29, First child of parents shows developmental defects want to take help from genetic counselor for their second child. The genetic counselor must know to predict the fate of second child
(1) Both parents are diseased
(2) Mode of inheritance of disease
(3) Marker for disease monomorphic
(4) Marker for disease is polymorphic
Option Analysis
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(1) Both parents are diseased: Parents of children with developmental defects are typically unaffected carriers for recessive traits, not diseased themselves. Knowing parental disease status alone fails to predict Mendelian risks like 1/4 affected offspring.
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(2) Mode of inheritance of disease: Essential for risk assessment. Autosomal recessive yields 25% recurrence if carriers confirmed; dominant gives 50% if one parent affected. Counselors use pedigrees and family history to classify as autosomal, X-linked, or mitochondrial.
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(3) Marker for disease monomorphic: Monomorphic markers lack variation, offering no linkage or heterozygote detection for carrier screening or prenatal prediction.
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(4) Marker for disease is polymorphic: Helpful for tracking alleles via linkage but insufficient without inheritance mode to compute exact probabilities like 1/2 or 1/4.
Why Mode of Inheritance Matters Most
Counselors analyze first child defects via karyotyping or sequencing, then infer inheritance from family patterns. For de novo mutations, risk stays low (1-2%); parental mosaicism raises it higher. Polymorphic markers aid prenatal diagnosis only after mode confirmed.
This CSIR NET-style question tests core genetics: recurrence risk hinges on inheritance pattern, not just parental status or markers. Parents seek counseling post-first child defects to inform decisions like prenatal testing.
