|
|
| Line 1: |
Line 1: |
| ==Overview==
| | I woke up last week and noticed - I've been single for a while today and after much intimidation from friends I today find myself opted for online dating. They promised me that there are lots of interesting, sweet and [http://Www.reddit.com/r/howto/search?q=regular+folks regular folks] to meet, so the pitch is gone by here!<br>My family and friends are amazing and spending some time together at bar gigabytes or [http://www.cinemaudiosociety.org luke brice] meals is always a must. As I see [http://lukebryantickets.asiapak.net luke bryan concert ticket] that you could not get a good conversation together with the sound I have never been into night clubs. Additionally, I got 2 really cute and definitely cheeky canines that are almost always enthusiastic to meet fresh individuals.<br>I try to maintain as physically fit as potential coming to the gym several times per week. I enjoy my athletics and endeavor to perform or watch because many a possible. Being winter I will frequently at Hawthorn suits. Notice: In case that you will contemplated buying an activity I really don't brain, I have seen the carnage of [http://search.huffingtonpost.com/search?q=wrestling+fits&s_it=header_form_v1 wrestling fits] at stocktake sales.<br><br> |
| Genetic load is the difference between the fitness of the theoretically optimal genotype in a population and the fitness of the observed average genotype in a population. This means that populations with a low genetic load will tend to be healthier, have fitness that is less dispersed, and have offspring that are more likely to survive to reproduction.<ref>Whitlock M., Bourguet D. 2000. Factors affecting the genetic load in Drosophila synergistic epistasis and correlations among fitness components, Evolution 54:5.</ref> On the other hand, populations with a high genetic load will have a greater variance in fitness levels; furthermore, a population with a high genetic load is not likely to have as many organisms at or near the optimal fitness.<ref> Saccheri I., Lloyd H., Helyar S., Brakefield P. 2005. Inbreeding uncovers fundamental differences in the genetic load affecting male and female fertility in a butterfly, Proceedings of the Royal Society 272:1.</ref> Genetic load can be looked at as the probability that an organism will reach its reproductive age.<ref> Crist K., Farrar D. 1982. Genetic load and long- distance dispersal in Asplenium platyneruon, Can. J. Botany 61:1. </ref> This is important to scientists because if genetic load gets too high the population will be in danger of going extinct because the organisms are not able to survive and reproduce.
| |
| ==Definition==
| |
| Genetic load is the reduction in selective value for a population compared to what the population would have if all individuals had the most favored genotype.<ref>{{cite journal|author=[[James F. Crow|JF Crow]]|year=1958|title=Some possibilities for measuring selection intensities in man|journal=Hum. Biol|volume=30|pages=1–13|pmid=13513111|issue=1}}</ref> It is normally stated in terms of [[Fitness (biology)|fitness]] as the reduction in the mean fitness for a population compared to the maximum fitness.
| |
| ==Causes==
| |
| ===Mutation===
| |
| This is the portion of the genetic load which is due to mutation, which is almost always negative since most mutations on a genome are deleterious. Mutation loads are higher in populations which experience a higher mutation rate. Mutation loads are also higher in populations with positive selection. This is because positive selection means that the genome is more likely to keep changes that are made through mutation than genomes that are not undergoing positive selection. This means that the population is more likely to keep mutations that are deleterious. It is important to remember that not all mutations are negative ones, and that mutations which are beneficial will also be kept, but they will occur much less often than the ones that are deleterious. If a population is lucky enough to receive a high amount of beneficial mutations, then that mutation load might actually help the population, though this is unlikely to happen. Mutation load also tends to be higher in larger populations as well as populations that have been around for longer.<ref> Crist K., Farrar D. 1982. Genetic load and long- distance dispersal in Asplenium platyneruon, Can. J. Botany 61:1. </ref> Mutation load is larger in big populations because there are more individuals which can generate and pass on the mutation. Mutation load is bigger in older populations because they have had a longer amount of time to accrue. Mutation load is the main contributing factor to genetic load overall.<ref> Klekowski E. 1988. Genetic load and its causes in long- lived plants, Trees 2:4. </ref>
| |
| ===Inbreeding===
| |
| Inbreeding increases genetic load in the short run, but actually decreases genetic load in the long run. Overall, inbreeding increases the rates of homozygosity. In the short run, this means that offspring are more likely to get two recessive and deleterious alleles, so they will have lower fitnesses and experience an [[inbreeding depression]]. The result is that you could begin to weed out deleterious alleles, some scientists have gone so far as to claim that if a population begins to decline because of higher mutation rates then getting the remaining organisms to breed with their relatives will help push out some of the more harmful mutations.<ref> Byers D., Waller D. 1999. Do Plant Populations Purge Their Genetic Load? Effects of Population Size and Mating, Annual Review of Ecology and Systematics 30:1. </ref> However, once the rate of the allele gets high enough the pairing frequency will get so high that it will start to weed out those alleles at a high rate since the organisms which get them will die before reproducing. This is referred to as a purging of the population as a result of inbreeding.<ref> Barrett S., Charlesworth D. 1991. Effects of a change in the level of inbreeding on the genetic load, Letters to Nature 352:1. </ref> This kind of load is dependent on sexual reproduction, so isn’t present in asexually reproducing species.
