AN75.1-5 | Principles of Genetics, Chromosomal Aberrations & Clinical Genetics — Summary & Reflection

REFLECT

KEY TAKEAWAYS

This guide covered the NMC competencies AN75.1-75.5 on chromosomal aberrations and clinical genetics. Numerical aberrations include euploidy (triploidy, tetraploidy) and aneuploidy (trisomy, monosomy) arising primarily from non-disjunction during meiosis, with maternal age being the strongest risk factor. Structural aberrations include translocations (reciprocal and Robertsonian), deletions, duplications, inversions, ring chromosomes, and isochromosomes — balanced rearrangements carry reproductive risks while unbalanced rearrangements cause phenotypic abnormalities. Mosaicism arises from post-zygotic mitotic errors in a single zygote, while chimerism involves cells from two or more zygotes. The six syndromes of AN75.3 are: Down syndrome (trisomy 21 — most common, with cardiac defects, intellectual disability, and early Alzheimer), Edward syndrome (trisomy 18 — overlapping fingers, lethal), Patau syndrome (trisomy 13 — holoprosencephaly, polydactyly, lethal), Turner syndrome (45,X — short stature, streak gonads, coarctation), Klinefelter syndrome (47,XXY — tall, small testes, infertility), and Prader-Willi syndrome (paternal 15q11 loss — hypotonia, then hyperphagia/obesity). Diagnostic techniques progress from karyotyping (5-10 Mb resolution) through FISH (100 kb) to PCR and NGS sequencing (single nucleotide resolution).