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Anthro Notes

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Western University
Anthropology 2235A/B
Eldon Molto

Anthro Notes Sept. 20/12 Lec. 3 - The field of genetics has a long history and key developments are the understanding of genetic transmission in families and populations, and the understanding of the molecular structure of genetic transmission. This together with advances in technology, particularly extracting DNA from tissue using PCR and sequencing the results, is the foundation of modern forensic genetic analysis. In the last two decades, the field of forensic science has changed dramatically due to genetic science and the admissibility of science in the courts. It is important to have a historical perspective to illustrate the long but disjointed development of the field and to emphasize the pace of change due to the exponential growth of molecular genetics. - History: 1865 Mendels Laws (discovered in 1900). 1908 Hardy-Weinberg Theorem. 1953 Watson-Crick. 1977 Sequencing (Sanger method). 1981 Human mitochondrial genome completely sequenced. 1985 PCR invented. 1987 First forensic case using RFLP. 1991 First paper on STRs. DNA Identification Act and TWGDAM. 1995-1999 First DNA STR databases in UK, US, and Canada. 2002 SNP technology. 2003 Human genome project completed. - Cytogenetics: Cytogenetics is the study of genetic mechanisms that operate within the context of a cell. In the human cell there are 23 pairs of chromosomes (46 total) in the nucleus, and in the cytoplasm another hereditary structure exists called the mitochondrion. Both chromosomes and mitochondria are made of DNA. DNA is present in the nucleus (nuclear DNA) and in the mitochondrion (mtDNA). In each cell there are hundreds of mitochondria. Because all the mtDNA is the same, the mitochondria is often called the 47 chromosome. mtDNA only comes from the mother. - In the human nuclear karyotype, DNA is located on long linear structures called chromosomes. When a new individual is formed by an egg being fertilized by a sperm, the new individual (zygote) receives 23 chromosomes from each parent. Females have two X chromosomes (receive X from each parent) and males have an X and a Y (Y comes from father). Since the new individual has 46 chromosomes and eventually has to transmit his or her DNA to the next generation, the 46 is reduced to 23 by a cell division process called meiosis. The Y chromosome is smaller and has fewer genes (exons). In forensics, junk DNA (introns) is studied, not the exons. The Y chromosome has many polymorphisms that are forensically valuable, especially in sexual assault cases. - Mendels Laws: In 1865, Mendel published two key laws of genetics. Law 1 Segregation: States that a simple genetic trait is determined by a pair of separate factors (now called alleles), one inherited from each parent intact (discrete) from generation to generation (do not blend). Law 2 Independent Assortment: Deals with the inheritance of two or more traits and states that each pair of alleles behaves independently of other pairs. Thus it is possible to predict the statistical probabilities of matings. This law provides basis for the product rule of probability. The laws are fundamental to forensic DNA analysis and form the basis for determining random match probabilities. The term gene is used to indicate any heritable unit and allele is used to represent variable forms (number of repeats in nuclear DNA testing) of the gene. Genes that code for proteins are called exons. - If there is mating between two homozygotes (BB and bb), the offspring will all be phenotypically B, but genotypically Bb. The recessive allele does not show in the heterozygote state. This is called a null allele. There are no null alleles in DNA analysis, but if a sample is highly degraded sometimes the DNA of one of the alleles is destroyed, resulting in an erroneous interpretation. The loss of an allele is called allelic dropout.- Hardy-Weinberg genetics: In 1908, Hardy and Weinberg detailed the laws of heredity at the population level which is a fundamental model for population genetics. The basic formula for a diallelc (two allele) system is: a + b = 1. The mating expansion of this is , where is the relative frequency of the aa genotype (homozygous STRs), is the relative frequency of the bb genotype (homozygous STRs), and 2ab is the relative frequency of the heterozygote. The HW equilibrium model assumes that populations are ideal (stable) and the statistical expansion is used to determine if subsets of forensic DNA data are suitable for making judgements about alleles sampled in forensic cases. Until DNA profiles are regarded as being sufficiently distinctive to establish individuality, forensic arguments are going to assign probabilities on the basis of proportions of marker types (STRs) in populations (blacks, whites, etc) that are stored in databanks and databases. - The allele pool: The collective genotype of the population is known as the allele pool. Even though allele frequencies remain constant, the genotypic relative frequencies differ through time. - Watson and Crick discovered the structure and function of DNA. In 1953, they published a paper in Nature titled Molecular Structure of Nucleic Acids: a structure for deoxyribonucleic acid. This article formed the basis for understanding heredity at the molecular level, including how proteins are made via a pathway between DNA, mRNA and transfer RNA. At one time it was thought that most of the DNA was used to code for proteins (protein synthesis), but we now know that less than 5% of our genome is made up of structural (exons) genes. Most of it is made up of non-coding repeated segments that are now used in forensic DNA analysis. Watson and Crick postulated that the base pairings they found (A-T and C-G) would have important implications. Today the base pair rule forms the basis of all our sequencing technology. - DNA structure and reactivity: The tightly wound helix protects the bases which are connected by a weak hydro
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