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Genetics is the study of how hereditary information is stored, transmitted, and expressed in living organisms. This information is encoded in the nucleotide sequence of deoxyribonucleic acid (DNA), which serves as the molecular foundation for all inherited traits. Although an individual's DNA sequence remains largely stable throughout life, DNA replication is not entirely error-free, allowing some nucleotide changes (mutations) to escape repair.[1] Many genetic variants do not affect the phenotype, but others may cause congenital or acquired disease and can be inherited by future generations if they occur in germ cells. In addition to traditional Mendelian inheritance, gene expression is influenced by epigenetic mechanisms that alter phenotype without changing the DNA sequence. Together, these concepts provide the basis for understanding chromosome behavior and abnormalities and underpin cytogenetic testing.[2] Cytogenetic testing involves examining chromosomes to detect abnormalities, including aneuploidy and structural abnormalities. A normal human cell contains 23 pairs of chromosomes, including 22 pairs of autosomes and a pair of sex chromosomes (XX or XY). (See Image. Human Male Karyotype). Aneuploidy is the presence of one or more extra chromosomes (47, XX,+21 or 48, XXXY) or having missing chromosomes (45, XO). The most common aneuploidies are Down syndrome (trisomy 21), Edward syndrome (trisomy 18), and Turner syndrome (monosomy X). The types of structural abnormalities are:[2] Duplication: Part of a chromosome is repeated Deletion: Part of a chromosome is missing Translocation: Material between 2 different chromosomes is exchanged (this exchange may be balanced or unbalanced) Inversion: Part of the chromosome is inverted within the chromosome Insertion: Addition of material from another chromosome

The types of structural abnormalities are:[2] Duplication: Part of a chromosome is repeated Deletion: Part of a chromosome is missing Translocation: Material between 2 different chromosomes is exchanged (this exchange may be balanced or unbalanced) Inversion: Part of the chromosome is inverted within the chromosome Insertion: Addition of material from another chromosome Cytogenetic testing is used across a wide range of clinical scenarios, including the evaluation of congenital disorders, prenatal assessment after abnormal ultrasound or biochemical screening, recurrent miscarriage, and postnatal investigations for mosaicism, intellectual disability, autism spectrum disorder, and developmental delay. Cytogenetic testing is also a key tool in identifying both solid tumor and hematologic malignancies, where cytogenetic findings provide crucial diagnostic and prognostic information. Conventional cytogenetic analysis involves examining metaphase chromosomes using Giemsa staining (G-banding) to visualize A · T and G · C regions.[3] Karyotyping provides a genome-wide overview and detects large chromosomal abnormalities (> 5 Mb), such as aneuploidy, major deletions, and translocations. However, more minor or cryptic alterations require more sensitive molecular techniques for accurate detection.[4]

Risks related to chorionic villi sampling are higher before 10 weeks of gestation. While the complication rate for chorionic villus sampling is approximately 1%, it is lower (0.5%) for amniocentesis. Chorionic villus sampling allows the patient to contemplate pregnancy termination earlier.[8][27] Amniocentesis may result in rupture of the amniotic sac, amniotic fluid leakage, and chorioamnionitis.