Human Pedigree Analysis
Human matings, like those of experimental organisms, provide many examples of single-gene inheritance. However, controlled experimental crosses cannot be made with humans, and so geneticists must resort to scrutinizing medical records in the hope that informative matings have been made (such as monohybrid crosses) that could be used to infer single-gene inheritance. Such a scrutiny of records […]
Shape, Arrangement, and Size
It might be expected that bacterial cells, being small and relatively simple, would be uniform in shape and size. This is not the case, as the microbial world offers considerable variety in terms of morphology. However, the two most common shapes are cocci and rods. Cocci Cocci (s., coccus) are roughly spherical cells. They can […]
Major Fields in Microbiology
Pathogenic microbes, though relatively few in number, have had and continue to have considerable impact on humans. Thus one of the most active and important fields in microbiology is medical microbiology, which deals with diseases of humans and animals. Medical microbiologists identify the agents causing infectious diseases and help plan measures for their control and […]
Microorganisms and Disease
Although Fracastoro and a few others had suggested that invisible organisms produced disease, most people believed that disease was caused by supernatural forces, poisonous vapors called miasmas, and imbalances among the four humors thought to be present in the body. The role of the four humors (blood, phlegm, yellow bile [choler], and black bile [melancholy]) […]
Microscopy and the Discovery of Microorganisms
The earliest microscopic observations of organisms appear to have been made between 1625 and 1630 on bees and weevils by the Italian Francesco Stelluti (1577-1652), using a microscope probably supplied by Galileo (1564-1642). Robert Hooke (1635-1703) is credited with publishing the first drawings of microorganisms in the scientific literature. In 1665 he published a highly […]
Introduction to Microbiology
Microorganisms are defined as those organisms and acellular biological entities too small to be seen clearly by the naked eye They are generally 1 millimeter or less in diameter. Although small size is an important characteristic of microbes, it alone is not sufficient to define them. Some cellular microbes, such as bread molds and filamentous […]
X-linked Recessive Disorders
Let’s look at the pedigrees of disorders caused by rare recessive alleles of genes located on the X chromosome. Such pedigrees typically show the following features: Males are affected more 1. Many more males than females show the rare phenotype under study. The reason is that a female can inherit the genotype only if both […]
PEDIGREE ANALYSIS OF AUTOSOMAL DOMINANT DISORDERS
What pedigree patterns are expected from autosomal dominant disorders? Here the normal allele is recessive, and the abnormal allele is dominant. It may seem paradoxical that a rare disorder can be dominant, but remember that dominance and recessiveness are simply properties of how alleles act and are not defined in terms of how common they […]
Penetrance and expressivity
Many mutations show incomplete penetrance: not every individual with the genotype expresses the corresponding phenotype. Thus penetrance is defined as the percentage of individuals with a given allele who exhibit the phenotype associated wit many mutations show incomplete penetrance: not every individual with the genotype expresses the corresponding phenotype.Thus penetrance is defined as the percentage […]
Carbon Cycle
Carbon is the element that defines life, and as such it is everywhere. The exchange of elements between sources and sinks is referred to as flux. Carbon is present in reduced forms, such as methane (CH4) and organic matter, and in more oxidized forms, such as carbon monoxide (CO) and carbon dioxide (C02). Carbon fixation Although […]