Spinal muscular atrophy carrier couple with normal child: Demonstrating Mendelian inheritance patterns

Spinal Muscular Atrophy (SMA) is an autosomal recessive neuromuscular disorder caused by mutations in the SMN1 gene, with a carrier frequency of approximately 1 in 40-60 individuals. When both parents are carriers, each pregnancy carries a 25% risk of producing an affected child, 50% chance of a carrier, and 25% probability of a completely unaffected offspring. We report the case of a 27-year-old primigravida and her 33-year-old husband, both confirmed SMA carriers through genetic testing. The wife had heterozygous deletions in SMN1 exons 7 and 8, while the husband showed heterozygous duplication with 3 copies. Despite having a family history of SMA (wife's sister died at 18 months with confirmed SMA), the couple elected to forego prenatal genetic testing after comprehensive genetic counseling. The pregnancy was monitored with serial ultrasounds, revealing bilateral renal pelvis dilatation but no other structural anomalies. The couple delivered a healthy male infant at term via normal vaginal delivery with appropriate growth parameters and normal neonatal examination findings. Postnatal genetic testing using Multiplex Ligation-Dependent Probe Amplification (MLPA) revealed normal copy numbers for both SMN1 and SMN2 genes in exons 7 and 8, with MLPA ratios within normal range (1.05-1.14), confirming the infant was neither affected nor a carrier of SMA. This case demonstrates the classical Mendelian inheritance pattern for autosomal recessive disorders, where two SMA carrier parents achieved the favourable 25% probability of having a completely unaffected child. The outcome emphasizes the importance of comprehensive genetic counseling in supporting diverse reproductive choices and highlights that positive outcomes remain possible even in high-risk genetic scenarios.