THE GENETICS LAB
The Fundamentals Of Genetics
Cells, whether through meiosis or mitosis, share genetic information through chromosomes. Chromosomes are the package for DNA and genes. A living organism has roughly 5 feet or 1.5 meters of compacted DNA within it's nucleus. To contain the DNA within the nucleus of a cell, the genetic information is wound up into strands like coiled rope. These strands take the shape of X's. Each place on a chromosome holds genetic information which pertains to the expression of a trait. The genetic information a chromosome holds, or one piece of genetic information a chromosome can hold, is known as a gene. An allele is a point or place on a chromosome. The locus is a place on a specific chromosome where a gene is found.
DeoxyriboNucleic Acid is commonly known as DNA. DNA at its most basic level is in essence the blueprint of an organism's genetic make-up. The shape of DNA, discovered by Francis Crick and James Watson, is a double helix. The composition of DNA is a series of nitrogenous bases (known as base pairs), and sugar and phosphate strands. Within the ladder model of DNA [see animated graphic above], the sugar and phosphate strands compose the sides of the DNA model, or molecule, while the actual rungs of the ladder are made up of the 4 nitrogen bases. For a molecule of DNA to maintain its shape there are a number of rules between pair bonds, and the strands of phosphates.
Each strand is formed by a backbone of deoxyribose sugar molecules linked by phosphate residues. Attached to each backbone are chemical structures called bases, which protrude away from the backbone towards the center of the helix, and which come in four types: Adenine, Cytosine, Guanine, and Thymine (designated A, C, G and T). In DNA a C can only hydrogen-bond with a G, and an A only with a T, these interactions, formed by so called hydrogen bonds, hold the two strands together. Each strand of DNA has a series of Gs, As, Ts and Cs attached to its backbone. It is the sequence of these bases that forms the code which is translated by cellular machinery to create a new protein. The other (complementary or antisense) strand always has a sequence that matches the first strand, with each C complemented by a G, and each A by a T, and vice versa.
GENETIC DISORDERS
A genetic disorder is a disease caused by a different form of a gene called a variation, or an alteration of a gene called a mutation. Many diseases have a genetic aspect. Some, including many cancers, are caused by a mutation in a gene or group of genes in a person's cells. These mutations can occur randomly or because of an environmental exposure, asbestos or cigarette smoke are notable examples.
Other genetic disorders are inherited. A mutated gene is passed down through a family and each generation of children can inherit the gene that causes the disease. Still other genetic disorders are due to problems with the number of chromosomes. In Down syndrome, for example, there is an extra copy of chromosome 21.
Although there are many thousands of identified genetic disorders, they can be broken down into 4 basic types:
(1) Single-gene (also called Mendelian or monogenic) This type is caused by changes or mutations that occur in the DNA sequence of one gene. Genes code for proteins, the molecules that carry out most of the work, perform most life functions, and even make up the majority of cellular structures. When a gene is mutated so that its protein product can no longer carry out its normal function, a disorder can result. There are more than 6,000 known single-gene disorders, which occur in about 1 out of every 200 births. Some examples are cystic fibrosis, sickle cell anemia, Marfan syndrome, Huntington’s disease, and hereditary hemochromatosis. Monogenic disorders are inherited in recognizable patterns: autosomal dominant, autosomal recessive, and X-linked.
(2)Multifactorial (also called complex or polygenic) This type is caused by a combination of environmental factors and mutations in multiple genes. For example, different genes that influence breast cancer susceptibility have been found on chromosomes 6, 11, 13, 14, 15, 17, and 22. Its more complicated nature makes it much more difficult to analyze than single-gene or chromosomal disorders. Some of the most common chronic disorders are multifactorial disorders. Examples include heart disease, high blood pressure, Alzheimer’s Disease, arthritis, diabetes, cancer, and obesity. Multifactorial inheritance also is associated with heritable traits such as fingerprint patterns, height, eye color, and skin color.
(3) Chromosomal Because chromosomes are carriers of genetic material, such abnormalities in chromosome structure as missing or extra copies or gross breaks and rejoinings (translocations), can result in disease. Some types of major chromosomal abnormalities can be detected by microscopic examination. Down syndrome or trisomy 21 is a common disorder that occurs when a person has three copies of chromosome 21 [see above].
(4) Mitochondrial This relatively rare type of genetic disorder is caused by mutations in the nonchromosomal DNA of mitochondria. Mitochondria are small round or rod-like organelles that are involved in cellular respiration and found in the cytoplasm of plant and animal cells. Each mitochondrion may contain 5 to 10 circular pieces of DNA. Leber's Hereditary Optic Neuropathy is one example of this rare genetic disorder.
The table below lists a number of genetic disorders, illustrating examples of all four basic types.
| A PARTIAL LIST OF GENETIC DISORDERS | ||
| Aarskog Syndrome | Achondroplasia Hereditary Dwarfism | Adams-Oliver Syndrome Published abstract |
| Bardet-Biedl Syndrome | Bjornstad Syndrome | Drummond's Syndrome Blue Diaper Syndrome |
| Camurati-Engelmann Disease | Fahr's Disease | Fragile X Syndrome | Fryns Syndrome | Gaucher Disease | Hypohidrotic Ectodermal Dysplasia Christ-Siemens-Touraine Syndrome |
| Lesch Nyhan Syndrome | Marfan Syndrome | Menkes Disease kinky hair disease |
| Rett Syndrome | Schindler Disease | Treacher Collins Syndrome |
This page last updated on
October 1, 2009
