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
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.
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
C
c
C
c
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).
Carrier screening detects the silent Cc parents
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.
Sweat chloride test (pilocarpine iontophoresis)Digital clubbing in CF lung disease
Nasal polyposisPseudomonas aeruginosa on culture
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.
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.
Bone Wizardry is an independent educational resource for visual learning in the medical sciences. It is not affiliated with, endorsed by, or sponsored by any licensing or examination board, contains no real or recalled examination questions, and does not guarantee any educational or examination outcome.
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