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

REFLECT

Consider the clinical scenario from the hook: a newborn with suspected Down syndrome born to young parents. You now know that translocation Down syndrome can occur at any maternal age and that karyotyping both the child and the parents is essential for accurate recurrence risk counselling. Reflect on how the diagnostic techniques you have learned — karyotyping, FISH, PCR, and sequencing — form a hierarchy of increasing resolution, each with specific indications and limitations. Think about how mosaicism can modify the clinical presentation of any chromosomal disorder and why it is important to consider when counselling families.

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).