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Download Importance of genetics: A brief History: Types of Genetic Diseases
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Importance of genetics: Genetics is playing an important role in the practice of clinical medicine. Medical genetics is now becoming central components of our understanding of most major diseases. These include not only pediatric diseases, but also common adult diseases such as heart disease, diabetes, and many cancers. Medical genetics involves any application of genetics to medical practice, it thus includes: * Studies of the inheritance of disease in families. * Mapping of disease genes to specific locations on chromosomes * Analysis of the molecular mechanisms through which genes cause disease * Diagnosis and treatment of genetic disease (ex. Gene therapy) A brief History: * In 1865: Gregor Mendel, performing a series of designed experiments on living organisms (garden peas). He then used this experimental information to formulate a series of fundamental principles of heredity. * In 1900: Landsteiner discovered the ABO blood groups. * In 1902: Garrod described "alkaptonuria" as the first "inborn error of metabolism". * In 1909: Johannsen use the term "gene" to denote the basic unit of heredity. * In 1944: Oswald Avery showed that genes are composed of Deoxyribonucleic acid (DNA). * In 1953: James Watson and Francis Crick describe the physical structure of DNA which formed the basis for what is now known as a molecular genetics (the study of the structure and function of genes at the molecular level). * In 1956: the human chromosome number was corrected from 48 to 46 chromosomes, and then * In 1959: scientists discovered that Down syndrome is caused by an extra copy of chromosome 21. * In 1990: the beginning of Human Genome Project which provided the complete human DNA sequences in the year 2003 * (the term genome refers to the entire DNA in an organism). Types of Genetic Diseases Each human is estimated to have approximately 30000 to 40000 different genes. Alterations in these genes, or in combinations of them, can produce genetic disorders. These disorders are classified into several major groups: 1. Chromosomal disorders: in which entire chromosomes (or large segments of them) are missing, duplicated, or altered. These disorders include diseases such as Down syndrome and Turner syndrome. 81 2. Single-gene disorders: single genes are altered; these are often termed "mendelian" conditions. Examples include Cystic fibrosis, Sickle cell disease, and hemophelia 3. Multifactorial disorders: result from combination of multiple genetic and environmental causes. Example, many birth defects, such as Cleft lip or Cleft palate as well, as many adult disorders including Heart disease, cancer, and Diabetes 4. Mitochondrial disorders: small number of disease caused by alteration in the small cytoplasm mitochondrial chromosome Concepts Defined Gene: a unit of hereditary information consisting of a particular nucleotide sequence found on a chromosome. Allele: an alternative form of a gene. Homozygous: describes a genetic condition characterized by the presence of two identical alleles for a given gene. Heterozygous: describes a genetic condition characterized by the presence of two different alleles for a given gene; the individual contains one dominant and one recessive allele in a gene pair. Chromosome: is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions. Chromatin: is the combination of DNA and proteins that make up the contents of the nucleus of a cell. * The primary functions of chromatin are: 1- package DNA into a smaller volume to fit in the cell 2- Strengthen the DNA to allow mitosis and meiosis 3- Prevent DNA damage, 4- Control gene expression and DNA replication. * The primary protein components of chromatin are histones that compact the DNA. Chromatin is only found in eukaryotic cells. Prokaryotic cells have a very different organization of their DNA which is referred to as a genophore (a chromosome without chromatin). Autosomes: The names for all the chromosomes that are not sex chromosomes; chromosomes that occur in homologous pairs in both males and females and do not bear the genes determining sex. 81 Homologous chromosomes: a pair of chromosomes that are matched because they have the same physical appearance and possess genes for the same characteristics; one homologous chromosome is inherited from the father and the other homologous chromosome is inherited from the mother. Non homologous chromosomes: chromosomes that are not alike; when compared, these chromosomes are different physically and bear genes for different characteristics. Sex chromosomes: the pair of chromosomes that determine sex in an organism; the X and Y chromosomes in humans. Sex determination: the combination of sex chromosomes that determine the sex of an offspring; in humans the sex chromosomes of a normal female are XX and the sex chromosomes of a normal male are XY. Genetic linkage: the inheritance of certain genes as a group because they are found closes together on the same chromosome; linked genes on sex chromosomes or autosomes do not show independent assortment. Crossing Over: homologous chromosomes exchange segments. Nucleic acids: includes DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). Together with proteins nucleic acids make up the most important macromolecules ; each is found in abundance in all living things, where they function in encoding, transmitting and expressing genetic information Deoxyribonucleic acid (DNA): is a nucleic acid containing the genetic instructions used in the development and functioning of all known living organisms (with the exception of RNA viruses). The DNA segments carrying this genetic information are called genes. Likewise, other DNA sequences have structural purposes, or are involved in regulating the use of this genetic information. Along with RNA and proteins, DNA are essential for all known forms of life. Ribonucleic acid (RNA): Like DNA, RNA is made up of a long chain of components called nucleotides. Each nucleotide consists of a nucleobase, a ribose sugar, and a phosphate group. The sequence of nucleotides allows RNA to encode genetic information. All cellular organisms use messenger RNA (mRNA) to carry the genetic information that directs the synthesis of proteins. In addition, many viruses use RNA instead of DNA as their genetic material. Recombinant DNA: DNA that has been altered as a result of the recombination of genes; the formation of new combinations of different alleles of each gene on a chromosome; the result of crossing over. 82