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| The Different Methods Used to Measure Relationships | |||||||||||||||||||||||||||||||||||||||||
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Autosomal STR: STRs are short tandem repeats found in the genetic material (or DNA) that makes up the human genome. Several different STRs, regions that are highly variable in length from individual to individual, have been located and examined. The size of the STR is what scientists look at to help answer questions regarding relatedness. The different sizes of DNA found at these STR regions correspond to, what geneticist call, alleles. These alleles are passed on from parents to offspring. By examining random individuals within a particular population, each allele is found to have a frequency associated with it within that group. It is these frequencies that researchers use to calculate the probabilities of relatedness. GeneTree uses STRs to measure probabilities of paternity, and most other DNA relationship test, including the Y-chromosome Haplotyping.  Y-chromosome Haplotype: Y-chromosome haplotypes are used for testing descendants with a common paternal link in their genealogical lineage. For example, if a group of males have strictly a male descent line (may have the same last name, such as Carmichael), and they are thought to all be related to a common male ancestor, examining the Y-chromosomes of the individuals in common makes it fairly easy to support or disprove this hypothesis. By looking at Y-chromosome markers from all of the terminal males with the same paternal descent (all the males with the Carmichael Surname) we would suspect that they should all share the same Y-chromosome markers (with allowances made for calculated mutation rates, which should be small given less than 8 or 9 generations). This test could also be used to distinguish non-paternity in the line, a question that one might not want answered. Although this method is quite a useful tool for genealogists, it is not without its limitations. For example, examining the Y-chromosome haplotypes doesn't give a lot of information about degree of relatedness, just a type of inclusion or exclusion from the family. The figure on the right, Y-chromosome Inheritance Pattern, shows how the Y-chromosome (red squares) is passed from father to son, exclusively (males are represented by squares and females are represented by circles). Explanation for Y-chromosome Haplotyping Results: Y chromosome mutations generally occur once every 500 generations / locus (Heyer et al. 1997). Therefore, if we were to examine 24 loci, we would expect that one locus would show a mutation every 20.8 generations (i.e., 500 generations / 24 markers = 20.8 generations).
Key Terms: MLE (most likely estimate): an estimate of when the most recent common
ancestor between two relatives lived. This is most commonly presented in
number of generations.  Mitochondrial DNA
(mtDNA) Analysis: You must note that in using this method, you are
tracing ancestors through the maternal, not paternal, line. This is
because mitochondrial DNA is passed from the mother to her children. As
long as there is a female lineage connecting some of these people, we
would expect that they would share the same mtDNA, from generation to
generation to generation, etc. (with allowances made for calculated
mutation rates, which are higher than what is observed from the
Y-chromosome) The figure on the right, mtDNA Inheritance Pattern,
shows how the mtDNA (red circles) is passed from mother to all her
children (males are represented by squares and females are represented by
circles).Explanation for mtDNA Haplotyping Results: mtDNA results are provided in a DNA sequence analysis format. The sequence variation from the Cambridge Reference Sequence is provided in a table for easy use in independent research studies (see below).
Mutation Rates and DNA: Because of the rate of mutation, one person's DNA that has been passed on to children, grandchildren, and great-grandchildren, etc. may have mutated several times over the many generations, yielding populations with different mutations. On a global scale, and over 1000's of generations, this can be observed, and best illustrated, by what is called gene pooling. Since populations that have diverged over 1000's of years ago show features that are different than other populations, the effect of gene pooling can be best illustrated by looking at different ethnic groups. For example, ethnic groups evolving closer to the equator, or where the sun is most damaging, tend to have darker skin than those that have evolved in areas that are more excluded from the harmful rays. This is because dark pigment provides them with less susceptibility to sunlight. This is an example of Darwin's Theory of Evolution, or survival of the fittest. Those with the genes best suited for survival will live long enough to reproduce, and pass their genes on to the next generation. Therefore, anyone with a mutation that makes them less susceptible to their environment will live longer and reproduce more. However, mutations related to genes, such as those governing pigmentation, are not passed on as often as mutation in areas of the DNA that don't have important functions, such as genes do. For example, certain areas of the human genome are highly variable from individual to individual. The variability associated with these regions are common because these regions allow for a higher rates of survivable mutations than most genes do. The areas that are most commonly looked at during relationship studies are called STRs (or 'short tandem repeats' of DNA). These regions do not code for genes, but rather make up some of the space between genes. At a specific STR, or genome loci, there might be 8 to 15 different alleles (or sizes of DNA in that region). For one individual, 2 alleles will be established (unless the locus examined is on the Y-chromosome, then you will find only one). Each allele, or size, has a different frequency in a given population, such as Northern Europeans. These frequencies are what are used to calculate probabilities of relatedness. Mutation rates:
How are the probabilities calculated? If we were to focus on testing only one locus, an area on the Y-chromosome, we can provide an easy illustration to help understand this process. At the y-chromosome locus there can be 7 - 20 different possible identification markers (or alleles). On the Y-chromosome each male will only have one of these identification markers (representing one Y-chromosome). Each of the possible 7 - 20 identification markers (found at one locus) have different frequencies in a given population. This is the key to understanding how these probabilities are calculated. One identification marker may have a frequency of 50% in the Caucasian population; another may have a frequency of 2% in the Caucasian population, and etc. Now 2 Caucasian people with a common identification marker of 50% frequency, are not calculated to have a high degree of relatedness. Therefore, the probability of relatedness is less than what would be calculated between 2 people that share the less common identification marker (i.e. the one with the 2% probability in the Caucasian population). Furthermore, the more loci that you examine between two individuals the more revealing your results will be. Therefore, the identification markers that scientists find at different loci when examining your DNA may be either a high frequency identification marker, which provides less to work with, or a low frequency identification marker, which provides more information when common between individuals. |
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