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Topic: Heterozygote advantage

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	Heterozygote Summary
		Sometimes the heterozygote is not phenotypically distinguishable because of the phenomenon of dominance and recessiveness.
		This advantage in fitness is termed a heterozygote advantage (also termed heterozygote superiority) and leads to a relatively stable balance (equilibrium) regarding the representation of alleles in a population (i.e., the percentage of alleles in a population).
		If the test is sensitive enough, the heterozygote has the phenotype of both parents: when a person who is homozygous for the A blood type gene marries a person who is homozygous for the B blood type gene, they produce heterozygous children who express both A and B antibodies (see, co-dominance).
	www.bookrags.com /Heterozygote (1579 words)

	BioMed Central \| Full text \| Effect of human leukocyte antigen heterozygosity on infectious disease outcome: The need ...
		Although population heterozygote advantage and allele-specific overdominance are different [10], reports of population heterozygote advantage have frequently been interpreted as confirmations of the Doherty-Zinkernagel hypothesis of allele-specific overdominance.
		Population heterozygote advantage (fl) may be observed even when resistance is not overdominant, but only dominant, additive, recessive or only underdominant, as long as allele frequencies are sufficiently far toward the lower right of the parameter space (high diversity of resistant alleles and low diversity of susceptible alleles).
		Specifically, population heterozygote advantage may be observed when resistance is additive (heterozygotes have risks equal to the average of the risks of the corresponding homozygotes), when resistance is recessive, or even when it is underdominant (heterozygotes have higher risk of disease than either corresponding homozygote).
	www.biomedcentral.com /1471-2350/4/2 (5257 words)

	Overdominance - Wikipedia, the free encyclopedia
		Overdominance is an alternate term for heterozygote advantage, a condition in genetics where the phenotype of the heterozygote is fitter than the phenotype of either homozygote.
		The theory that heterosis or hybrid vigor could be explained by heterozygote advantage became known as the overdominance hypothesis.
		The overdominance hypothesis attributes to heterozygote advantage the survival of many alleles that are recessive and harmful in homozygotes.
	en.wikipedia.org /wiki/Overdominance (693 words)

	sociology - Heterozygote advantage (Site not responding. Last check: 2007-11-04)
		A heterozygote advantage (heterozygous advantage or overdominance) describes the case in which the heterozygote genotype has a higher relative fitness than either the homozygote dominant or homozygote recessive genotype.
		This selection favoring the heterozygote is one of the mechanisms that maintains polymorphism and helps to explain some kinds of genetic variability.
		This advantage is dominant, and conveyed in the heterozygote.
	www.aboutsociology.com /sociology/Heterozygote_advantage (1371 words)

	MCDB 2150 -- Lecture 40
		Heterozygote advantage: If a heterozygous genotype has the highest relative fitness, the population will reach an equilibrium with the heterozygote stabilized.
		One example of heterozygote advantage that we have touched on before is sickle cell anemia in parts of Africa.
		The heterozygotes do not exhibit the recessive sickle cell disease, but are resistant to malaria, and thus form the healthy population that is responsible for most of the reproduction.
	www.colorado.edu /MCDB/MCDB2150Fall/notes/L40.html (2212 words)

	Heterozygote advantage - Wikipedia, the free encyclopedia
		A well-established case of heterozygote advantage is that of the gene involved in sickle cell anaemia.
		The first experimental confirmation of heterozygote advantage was with Drosophila melanogaster, a fruit fly that has been a model organism for genetic research.
		The homozygote wild type was perfectly healthy, but did not possess the improved viability of the heterozygote, and was thus at a disadvantage compared to the heterozygote in survival and reproduction.
	en.wikipedia.org /wiki/Heterozygote_advantage (1665 words)

