BIOLOGY TEST-Cystic Fibrosis

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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.

The symptoms of cystic fibrosis can vary in severity from person to person. The thick mucus in the body affects a number of organs, particularly the lungs and digestive system.

Most cases of cystic fibrosis in the UK are now identified through screening tests carried out early in life, before symptoms appear. Read more about screening for cystic fibrosis.

The main symptoms and related problems are detailed below.

Symptoms at birth

Around 10% of babies with cystic fibrosis are born with a serious bowel obstruction, known as meconium ileus. This can make them very ill in their first few days of life.

All babies are born with meconium in their bowels. This is a thick, black substance normally passed out of the body a day or two after the birth.

However, in some babies with cystic fibrosis, the meconium is too thick to be passed through the bowels and causes a blockage. If a baby is born with meconium ileus, an operation to remove the blockage will probably be required.

A few babies will also have symptoms of jaundice (yellowing of the skin, eyes and mucus membrane), which can last for a long time. However, there are many other, more common, causes of jaundice.


It is common for people with cystic fibrosis to experience persistent coughing as the body tries to remove the thick mucus from the lungs. Sudden coughing fits can occur, which sometimes lead to vomiting.

Inflammation in the lungs may cause wheezing, shortness of breath and breathing difficulties, particularly after exercise.


Some people with cystic fibrosis also experience recurring chest and lung infections, which can be severe. These infections are caused by the continual build-up of mucus in the lungs, which provides an ideal breeding ground for bacteria.

The danger is that a person with cystic fibrosis who has such an infection can pass it on to another person with cystic fibrosis through close personal contact…


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