Genetics is an exciting and rapidly evolving field that has significant relevance to the understanding of human embryology, physiology, and disease processes. Tremendous advances in molecular biology and biochemistry are allowing more comprehensive understanding of mechanisms inherent in genetic disorders as well as improved diagnostic tests and management options. Many of the newer technologies and terms may be unfamiliar to the clinician in practice. Thus, the topics in the first part of the chapter serve as an introduction and review of the basic principles of genetics, including basic knowledge of cytogenetics and molecular biology. The second part discusses principles of inherited human disorders, encompassing different genetic mechanisms as well as how to obtain a genetic history and pedigree. Topics in the third part of the chapter focus on applied clinical genetics, which include dysmorphology, teratology, and perinatology. Common clinical disorders with descriptions of the diseases and discussion of their pathogenesis, diagnosis, and management are also included.
FOUNDATIONS OF GENETIC DIAGNOSIS
Cytogenetics is the study of genetics at the chromosome level. Chromosomal anomalies occur in 0.4% of all live births and are a common cause of intellectual disabilities and congenital anomalies. The prevalence of chromosomal anomalies is much higher among spontaneous abortions and stillbirths.
Human chromosomes consist of DNA (the blueprint of genetic material), specific proteins forming the backbone of the chromosome (called histones), and other chromatin structural and interactive proteins. Chromosomes contain most of the genetic information necessary for growth and differentiation. The nuclei of all normal human cells, with the exception of gametes, contain 46 chromosomes, consisting of 23 pairs (Figure 37–1). Of these, 22 pairs are called autosomes. They are numbered according to their size; chromosome 1 is the largest and chromosome 22 the smallest. In addition, there are two sex chromosomes: two X chromosomes in females and one X and one Y chromosome in males. The two members of a chromosome pair are called homologous chromosomes. One homolog of each chromosome pair is maternal in origin (from the egg); the second is paternal (from the sperm). The egg and sperm each contain 23 chromosomes (haploid cells). During formation of the zygote, they fuse into a cell with 46 chromosomes (diploid cell).
Normal male and female human karyotype. (Used with permission from the Colorado Genetics Laboratory.)
Chromosome Preparation & Analysis
Chromosome structure is visible only during mitosis, most often achieved in the laboratory by stimulating a blood lymphocyte culture with a mitogen for 3 days. Cells processed for routine chromosome analysis are stained on glass slides to yield a light-and-dark band pattern across the arms of the chromosomes (see Figure 37–1). This band pattern is characteristic and reproducible for each chromosome, allowing the ...