0.2 Eukaryotic genetics  (Page 6/9)

There are two kinds of tetrads in fungi: ordered and unordered tetrads. Ordered tetrads contain the spores (the products of a single meiosis) inside the sac (ascus) in a linear order according to the moving behaviour of chromosomes in meiosis. The tetrads of the kind are available in Neurospora crasa, for example. Unordered tetrads contain the spores inside the ascus in a disorder without any sequence, which are available, for example, in Saccharomyces cerevisiae. Genetic analysis of ordered tetrads technically give more information than that of unordered tetrads. A demonstration of genetic analysis in ordered tetrads is given in MITOPENCOURSEWARE ( PDF ).

Lecture 28. human dna polymorphisms

One of the most important tools underlying the revolution in medical genetics is the ability to visualize sequence differences directly in DNA. When studied in the context of a population, these differences in DNA sequences are called polymorphisms; they may occur in coding regions (exons) or noncoding regions of genes. The ability to visualize thousands of DNA polymorphisms has made possible family studies for tracking genes of medical importance. This technique has located and identified genes for many disorders with a clear pattern of mendelian inheritance, such as cystic fibrosis, the inherited muscular dystrophies, and neurodegenerative disorders such as Huntington's disease. Methods that exploit genetic polymorphism will also be essential for finding genes that predispose people to more common conditions in which inheritance patterns are complex, such as diabetes, atherosclerosis, and hypertension.

DNA polymorphisms are also playing a crucial part in unraveling the genetic basis of tumor formation and progression in cancer. They provide markers for the loss of specific chromosomal segments during the evolution of a tumor. DNA polymorphisms have already been crucial in the identification of genes important for susceptibility to common forms of cancer, such as colon cancer, as well as susceptibility to less common childhood tumors, such as retinoblastoma and Wilms' tumor.

The most useful DNA sequence polymorphisms have many alternative forms. The value of highly variable DNA sequences as genetic markers rests on straightforward principles. Every person carries two copies of each chromosome except the sex chromosomes. If a DNA polymorphism is to be useful in analyzing the transmission of the two chromosomes in a family or the loss of one of the chromosomes during tumorigenesis, then the DNA copies at the polymorphic site of the person under study must be different in the two chromosomes ( Figure 1A ), Figure 1B ), Figure 1C ), and Figure 1D ). The likelihood that a given person will have different DNA sequences at the polymorphic site directly determines the usefulness of that site in genetic studies. Chromosomal sites at which the DNA sequences can have many alternative forms are thus ideal sites for genetic markers. At these sites, a person is most likely to carry two alternative DNA sequences, accurately marking the two alternative chromosomes.