(3.5) Cystic fibrosis

Know and understand that some disorders like cystic fibrosis are inherited.

You need to be able to evaluate the outcomes of pedigree analysis when screening for genetic disorders e.g. cystic fibrosis.

The genetically inherited disorder called cystic fibrosis is described and genetically explained below, and there are serious medical consequences.

 

Genomics and cystic fibrosis inherited disease

It is now known most of our characteristics are controlled by more than one gene and this is also true for the genetically inherited disease cystic fibrosis.

Single-gene disorders like cystic fibrosis comply with what is called 'Mendelian inheritance' and genetic diagrams and Punnett squares are quite easy to work out - as I hope you will see below.

(Most diseases with a 'genetic connection' like diabetes, obesity and cardiovascular diseases (heart disease) involve the interaction of many genes including non-coding sections of the genome's DNA and environmental factors e.g. lifestyle choice - diet and exercise.)

Cystic fibrosis is a genetic disorder of cell membranes, the disease is passed down through families.

Cystic fibrosis can be caused by the deletion of only three bases, but this has a dramatic effect on the phenotype.

The faulty gene should code for a protein that controls the movement of salt and water in and out of cells.

Unfortunately, the protein produced by the faulty gene doesn't work properly and leads to excess mucous production.

Cystic fibrosis causes this thick, sticky mucus to build up in the air passages, lungs, digestive tract, pancreas and other areas of the body - affected people suffer from breathing and digestion difficulties and patients are on a complex mixture of medications.

It is one of the most common chronic lung diseases in children and young adults and sadly, it is a life-threatening disorder caused by a defective gene which causes the body to produce abnormally thick and sticky fluid, called mucus.

The thick mucus builds up in the breathing passages of the lungs (causing lung infections) and in the pancreas, the organ that helps to break down and absorb food (causing digestion problems).

 

The parents may be carriers of the cystic fibrosis disorder without actually having the disorder themselves.

In the above diagram, imagine the 'arrowed' yellow band represents the allele that codes for the essential protein required to avoid suffering from cystic fibrosis.

A represents the normal dominant alleles in the pair of chromosomes (notated as FF below in the genetic analysis).

This person is not a carrier or sufferer of cystic fibrosis.

B represents a dominant normal and a defective recessive allele (notated as Ff below in the genetic analysis).

This person is a carrier, but not a sufferer of cystic fibrosis because normal is dominant.

C represents a person with a pair of defective recessive alleles (notated as ff below in the genetic analysis).

This person is both a carrier and sufferer of cystic fibrosis - the double recessive gene prohibits the production of the vital protein.

 

It is caused by a recessive allele (denoted by f) of a gene and can therefore be passed on by parents, neither of whom has the disorder.

About 1 in 25 people carry the recessive allele of f cystic fibrosis.

About in 3000 newborn babies have the condition.

In order to be affected by cystic fibrosis, you must inherit the double recessive gene ff.

 

Punnett square and genetic diagram for cystic fibrosis

Punnett square genetic table for cystic fibrosis
1. Genotypes of parents: Ff x Ff, normal but both carriers
Gametes: F, f, F and f (alleles)
Genotypes of children	F	f
F	FF	Ff
f	Ff	ff

FF = homozygous alleles (dominant)

Ff = heterozygous alleles

ff = homozygous alleles (recessive)

 

Cystic fibrosis is caused by a recessive allele f (so it needs genotype ff, a double recessive allele, for the person to suffer from cystic fibrosis.

For someone to suffer from cystic fibrosis, they must inherit the faulty allele (f) from both parents.

 

The genetic diagrams above and below show that when both parents are carriers of the recessive allele, but NOT affected (Ff, heterozygous), there is a 3 in 4 (75%) chance of having a normal child (FF non-carrier or Ff carrier) and a 1 in 4 (25%) chance of having a child with cystic fibrosis (recessive and homozygous ff sufferer and carrier).

Genetic diagram for normal, but carrier of  cystic fibrosis and another normal, but carrier of cystic fibrosis.

 

Five other possible parental crosses involving the recessive allele f for cystic fibrosis

I've shown below the analyses for cystic fibrosis using a basic Punnett square of the two pairs of gametes of the parents and the four possible genotypes of offspring (children).

