Chapter 4 - Cytogenetics: Part 1, General Concepts and Aneuploid Conditions

Abstract

This chapter describes traditional cytogenetic testing. Testing methods for detection of numerical chromosome abnormalities are reviewed. Emphasis is on methods that are preferred for prenatal diagnosis of chromosome abnormalities, banding methods, and use of FISH testing. Limitations of use of direct chromosome testing are discussed. Expectations for information provided in chromosome testing reports are described. The frequency and types of abnormalities observed at various stages and outcomes of pregnancy, including early pregnancy loss, stillbirth, and live births, are described. The basis for variations in syndromes associated with particular chromosome disorders and the critical evaluation of data for counseling are discussed. The mechanism for development of mosaicism and the critical evaluation and uncertainties associated with prenatal diagnosis of mosaicism and counseling challenges are discussed. Clinical associations with risk of conceptions with numerical chromosome abnormalities are discussed relative to maternal age, recurrent early pregnancy loss, and recurrence of trisomy in live born offspring. Clinical value of testing early losses and parents of children with numerical chromosome abnormalities are discussed. Concepts of uniparental disomy (UPD) and imprinting are introduced. Guidance regarding cytogenetic results that should prompt consideration of UPD is provided.