Why is cystic fibrosis autosomal recessive instead of X-linked?

The CFTR gene sits on chromosome 7, an autosome, not on a sex chromosome. Disease requires two defective CFTR alleles, one inherited from each parent, so both parents are unaffected carriers. Because the gene is autosomal, males and females are affected equally. X-linked recessive disease, such as chronic granulomatous disease, is carried on the X chromosome, affects mainly sons of carrier mothers, and shows no father-to-son transmission.

If both parents are cystic fibrosis carriers, what is the chance each child is affected?

Two carrier parents (each Cc) have a 25 percent chance of an affected child (cc), a 50 percent chance of an unaffected carrier (Cc), and a 25 percent chance of a child with two normal alleles (CC) at each pregnancy. Among children who are not affected, two out of three are silent carriers.

How do you tell cystic fibrosis apart from chronic granulomatous disease on a board question?

Cystic fibrosis presents with meconium ileus at birth, thick purulent sputum, recurrent pulmonary infiltrates, failure to thrive, pancreatic insufficiency, and an elevated sweat chloride; it is autosomal recessive (CFTR on chromosome 7). Chronic granulomatous disease is X-linked recessive, caused by defective NADPH oxidase, and presents with recurrent catalase-positive infections and granulomas, diagnosed by a dihydrorhodamine flow cytometry or nitroblue tetrazolium test. Recurrent infections alone do not make a disease X-linked.

What does the CFTR protein do and why does its loss cause thick mucus and salty sweat?

CFTR is a cyclic AMP regulated chloride channel. In the airways, pancreas, and gut it secretes chloride with water following, keeping secretions thin; loss of CFTR leaves dehydrated, viscous mucus that obstructs and gets infected. In the sweat duct CFTR normally reabsorbs chloride from sweat, so when it fails chloride and sodium stay in the sweat, producing the salty skin and elevated sweat chloride that confirm the diagnosis.

What are the classic complications of cystic fibrosis to know for boards?

Pancreatic insufficiency causing malabsorption and fat-soluble vitamin A, D, E, and K deficiency; chronic lung infection with Staphylococcus aureus early and mucoid Pseudomonas aeruginosa later; nasal polyps and chronic sinusitis; male infertility from congenital bilateral absence of the vas deferens; and an elevated sweat chloride above 60 mmol per liter that confirms the diagnosis.

Genetics · Mendelian Inheritance

Cystic Fibrosis:
Both Parents, Not the X

A child with thick green sputum, meconium ileus at birth, and failure to thrive is cystic fibrosis. The trap is the genetics question that follows. CFTR sits on chromosome 7 and CF is autosomal recessive, so the disease allele came from BOTH parents. Panic and call it X-linked and you just confused it with chronic granulomatous disease. Learn the decoder so the inheritance pattern is never the part you miss.

Medically reviewed by Fatima Ali, DO & Kaitlyn Cocuzzo, MD ✦elite

Before you scroll

A 7-year-old boy is brought to the office because of months of cough productive of thick green sputum and poor weight gain. He was born with meconium ileus that required surgery and has had several episodes of pneumonia. Vital signs are within normal limits. Physical examination reveals digital clubbing and scattered crackles. Sweat chloride testing is elevated at 92 mmol/L, and genetic testing confirms two CFTR mutations. From which of the following did the two disease-causing alleles most likely come?

Where does CFTR live?

On the long arm of chromosome 7 (7q31). Chromosome 7 is an autosome, not a sex chromosome. That single fact rules out X-linked and mitochondrial inheritance immediately.

How many bad copies make CF?

Two. CF is autosomal recessive, so an affected child is cc and needs one mutant allele from each parent. Each parent is a healthy Cc carrier. One parent alone cannot do it.

So why did the student say X-linked?

He saw recurrent infections and reached for chronic granulomatous disease, which IS X-linked. But CF carries meconium ileus, salty sweat, and pancreatic failure with it. CFTR is autosomal recessive, so the disease allele comes from chromosome 7 of both parents.

Scroll ↓ recognize CF from the cluster

Pattern Recognition

Read the Whole Cluster

No single clue makes cystic fibrosis. The diagnosis lives in the constellation: a newborn gut obstruction, thick infected lungs, a failing pancreas, and salt. Tap each finding to build the case, then read the route.

Tap the findings that point to CF

Select findings to see whether they assemble into cystic fibrosis.

From the Attending

A baby that did not pass meconium, a child with thick green sputum and pneumonia after pneumonia, greasy stools, and skin that tastes like salt. Stop hunting for one magic finding. One organ is a symptom; four organs that all secrete is a channel disease. That channel is CFTR. Know your cluster.

