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INTRODUCTION

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Few specialties of medicine, if any, can match the unprecedented advances that have occurred in the field of human and medical genetics during the last 2 decades. Since completion of the sequencing of the human genome in 2003, our understanding of the genetic bases of rare single-gene disorders as well as common multifactorial disorders has increased tremendously. Molecular cytogenetic and sequencing techniques are not only being widely used for diagnosis of many disorders but also to help guide management and therapy, particularly cancer therapy. This chapter outlines the key principles of genetics that are relevant for the practice of medicine.

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THE HUMAN GENOME

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Deoxyribonucleic acid (DNA) contains within its sequence a remarkable amount of information that is essential for all developmental and physiologic processes. DNA is composed of purine (adenine and guanine) and pyrimidine (thymine and cytosine) bases bound to deoxyribose sugar moieties that are covalently linked by phosphodiester bonds. The double helix of DNA consists of 2 complementary strands coiled around a common axis in an antiparallel fashion (the 3′ end of one strand is paired with the 5′ end of the second strand). The pairing of bases between the 2 strands is highly specific; adenine is paired with thymine and guanine with cytosine. The double-stranded structure and complementarity afforded by Watson-Crick base pairing not only permit faithful replication and passing of genetic information during meiosis and mitosis, but also allow for flow of information from DNA to ribonucleic acid (RNA) to protein.

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DNA associates with numerous proteins including histones to form higher order structures called chromatin that facilitates packaging of long DNA molecules within the nucleus of the cell. The basic unit of chromatin is the nucleosome, which consists of a segment of DNA wound around 8 histone proteins. The organization of chromatin is complex and involves dynamic interactions between various segments of the DNA. Topologically associating domains (TADs) are regions of DNA that preferentially interact resulting in discrete 3-dimensional chromatin structures that can affect the regulatory architecture of the region. The human genetic material or genome is organized into 23 pairs of chromosomes: 22 pairs of autosomes (numbered 1 to 22 from largest to smallest) and 1 pair of sex chromosomes (XX in females and XY in males). Each chromosome consists of a single, continuous, double-stranded DNA molecule, and thus, the nuclear genome is essentially composed of 46 linear DNA strands. Almost all cells are diploid (2n), meaning they contain homologous pairs of each chromosome; however, germ cells are haploid (n) and contain only 1 copy of each chromosome. Human chromosomes can be visualized under light microscopy and distinguished based on their size and characteristic staining patterns (e.g., G-banding with Giemsa stain). During metaphase, each chromosome consists of 2 chromatids that have a short (p for petit) and a long arm (q) connected by a centromere. The position of the centromere can be used to classify chromosomes as metacentric, submetacentric, ...

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