| |
| ===Sexual vs. Asexual reproduction===
| |
| Sexual and asexual reproduction both have ways that they directly increase or decrease genetic load. Sexual reproduction can increase genetic load by pairing together two different deleterious alleles who’s net effect is higher than just the sum of their two deleterious effects, this is referred to as a concealed portion of the genetic load.<ref> King J. 1965. The gene interaction component of the genetic load, Donner Laboratory and Lawrence Radiation Laboratory, University of California, Berkeley </ref> However, asexual reproduction can also prevent deleterious alleles from spreading and building up in a population, because less fit individuals won’t have as much reproductive success. This can also apply to those organisms which reproduce asexually because all organisms will create copies of themselves, thus passing on all of their deleterious mutations, causing a buildup in the population.<ref> Zimmer C., Emlen D. 2013. Evolution: Making Sense of Life- Sex: Causes and Consequences. </ref> This is called [[Muller’s ratchet]]. If the mutations that are spread to all offspring are only slightly deleterious to the point where the organisms can still live and reproduce then the genetic load will increase at a steady rate.<ref> Takhtajan A. 2009. Flowering Plants: Asexual vs. Sexual Reproduction: Advantages and Disadvantages. Springer Verlag: New York. </ref> If the mutations are quite harmful then eventually this may lead to a rapid fall in the population size. With both of these in mind, sexually reproducing species are still expected to have lower genetic loads.<ref> Marriage T. 2009. Mutation, asexual reproduction and genetic load: A study in three parts, University of Kansas. </ref>
| |
| ===Recombination/ Segregation load===
| |
| This is the fitness cost of breaking down genetic correlations and linkage disequilibria that were established by natural selection.<ref> Haag C., Roze D. 2007. Genetic Load in Sexual and Asexual Diploids: Segregation, Dominance and Genetic Drift, Genetics 176:3. </ref> Especially useful linkages or correlations happen when two beneficial alleles co-occur, organisms that do inherit them are well off, and those who do not experience a much depleted fitness. By breaking down this relationship you open up the possibility of inheriting one of the “good” genes and one of the “bad” ones, which will increase genetic load as opposed to receiving both of the good ones. It is known as recombination load when this acts on two different loci and segregation load when it happens at a single locus.<ref> Haag C., Roze D. 2007. Genetic Load in Sexual and Asexual Diploids: Segregation, Dominance and Genetic Drift, Genetics 176:3. </ref> Because this is the result of the joining together of two sex cells, this kind of load is only present in sexual reproduction.
