Anticipation in myotonic dystrophy can occur through either parent — but the congenital form, which presents at birth with profound hypotonia and respiratory failure, almost exclusively comes from the mother. The reason is biological: maternal transmission of the CTG repeat expansion in the DMPK gene tends to produce the largest expansions across generations, and the congenital form requires a massive expansion (typically greater than 1000 CTG repeats). Paternal transmission rarely achieves expansions of that magnitude.
The common mistake
Working through the anticipation module in Step 1 genetics, Maya was given a follow-up question after correctly identifying the anticipation mechanism in a grandfather-to-son-to-grandson vignette. The follow-up: a newborn is profoundly hypotonic and needs respiratory support at birth. The mother is later found to have mild grip myotonia she had never noticed. Which parent almost always transmits the congenital form?
Maya chose: "Either parent equally — anticipation is parent-of-origin independent."
The reasoning is understandable. Anticipation — the worsening of a trinucleotide repeat disease across generations — genuinely can happen through either parent in many contexts. Huntington's disease, for instance, can show paternal anticipation. The statement "anticipation is parent-of-origin independent" is not entirely wrong as a general rule. The trap is that myotonic dystrophy is an exception, and the congenital form is a specific exam target precisely because students apply the general anticipation rule and miss the nuance.
A lot of students who know anticipation cold still miss this question because the general rule feels complete. It is not — at least not for myotonic dystrophy's most severe presentation.
The actual mechanism
Myotonic dystrophy type 1 is caused by a CTG repeat expansion in the 3' UTR of the DMPK gene on chromosome 19. The disease shows anticipation: each generation tends to inherit a longer repeat than the parent, producing earlier onset and more severe disease.
The severity of disease correlates directly with repeat length. Normal alleles carry 5–37 CTG repeats. Mild disease starts around 50–150. Classic adult disease runs 100–1000. The congenital form requires expansions typically exceeding 1000 repeats.
Maternal transmission in myotonic dystrophy is uniquely prone to producing these massive expansions. The biology is not fully understood at every mechanistic step, but the germline instability of long CTG repeats is substantially higher in female meiosis than in male meiosis for this locus. Paternal transmission tends to produce smaller intergenerational expansions. The congenital form — which requires an expansion so large it disrupts neonatal muscle function and respiratory drive — almost always reflects a maternal lineage where the repeat grew to an extreme length.
This is distinct from other repeat expansion diseases. Huntington's disease, also an anticipation disease, shows the opposite pattern: the juvenile form (which also requires a very large expansion of CAG repeats in HTT) tracks more commonly through paternal transmission. Step 1 exploits the contrast between these two.
The clinical picture of congenital myotonic dystrophy — floppy infant, respiratory failure requiring NICU support, poor suck — should immediately trigger: maternal myotonic dystrophy, massive CTG expansion, maternal transmission.
For context, the parent-of-origin effect here is a transmission asymmetry, not epigenetic silencing. It should not be confused with genomic imprinting, where methylation marks set during gametogenesis silence one parental copy permanently.
How to remember it
Contrast pair for Step 1:
- Huntington's juvenile form → more often paternal (large CAG expansion more common through male germline)
- Myotonic dystrophy congenital form → almost always maternal (large CTG expansion more common through female germline)
The anchor: floppy newborn + respiratory failure + myotonic dystrophy = maternal.
Check yourself
A woman with mild cataracts and grip myotonia gives birth to a child with profound hypotonia, respiratory failure, and poor feeding. She had not previously been diagnosed.
Which of the following best explains why the child is so much more severely affected than the mother?
A) Genomic imprinting — the paternal chromosome was silenced, leaving only the maternal copy active
B) Heteroplasmy — the child inherited a higher proportion of mutant mitochondria from the mother
C) Anticipation via maternal transmission — the CTG repeat expanded massively in the child, producing the congenital form
D) Incomplete penetrance — the mutation was more fully expressed in the child than in the mother
C. This is the classic congenital myotonic dystrophy presentation. The mother has mild classic disease (modest CTG expansion), but the child has the congenital form (massive expansion). Anticipation through maternal transmission is the mechanism — maternal germline instability drives the largest CTG repeat expansions. Genomic imprinting (A) and heteroplasmy (B) are different parent-of-origin mechanisms. Incomplete penetrance (D) describes binary expression, not progressive severity.
Close the gap
The tutor that flagged Maya's "either parent equally" answer — and then tested the correction again in a completely different floppy-newborn vignette to verify the distinction held — is the same tutor available every time you open Gradual Learning. It tracks which nuances you're missing and comes back to them.