Consanguinity and a recessive disorder in a village pedigree
In a rural union council, 9 of 1,400 children born over five years have a severe neuromuscular disorder. The population is endogamous; 41 percent of marriages are between first cousins. A pedigree of one extended family shows two affected siblings whose parents are first cousins and unaffected; the paternal grandmother and maternal grandfather are siblings. No affected child has an affected parent. A pilot screening finds carrier frequency 1 in 28 in this village versus 1 in 300 nationally. A health officer proposes universal newborn screening; a geneticist proposes carrier screening of couples before marriage.
1. Using the pedigree, infer the mode of inheritance and calculate the expected recurrence risk for the next child of the affected siblings' parents.
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Mode of inheritance: autosomal recessive. Evidence:
- Affected children have unaffected parents (skips generation).
- Both sexes affected (rules out X-linked recessive as sole explanation, since an affected daughter would need an affected father).
- Parental consanguinity (first cousins) and a pedigree loop through the grandmother/grandfather sibship raise the probability that both parents carry the same ancestral allele.
Recurrence risk: for two confirmed carriers, each child has 1/4 risk of being affected, 1/2 of being a carrier, 1/4 of being homozygous normal. So the next child's risk is 25 percent, independent of previous children.
Why the population matters: the village carrier frequency of 1/28 is roughly tenfold the national figure. Under Hardy-Weinberg, expected affected frequency would be q squared with q approximately 1/56, giving about 1 in 3,136 - far below the observed 9/1,400 (about 1 in 156). The excess is explained by consanguinity: for first-cousin marriages the offspring risk is q squared plus additional identity-by-descent terms, so the coefficient of relationship must be included. Report the risk as conditional on carrier testing, not as a population average.
2. Compare the two screening proposals on yield, ethics and feasibility in this setting, and recommend a combined strategy.
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Universal newborn screening
- Yield: detects affected infants early, allowing supportive care and avoidance of harm, but does not prevent births. With 9 cases in five years, the absolute number is small; a test with imperfect specificity in 1,400 births will generate false positives and parental anxiety.
- Feasibility: requires a validated assay for this specific disorder, sample transport, and follow-up services that may not exist. Poor for a condition with no early treatment.
Pre-marital carrier screening
- Yield: higher preventive value. At 1/28 carrier frequency, screening both partners identifies at-risk couples before reproduction; each such couple can be counselled on 25 percent risk, prenatal diagnosis, or avoidance.
- Ethics: voluntary consent, confidentiality, and protection from stigma are essential, especially where marriage prospects could be harmed. Community engagement through religious and local leaders is needed; a directive approach would be unethical.
- Feasibility: cheaper per case prevented, but requires laboratory capacity and genetic counselling in the local language.
Recommendation: staged combined approach - pilot carrier screening with counselling in this village, link confirmed at-risk couples to prenatal diagnosis, and introduce targeted newborn screening only if a treatment or management pathway exists. Add a registry to measure outcomes. Do not mandate either programme.