| |
|
| |
|
| ===Migration Load===
| | Also [http://okkyunglee.com luke bryan concert tour schedule] visit my weblog - [http://www.netpaw.org concert luke bryan] |
| Migration load is the result of nonnative organisms that aren’t adapted to a particular environment coming into that environment and breeding with individuals who are adapted to that environment. This will mean that their offspring will not be as fit as they would have been if both of their parents had been adapted to that particular environment.<ref> Bolnick D., Nosil P. 2007. Natural selection in populations subject to a migration load, Evolution 61:9. </ref> The offspring will have lower viability rates as well as a lower overall fitness.<ref> Hu X., Li B. 2003. On migration load of seeds and pollen grains in a local population, Heredity 90:1. </ref> This can also be referred to as an [[outbreeding depression]]. This will create a higher load the more different the two environments are that the organisms are from, as well as if there is either a small population that is native, or a large population of immigrating organisms.<ref> Hanski,I., Gaggiotti O. 2004. Ecology, Genetics, and Evolution of Metapopulations. Elsevier Academic Press: London, UK. </ref> This can also happen in asexually reproducing organisms if they come into the area. However, since the adapted and unadapted genes don’t mix, the effect won’t be as drastic. In either case, this means that there is often a steady selection against immigrant phenotypes and genotypes.<ref> Bolnick D., Nosil P. 2007. Natural selection in populations subject to a migration load, Evolution 61:9. </ref>
| |
| == Mathematics ==
| |
| Consider a single [[gene locus]] with the [[allele]]s <math> \mathbf{A} _1 \dots \mathbf{A} _n</math>, which have the [[fitness (biology)|fitness]]es <math>w_1 \dots w_n</math> and the [[allele frequency|allele frequencies]] <math>p_1 \dots p_n</math> respectively. Ignoring [[frequency-dependent selection]], then genetic load (<math>L</math>) may be calculated as:
| |
| | |
| :<math>L = {{w_\max - \bar w}\over w_\max}~~~~~~~~~~(1)</math>
| |
| | |
| where <math>w_\max</math> is the [[maximum]] value of the fitnesses <math>w_1 \dots w_n</math> and <math>\bar w</math> is [[mean]] [[fitness (biology)|fitness]] which is calculated as the mean of all the fitnesses weighted by their corresponding allele frequency:
| |
| | |
| :<math>\bar w = {\sum_{i=1}^n {p_i w_i}} ~~~~~~~~~~(2)</math>
| |
| | |
| where the <math>i^\mathrm{th}</math> allele is <math>\mathbf{A}_i</math> and has the fitness and frequency <math>w_i</math> and <math>p_i</math> respectively.
| |
| | |
| When the <math>w_\max = 1</math>, then (1) simplifies to
| |
| | |
| :<math>L = 1 - \bar w. ~~~~~~~~~~(3)</math>
| |
| | |
| | |
| ==Problem==
| |
| One problem with calculating genetic load is that in order to do so you have to a have a “perfect” or “optimal” genotype with which to compare the population to; this kind of genotype simply does not exist. This is problem because it means that it is harder for scientists to gauge with accuracy how much load a population has, and how much load it can bear without being in danger. This means that all perceptions of genetic load should be taken with a grain of salt.<ref> Agrawal A., Whitlock M. 2012. Mutation Load: The Fitness of Individuals in Populations Where Deleterious Alleles Are Abundant, Annual Review of Ecology, Evolution, and Systematics 43:1. </ref>
| |
| ==References==
| |
| {{reflist}}
| |
| {{DEFAULTSORT:Genetic Load}}
| |
| [[Category:Population genetics]]
| |
I woke up last week and noticed - I've been single for a while today and after much intimidation from friends I today find myself opted for online dating. They promised me that there are lots of interesting, sweet and regular folks to meet, so the pitch is gone by here!
My family and friends are amazing and spending some time together at bar gigabytes or luke brice meals is always a must. As I see luke bryan concert ticket that you could not get a good conversation together with the sound I have never been into night clubs. Additionally, I got 2 really cute and definitely cheeky canines that are almost always enthusiastic to meet fresh individuals.
I try to maintain as physically fit as potential coming to the gym several times per week. I enjoy my athletics and endeavor to perform or watch because many a possible. Being winter I will frequently at Hawthorn suits. Notice: In case that you will contemplated buying an activity I really don't brain, I have seen the carnage of wrestling fits at stocktake sales.
Also luke bryan concert tour schedule visit my weblog - concert luke bryan