Measuring variation in natural populations: a primer (Genetics)

1. Sources of data Most measures of variation (synonymous with diversity) in natural populations quantify the average amount of difference between two entities from a defined population. For example, biologists may be interested in differences in size or length among organisms in a population, differences among DNA or protein sequences from those organisms, and so […]

The visualization of chromosomes (Genetics)

Chromosomes have been both understood and misunderstood for over the past century. As early as 1882, Walther Flemming was believed to have started the field of human cytogenetics by describing chromatin (chromosome) as the rod-shaped bodies that he microscopically saw in the nucleus of cells. He also coined the term mitosis as the division of […]

Human cytogenetics and human chromosome abnormalities (Genetics)

1. Introduction Modern human cytogenetics can be divided into three distinct eras, on the basis of the technologies that were available at the time. The first of these, the “golden era” extended from the late 1950s to about 1970. During this time, methodology was introduced, that made it possible to obtain karyotypes from a variety […]

Meiosis and meiotic errors (Genetics)

1. The general scheme of meiosis The meiotic process involves two cell divisions, and is accordingly divided into two parts, meiosis I and meiosis II (Figure 1). As in mitosis, these divisions are subdivided into the stages Prophase, Metaphase, Anaphase, and Telophase, and thus a full meiotic process includes two of each of these stages. […]

Human X chromosome inactivation (Genetics)

1. X chromosome inactivation 1.1. The mammalian sex chromosomes Dioecy, the presence of two distinct sexes, is common amongst animals. In mammals, sex determination is controlled by distinct sex chromosomes (the X and Y), which are believed to have originated from an autosomal chromosome pair by the acquisition of a major sex-determining locus and subsequent […]

Acquired chromosome abnormalities: the cytogenetics of cancer (Genetics)

1. Introduction Normal cellular growth is a delicate balance between cellular proliferation and cell death. Cancer is a disruption in this homeostasis, resulting in an imbalance favoring cellular proliferation over cell death. In the last several years, it has become clear that this homeostasis is controlled by genes (see Article 65, Complexity of cancer as […]

Nondisjunction (Genetics)

Nondisjunction is the improper segregation of chromosomes during either meiotic or mitotic cell divisions. The result of nondisjunction is dosage imbalance – either one extra or one missing chromosome for some proportion of the resulting cells. Mitotic nondisjunction is commonly observed in cancer cells (see Article 14, Acquired chromosome abnormalities: the cytogenetics of cancer, Volume […]

Uniparental disomy (Genetics)

  In the mid-nineteenth century, Gregor Mendel discovered that when round garden peas were crossbred with wrinkled garden peas, all of the offspring were round, regardless of the parental origin of each trait in the cross. The resultant principle of equivalence – that genes are expressed equally, no matter what the parental origin is – […]

Mosaicism (Genetics)

1. Mosaicism-overview Chromosome mosaicism, the existence of two cell lines with differing chromosomal constitutions derived from a single fertilization, may be observed in amniotic fluid (AF) or chorionic villus (CVS) samples at prenatal diagnosis or in blood or skin samples of individuals referred for a variety of medical conditions. The diagnosis of mosaicism, particularly when […]

Microdeletions (Genetics)

In the 1980s, the development of in situ hybridization techniques, particularly those utilizing fluorescent markers (FISH) (see Article 22, FISH, Volume 1) (Pinkel et al., 1988), led to the discovery of the first cryptic deletion syndromes, that is, those in which the missing material was not visible using conventional microscopy. Discovery of most microdeletion syndromes […]