 

3. genotypes of parents: FF x Ff			Comments on cross 3. for cystic fibrosis A non-carrier crossed with a carrier of the cystic fibrosis recessive gene f. 2 in 4 chance (50%) of the offspring will be carriers of the recessive gene f. Non of the offspring will be affected by cystic fibrosis.
genotypes of children	F	F
F	FF	FF
f	Ff	Ff

 

4. genotypes of parents: FF x ff			Comments on cross 4. for cystic fibrosis A non-carrier crossed with someone suffering from cystic fibrosis. All the offspring will be carriers of the recessive gene f. Non of the offspring will be affected by cystic fibrosis.
genotypes of children	F	F
f	Ff	Ff
f	Ff	Ff

Extra note on cystic fibrosis:

(i) For 5. DD x DD, all offspring will be DD non affected, similarly, for 6. ff x ff, all offspring will be ff affected and carriers.

(ii) For couples who may carry the recessive gene, certain crosses carry an increased risk that their child might suffer from cystic fibrosis.

Genetic screening for potentially harmful alleles may inform potential parents of the risk, but this may in itself lead to agonising decisions.

For lots more examples of genetic analysis of offspring see the index with lots of diagrams and explanations.

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Key points Source of information is based on textbooks & syllabus-specifications for students taking the AQA GCSE, Edexcel GCSE and OCR GCSE level biology examinations (~US grades 9-10).

Key points in understanding the genetics of cystic fibrosis

Cystic Fibrosis: Inherited Genetic Disorder and Genetic Outcomes

Cystic fibrosis (CF) is a recessive genetic disorder affecting the lungs, digestive system, and sweat glands.

It is caused by a mutation in the CFTR gene and leads to excessive thick mucus production, resulting in respiratory and digestive complications.

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1. Causes of Cystic Fibrosis

Cystic fibrosis is caused by a mutation in the CFTR (Cystic Fibrosis Transmembrane Conductance Regulator) gene, located on chromosome 7.

• The CFTR gene controls the movement of chloride ions in and out of cells, helping to maintain the balance of salt and water.

• Mutations disrupt this process, causing thick mucus to build up in organs like the lungs and pancreas.

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2. Genetic Inheritance and Possible Outcomes

CF follows a recessive inheritance pattern, meaning an individual must inherit two faulty CFTR alleles (one from each parent) to develop the disease.

Punnett Square Example – Carrier Parents (Both Heterozygous, Ff):

Genetic Outcomes:

• 25% (FF) – Normal, no CF mutation

• 50% (Ff) – Carrier, but unaffected

• 25% (ff) – Has cystic fibrosis

Key Terms:

• Homozygous recessive (ff): Individual develops cystic fibrosis.

• Heterozygous (Ff): Carrier but does not show symptoms.

• Homozygous dominant (FF): No mutation, fully healthy.

If one parent is a carrier (Ff) and the other parent does not carry the mutation (FF), none of the offspring will have CF, but 50% may be carriers.

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3. Importance in Understanding Human Genetics

Studying cystic fibrosis enhances knowledge in:

• Disease Screening & Genetic Testing: Detecting CF carriers in families before pregnancy.

• Medical Advancements & Treatment: Developing gene therapies and personalized medicine.

• Inheritance & Genetic Counseling: Assisting families in understanding reproductive risks.

• Evolutionary Biology: Understanding how recessive diseases persist in human populations.

Cystic fibrosis is a key example of recessive genetic disorders, highlighting the role of inheritance, genetic variation, and medical research in human biology.

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Summary of learning objectives and key words or phrases

Know how to explain the genetics cause of the serious medical condition of cystic fibrosis.

Using genetics explain the inherited genetic disorder cystic fibrosis, including a faulty gene allele, know how to work out the possible genetic outcomes from Punnett squares and cross bred diagrams for cystic fibrosis.

For cystic fibrosis you need to be able to construct genetic diagrams of monohybrid crosses and predict the outcomes of monohybrid crosses and be able to use the terms homozygous (same alleles eg FF, ff) genes or heterozygous (different alleles eg Ff), understand the phenotype (gene expression - the outcome!) from the genotypes (gene type) for cystic fibrosis.

Know how do you draw monohybrid genetic diagrams for cystic fibrosis.

Know how to construct Punnett squares for monohybrid crosses for cystic fibrosis.

Be able to predict and/or explain the outcome of crosses between individuals for each possible combination of dominant and recessive alleles of the same gene for the genetic disorder cystic fibrosis and explain the observed phenotypes from the genotype carriers of cystic fibrosis

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