Alleles are different forms of a gene. They can be dominant or recessive. Genetic diagrams help us to understand the possible outcomes when parents produce offspring. Huntington’s disease is a disorder of the nervous system that is caused by a dominant allele. Cystic fibrosis is a disorder of the cell membranes caused by a recessive allele.
Some characteristics, such as eye colour and the shape of the earlobe, are controlled by a single gene. These genes may have different forms.
Different forms of the same gene are called alleleallele: One form of a gene.s (pronounced al-eels). The gene for eye colour has an allele for blue eye colour and an allele for brown eye colour.
Alleles are dominantdominant: An allele that always expresses itself whether it is partnered by a recessive allele or by another like itself or recessiverecessive: 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:
For example, the allele for brown eyes is dominant, while the allele for blue eyes is recessive. An individual who inherits one or two alleles for brown eyes will have brown eyes. An individual will only have blue eyes if they inherit two copies of the allele for blue eyes.
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. Genetic diagrams help to show how this works.
In a genetic diagram, you show all of the possible alleles for a particular characteristic. There will be two alleles from one parent, and two from the other parent, making four altogether. You then draw lines to show all the possible ways that these alleles could be paired in the offspring. There will be four possible ways, but some or all of them could be repeated.
In genetic diagrams, the dominant allele is shown as a capital letter, while the recessive allele is shown as a lower-case letter.
Huntington’s disease is an inherited disorder that affects the nervous system. It is caused by a dominant allele. This means it can be passed on by just one parent if they have the disorder. The genetic diagram shows how this can happen.
Note that if you are doing the Foundation Tier paper you are expected to be able to interpret genetic diagrams. If you are doing the Higher Tier paper, you are expected to be able to draw genetic diagrams for any combination of dominant and recessive alleles.
Cystic fibrosis is an inherited disorder that affects the cell membranes, causing the production of thick and sticky mucus. It is caused by a recessive allele. This means that it must be inherited from both parents. The genetic diagram shows how this can happen.
Notice that the offspring with Ff are labelled 'carriers'. A carrier has one copy of the faulty allele, but does not have the disorder themselves. In this example above, both parents are carriers. They may not know they are, but there is a one in four chance of them producing a child who has cystic fibrosis. It is possible to screen embryos to see if they carry alleles for genetic disorders.
In the example, one parent is a carrier, while the other does not carry the allele for cystic fibrosis. They cannot produce a child with the disorder, but they can produce children who are carriers.