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Science

Genetic variation and genetic disorders

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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.

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