One Broken Channel, Two Faces

CFTR On Chromosome 7

CFTR is a cyclic AMP gated chloride channel. The same broken channel makes thick mucus in the lung and salty sweat at the skin, because chloride moves in opposite directions in those two tissues. Toggle the tissue and watch the chloride.

In the airway and gut, working CFTR pushes chloride OUT into the lumen and water follows, keeping mucus thin and sweepable. Lose CFTR and the surface dehydrates into thick mucus that traps bacteria.

Chloride (Cl)Cell membraneMucus

Human chromosome 7 ideogram, location of the CFTR gene — Chromosome 7: home of CFTR (7q31)

Why the sweat is salty

Flip the tissue and chloride flips direction. In the sweat duct, working CFTR reabsorbs chloride out of the forming sweat, and sodium follows it back into the body. Broken CFTR leaves chloride and sodium stuck in the sweat, so the skin tastes salty and the sweat chloride climbs above 60 mmol/L, the value that confirms the diagnosis.

Lung: Cl out, water out, thin mucus. Sweat: Cl back in. Lose CFTR and the lung clogs while the sweat turns salty.

The most common mutation

About 70 percent of CF alleles are F508del (a deletion of phenylalanine at position 508). The protein misfolds and is destroyed before it ever reaches the membrane. It is a single recurring defect, which is exactly why a carrier screen can scan for it.

F508del: misfolded CFTR, degraded early, never reaches the surface.

The New Interaction

Inheritance Pattern Decoder

A board question hands you a family, not a label. Read the pedigree clue, predict the pattern, then watch the family tree resolve. Four patterns, five families. CF is hiding in here as the autosomal recessive one.

Family 1 of 5

From the Attending

Stop memorizing disease lists and read the family. Two healthy parents with an affected child of either sex is recessive. Affected in every generation is dominant. Only boys, passed through unaffected mothers, is X-linked. Mom passing it to every single child is mitochondrial. The pedigree tells you the pattern before you ever name the disease.

Run The Cross

Two Carriers, One In Four

Both CF parents are silent carriers, each one Cc. Press run and watch the alleles drop into the square. The recurrence risk is 25 percent at every pregnancy, and the quiet truth is hiding in the unaffected column.

Father

Mother (rows) × Father (columns)

25%Clear (CC)

50%Carrier (Cc)

25%Affected (cc)

Carrier times carrier: 1 in 4 affected. Now look only at the healthy children. Two of every three of them are silent carriers (the hidden 2 in 3).

The 2 in 3 trap

A healthy sibling of a child with CF has a 2 in 3 chance of being a carrier, not 1 in 2.

The affected cc outcome is already excluded because the sibling is healthy. That leaves three boxes (CC, Cc, cC), and two of them carry the mutant allele. Boards love this conditional probability because the reflex answer of one half is wrong.

Healthy sib of a CF patient: remove the cc box, 2 of the remaining 3 are carriers.

The Look-Alike That Started This

CF Versus X-Linked CGD

Both are children with recurrent infections, and that surface match is the whole trap. One is autosomal recessive on chromosome 7; the other is X-linked on a totally different axis. Toggle the two and watch the genetics split.

The discriminator in one line

Cystic fibrosis: meconium ileus, thick mucus, pancreatic failure, salty sweat. Autosomal recessive on chromosome 7, so both parents are carriers. Chronic granulomatous disease: recurrent catalase-positive abscesses and granulomas from a dead respiratory burst. X-linked recessive, so a carrier mother passes it to her sons.

Recurrent infection is the shared bait. The cluster and the chromosome are the answer.

From the Attending

Recurrent infection does not mean X-linked. It means stop and read the rest of the chart. Meconium ileus, salty sweat, and a failing pancreas is CFTR on chromosome 7, autosomal recessive, both parents. Catalase-positive abscesses with granulomas and a negative dihydrorhodamine test is NADPH oxidase, X-linked, a carrier mother. Two doors. The cluster picks the door.

What The Thick Mucus Breaks

The Standard CF Complications

Every CF complication is the same story in a different tube: a duct that needed thin fluid got thick mucus instead. Tap each to open the board-relevant detail.

🦦

Pancreatic insufficiency

malabsorption and ADEK

▼

Tell

Thick secretions plug the pancreatic ducts, so enzymes never reach the gut. Result: steatorrhea (greasy, bulky, foul stools), failure to thrive, and deficiency of the fat-soluble vitamins A, D, E, and K.

Move

Pancreatic enzyme replacement with meals plus ADEK supplementation. Watch for vitamin K driven bleeding and vitamin D driven bone loss.

