Dr. Suha Saleh, HSC 4555
Copstead-Kirkhorn: Pathophysiology, 4th Edition
Chapter 6: Genetic and Developmental Disorders
PRINCIPLES OF INHERITANCE
Human DNA is organized into 46 chromosomes (23 pairs). Paired chromosomes look similar under the microscope but differ in DNA sequence. One member of each pair is inherited from the mother, and the other from the father.
Twenty-two pairs of chromosomes are autosomes. The remaining pair, the sex chromosomes, confers maleness (XY) or femaleness (XX).
During meiotic cell division, the chromosomes are distributed to daughter cells. Meiosis results in four daughter cells, each having one-half the normal number of chromosomes (23). Genes that code for a particular trait come in several forms or alleles. Genotype refers to the particular set of alleles an individual receives. Phenotype refers to an individual’s observable attributes. People with different genotypes may have similar phenotypes. Some traits involve only one gene locus and are called single-gene traits. The transmission of these traits from parent to offspring follows predictable patterns. The expression of singlegene traits is determined by whether the gene is dominant or recessive. Dominant genes usually code for functional enzymes; recessive genes do not. Traits resulting from the interaction of several genes are polygenic and do not follow predictable patterns of inheritance.
CHROMOSOMAL ABNORMALITIES: MENDELIAN SINGLE-GENE DISORDERS,
NONMENDELIAN SINGLE-GENE DISORDERS, MULTIFACTORIAL (POLYGENIC) DISORDERS
Genetic disorders are of three general types: chromosomal aberrations, single-gene disorders, and polygenic disorders.
Chromosome disorders result from an abnormality in number or structure. The presence of only one chromosome of a homologous pair is termed monosomy (e.g., Turner syndrome), and the presence of an excessive number of chromosomes is called polysomy (e.g., Down syndrome). Abnormal rearrangement of portions of the...
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