Gene therapy
Gene therapy involves inserting copies of a normal allele into the chromosomes of an individual who carries a faulty allele. It is not always successful, and research is continuing to try and develop this possible treatment further.
Gene therapy involves these basic steps:
- identify the gene involved in the genetic disorder
- restriction enzymes cut out the normal allele
- many copies of the allele are made
- copies of the normal working allele are put into the cells of a person who has the genetic disorder due to a mutated or faulty copy of an allele
Problems in the process
The problems involved in the process:
- the alleles may not go into every target cell, which are cells that need the new non-faulty cell
- the alleles may be inserted into the chromosomes in random places, rather than in the required position, so they do not work properly
- some treated cells may be replaced naturally by the patient’s own untreated cells, as cells are frequently replaced through the process of mitosis during growth and repair
Different methods
Different methods are used to get the alleles into the patient’s cells, including:
- using nose sprays, which allow a patient to introduce the working allele up their nose and it will be taken into their body and incorporated
- using cold viruses that are modified to carry the allele – the viruses go into the cells and infect them
- the direct injection of DNA
Gene therapy can have major ethical implications in society as people disagree with gene alteration in people, as they believe it is unnatural.
Other people think that gene therapy is a good idea, as it prevents unnecessary suffering in affected individuals. Gene therapy only affects the individual involved in the process and not any future generations who would be likely to inherit similar diseases.
