Science

Genetic variation and genetic disorders

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. Polydactyly - having extra fingers or toes - is caused by a dominant allele. Cystic fibrosis is a disorder of the cell membranes caused by a recessive allele.

Gender determination

Human body cells have 23 pairs of chromosomeschromosome: One of the rod shaped bodies found in the nucleus of cells that contain genetic information (DNA). in the nucleus [nucleus: Controls what happens inside the cell. Chromosomes are structures found in the nucleus of most cells. ]. One of these pairs controls the inheritance of gender - whether offspring are male or female:

  • In males, the two sex chromosomes are different. They are XY
  • In females, the two sex chromosomes are the same. They are XX
23 pairs of of chromosomes

Chromosomes from a male

23 pairs of chromosomes

Chromosomes from a female

This animation shows how gender determination works:

Alleles

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 alleles (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:

  • The characteristic controlled by a dominant allele develops if the allele is present on one or both chromosomes in a pair
  • The characteristic controlled by a recessive allele develops only if the allele is present on both chromosomes in a pair

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.

individual A is heterozygous and has one allele for blue eyes (recessive). B is homozygous and has two alleles for brown eyes (dominant). C is homozygous recessive and has two alleles for blue eyes (recessive)

Individuals A and B have brown eyes - only individual C has blue eyes

The cells of offspring produced by asexual reproduction are produced by mitosismitosis: A type of cell division which produces daughter cells identical to the parent. from the parental cells. They contain the same alleles as the parent. However, sexual reproduction causes variation because when two gametes join together one of each pair of alleles comes from each parent.

DNA fingerprinting

Each person (apart from identical twins) has unique DNA [DNA: The material inside the nucleus of cells, carrying genetic information. DNA stands for Deoxyribonucleic Acid. ]. This can be used to identify individuals by DNA fingerprinting. This technique uses some of the small differences between the DNA from different people to make a picture rather like a barcode.

If enough parts of the DNA are tested, it is very unlikely that two identical DNA fingerprints would belong to two different people. This makes the method very useful for matching samples found at the scene of a crime to people suspected of committing the crime.

Genetic diagrams

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.

Genetic diagram of FF x ff for flower colours

A genetic diagram showing the outcome of Mendel's first cross. All the offspring have red flowers, even though they carry the recessive allele for white flowers

In genetic diagrams, the dominant allele is shown as a capital letter, while the recessive allele is shown as a lower-case letter.

Genetic diagram of Ff x Ff for flower colours

A genetic diagram showing the outcome of Mendel's second cross. Three-quarters of the offspring have red flowers and a quarter have white flowers

Polydactyly

Polydactylypolydactyly: A condition in which there are more than five fingers or toes on a hand or foot. is an inherited condition in which a person has extra fingers or toes. It is caused by a dominant allele of a genegene: 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.. This means it can be passed on by just one parent if they have the disorder.

The genetic diagram shows how this can happen.

Polydactyly allele chart

Offspring need to carry just one dominant allele from their parents to inherit the polydactyl condition

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

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.

Genetic diagram to show the inheritance of cystic fibrosis

The cystic fibrosis allele is represented by f. The normal allele is F. Suppose both parents have alleles Ff. The possible combinations of alleles in the children are FF, Ff, Ff and ff. The alleles ff will cause the disease. So, although the parents do not have cystic fibrosis, they can produce children with the disease. The parents are called 'carriers' of the disease

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.

Genetic diagram to show the inheritance of cystic fibrosis when only one parent is a carrier

The cystic fibrosis allele is represented by f. The normal allele is F. Suppose one parent is FF and the other is a carrier, Ff. The possible combinations of alleles in the children are FF, FF, FF and Ff. So the parents cannot produce children with cystic fibrosis (ff). But they can produce children with alleles Ff, who will be carriers

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.

Embryo screening

Embryos can be screened for the alleles that cause polydactylypolydactyly: A condition in which there are more than five fingers or toes on a hand or foot., cystic fibrosis and other genetic disorders. Note that you do not need to know or understand how embryo screening works for the examination.

Back to Revision Bite