Genetics and inheritance
The Rules of Inheritance (Magill’s Medical Guide, Sixth Edition)
The primary genes of interest to heredity consist of a set of coded directions for making proteins. Each gene codes for a protein; distinct versions of a gene, which encode slightly different versions of the protein, may be carried in the same or different individuals. The distinct versions of a gene, called alleles, are responsible for differences in hereditary traits among individuals. Each individual receives a combination of alleles encoding proteins that directly or indirectly determine traits such as eye, skin, and hair color; height; and, to a degree, characteristics such as personality, behavior, and intelligence.
In molecular terms, genes consist of a sequence of chemical units called nucleotides, linked end to end in long, linear deoxyribonucleic acid (DNA) molecules. There are four kinds of nucleotides in DNA; each gene has its own nucleotide sequence. The alleles of a gene differ slightly in nucleotide sequence—some alleles differ in the substitution of only a single nucleotide. There are many thousands of genes arranged in tandem on the DNA molecules of a human cell; each DNA molecule is known as a chromosome. In humans, the chromosomes occur in twenty-three pairs, for a total of forty-six chromosomes. The two members of a chromosome pair contain the same genes in the same order, but different alleles of a gene may be present in the two members of a pair. One member of a chromosome pair is derived...
(The entire section is 1297 words.)
Disorders and Diseases (Magill’s Medical Guide, Sixth Edition)
Many human diseases, involving every system in the body, depend on the presence of particular dominant or recessive alleles and are directly inherited. Only the disposition for development of other diseases is inherited—that is, some individuals inherit a combination of alleles that increases the possibility that a genetically based disease will develop during their lifetimes.
The list of diseases contracted through inheritance of a dominant allele is long and impressive. Among the more important of these diseases are achondroplasia, in which individuals are short-statured; familial hypercholesterolemia, in which cholesterol concentration in the blood is abnormally high, leading to vascular disease, particularly of the coronary arteries; Huntington’s disease, a disease characterized by dementia, delusion, paranoia, and abnormal movements that begins in persons between twenty and fifty years of age and progresses steadily to death in about fifteen years; Marfan syndrome, a disease of connective tissues involving the skeleton, eyes, and cardiovascular system, characterized by elongated limbs, abnormal position of the eye lens, and structural weakness of blood vessels, particularly of the aorta; neurofibromatosis, characterized by tumors dispersed throughout the body and coffee-colored skin lesions; polycystic kidney disease, in which dilated cysts grow in the kidneys and interfere with kidney function, leading to...
(The entire section is 1001 words.)
Perspective and Prospects (Magill’s Medical Guide, Sixth Edition)
The primary features of meiosis and fertilization, random segregation of chromosome pairs in meiosis and random rejoining of pairs in fertilization, makes heredity subject to analysis by mathematical techniques. In fact, mathematical analysis of heredity was carried out successfully before there was any understanding concerning meiosis or DNA. The groundwork for this analysis was laid down in the 1860’s by an Austrian monk, Gregor Mendel. Mendel’s research approach and his conclusions were so advanced that they were misunderstood and unappreciated during his lifetime.
Mendel chose garden peas for his research because they could be grown easily and they possessed several hereditary traits that were known to breed true—that is, to appear dependably in offspring. Mendel crossed pea plants with different traits in various combinations. On analyzing the results of his crosses, Mendel realized that the numbers of offspring exhibiting different traits could be explained mathematically if he assumed that parents contain a pair of factors governing the inheritance of each trait. Furthermore, he concluded that the factors separate, or segregate, independently as gametes are formed and are reunited randomly at fertilization. He also discovered that some traits are inherited as dominant and some as recessive. Mendel’s factors were later called genes.
Until Mendel’s time, inheritance was commonly believed to...
(The entire section is 574 words.)
For Further Information: (Magill’s Medical Guide, Sixth Edition)
Campbell, Neil A., et al. Biology: Concepts and Connections. 6th ed. San Francisco: Pearson/Benjamin Cummings, 2008. This classic introductory textbook provides an excellent discussion of essential biological structures and mechanisms. Of particular interest are the chapters “Mendel and the Gene Idea,” “The Chromosomal Basis of Inheritance,” and “The Molecular Basis of Inheritance.”
Lewin, Benjamin. Genes IX. 9th rev. ed. Sudbury, Mass.: Jones and Bartlett, 2008. A college textbook that discusses the entire field of molecular biology and genetics, with many references to the structure and activity of the cell nucleus. Although written at the college level, it is readable and accessible to a general audience. Many highly informative illustrations and diagrams are included.
Lewis, Ricki. Human Genetics: Concepts and Applications. 9th ed. Dubuque, Iowa: McGraw-Hill, 2009. A very accessible undergraduate text that covers the fundamentals, transmission genetics, DNA and chromosomes, and the latest genetic technology, among other topics.
Marieb, Elaine N. Essentials of Human Anatomy and Physiology. 9th ed. San Francisco: Pearson/Benjamin Cummings, 2009. This introductory anatomy and physiology textbook, easily accessible to those with little science background, is richly illustrated with diagrams and photographs, which help to illuminate body systems and...
(The entire section is 338 words.)