🦠

Chronic lung infection

Staph early, Pseudomonas later

▼

Tell

Early years: Staphylococcus aureus and Haemophilus influenzae. Older child and adult: chronic mucoid Pseudomonas aeruginosa, the classic CF colonizer. Burkholderia cepacia signals a worse course.

Why

Dehydrated airway mucus cannot be cleared, so it becomes a permanent culture dish. Repeated infection scars the airways into bronchiectasis and drives the digital clubbing.

👨

Male infertility (CBAVD)

obstructive azoospermia

▼

Tell

Almost all men with CF have congenital bilateral absence of the vas deferens, so sperm are made but cannot exit. The result is obstructive azoospermia with normal testosterone and normal spermatogenesis.

Pearl

A young man with absent vas deferens and infertility, even with mild or no lung disease, should be evaluated for CFTR mutations. CBAVD can be the only sign.

👃

Nasal polyps and sinusitis

a red flag in a child

▼

Tell

Chronic thick sinus secretions cause recurrent sinusitis and nasal polyps. Polyps are uncommon in young children, so a child with nasal polyps deserves a CF workup.

Pearl

Nasal polyps in a child point to CF; in an adult with asthma think aspirin-exacerbated disease. Age and context separate the two.

💧

Sweat chloride test

the confirmatory study

▼

Tell

Pilocarpine iontophoresis stimulates sweat, then chloride is measured. A value above 60 mmol/L confirms CF; 30 to 59 is intermediate and needs genetics. Newborn screening flags a high immunoreactive trypsinogen first.

Pearl

High immunoreactive trypsinogen on the newborn screen, then a high sweat chloride, then CFTR genotyping. Screen, confirm, genotype.

Digital clubbing of the fingers in cystic fibrosis — Digital clubbing in CF lung disease

Walk The Counseling

Counsel A Carrier Couple

One couple, one decision at a time. Recurrence risk, carrier math for the healthy sibling, and the look-alike trap, all in the order the exam asks them.

From the Attending

Both parents are carriers, so every pregnancy is an independent 1 in 4. The last healthy baby does not change the next one. The healthy sibling is 2 in 3 to carry. And if someone in the room calls it X-linked, you walk them back to chromosome 7. Same numbers, every time.

The Osteopathic Lens

Clearing The Thick Airway

CF airway clearance is mechanical: thin what you can, then move it out. Osteopathic technique supports clearance and lymphatic return. It never replaces enzymes, antibiotics, or CFTR modulators.

Autonomic and lymphatic supply

Sympathetics to the lungs arise from T1 to T6; high sympathetic tone thickens secretions, so rib raising can lower that tone. Parasympathetics travel with the vagus and drive bronchial gland secretion. The thoracic and abdominal diaphragms are the pumps that move thick lymph and mucus.

T1 to T6 sympathetics, vagus parasympathetics, diaphragms as the pump.

OMT as adjunct only

Rib raising, thoracic lymphatic pump, doming the diaphragm, and suboccipital release support mucus clearance and ease the work of breathing during a CF exacerbation. This is an adjunct that supports airway clearance therapy, inhaled mucolytics, antibiotics, and CFTR modulators. It does not change the genotype or replace medical therapy.

Lymphatic pump and rib raising move the mucus. The medicine still does the curing.

Rapid Fire

Five Quick Calls

Five questions pulled from a larger pool and reshuffled each visit. Cross out (right-click or long-press) and highlight (double-click or double-tap) as you read.

clinical Practice

Walk The Genetics Cases

Full board-style vignettes, one at a time, in shuffled order that never repeats until the bank is exhausted. Progress saves on this device. Cross out wrong choices and highlight clues as you go.

Tip: kill the wrong choices first, then read every explanation chain.

References

First Aid for the Step 1 board exam. Genetics: modes of inheritance, autosomal recessive disorders, and cystic fibrosis; Immunology: chronic granulomatous disease.
Robbins and Cotran, Pathologic Basis of Disease. Cystic fibrosis and the CFTR chloride channel.
Thompson and Thompson, Genetics in Medicine. Mendelian inheritance patterns and pedigree analysis.
Nelson, Textbook of Pediatrics. Cystic fibrosis diagnosis, sweat testing, and complications.

Reviewed by Fatima Ali DO and Kaitlyn Cocuzzo MD. Vignettes are original clinical teaching cases. Confirm management and genetic counseling against current guidelines at the point of care.

Cystic Fibrosis:Both Parents, Not the X

Cystic Fibrosis:
Both Parents, Not the X