	Modelling the effects of memetic taboos on genetic homosexuality
		The models for heterozygote advantage and sex-specific advantage (the latter not explicitly discussed here, but see Ruse 1981; Gallup and Suarez 1983; Dickemann M 1995; Miller 2000) require a fairly extreme set of parameters in order for them to be able to sustain high levels of homosexuality in populations.
		The magnitude of the required t is therefore scarcely plausible, and one might be tempted to exclude the heterozygote advantage hypothesis on this basis.
		Furthermore, it is clear that although heterozygote advantage can (at least theoretically) sustain levels of homosexuality, and female-specific selection is even capable of increasing it, both of these mechanisms are unstable and vulnerable to sudden fluctuations in taboo (but not to frequency-dependent taboos).
	cfpm.org /jom-emit/2001/vol4/gatherer_d.html (5123 words)

	Overdominant Selection
		The logic behind heterozygote advantage at MHC is that the possession of two different alleles, each with specific binding specificities, would increase the number of antigen sequences that can be presented by the MHC molecule.
		The claim is that the substitution is favoured because the mutant allele almost always produces a heterozygote that has a selective advantage over the homozygotes and more common alleles with respect to their immune responses (6).
		There is also heterozygote advantage because being heterozygous prevents the possibility of a parasite "attacking" a particular MHC allele and driving it to extinction.
	www.science.mcmaster.ca /Biology/CBCN/genetics/hog_overdom.htm (617 words)

	Lecture #27, 4/29/03
		A situation in which the heterozygote is the most fit type (heterozygote advantage) will maintain both alleles in a population.
		In addition to heterozygote advantage, other factors that could be contributing to maintenance of genetic variation in populations could include the neutral nature of many mutations, and the fact that many new mutations may only be deleterious in the homozygous state.
		This is an aspect of the field of "molecular evolution".Mitochondrial DNA has advantageous properties for the study of evolution: small circular DNA about 16 kb in size in animals, inherited through the mother, no apparent recombination, DNA sequences change quickly in evolution, multiple copies (100-1000) per cell.
	www.wsu.edu /~thorglab/biol301/lectures/lecture0429.html (738 words)

	Medscape MEDLINE search: Heterozygote
		Heterozygote deficiencies in small lacustrine populations of brook charr Salvelinus Fontinalis Mitchill (Pisces, Salmonidae): a test of alternative hypotheses.
		Heterozygote excess in small populations and the heterozygote-excess effective population size.
		A quantitative assay for JAK2(V617F) mutation in myeloproliferative disorders by ARMS-PCR and capillary electrophoresis.
	search.medscape.com /uslclient/searchMedline.do?queryText=Heterozygote (1112 words)

	PhD position: Evolution of major histocompatibility complex (MHC) polymorphisms
		Panels (b) and (c) show that the average fitness of an MHC allele decreases with its frequency in the population in both in the co-evolution scenario, and in the heterozygote advantage scenario, respectively.
		The most important result is that both the heterozygote advantage and the co-evolution have a large contribution to the MHC polymorphism.
		In Fig2 we depict the dynamics of the allele frequencies for the co-evolution (a) and the heterozygote advantage (b) scenario for a 100 additional host generations.
	www-binf.bio.uu.nl /rdb/PhD/PhD.html (2702 words)

	[No title]
		heterozygote advantage) because the heterozygote has the highest probability of survival.
		Individuals that are heterozygous for the sickle-cell allele in areas where there is malaria have a fitness advantage over people that lack the sickle-cell allele and those who are homozygous for the sickle-cell allele.
		Draw diagrams for the patterns of fitness differences among genotypes that illustrate the relationship between mean fitness of the population and allele frequency.
	darwin.eeb.uconn.edu /eeb245/supplements/Problem-Set-2-Answers.doc (788 words)

	[No title]
		Population genetics — the study of the distribution of genes in a population and of how the frequencies of genes and genotypes are maintained or changed.
		Heterozygote advantage — heterozygotes have a higher reproductive fitness relative to either type of homozygote.
		Balanced polymorphisms are maintained through selective advantage of the heterozygote over either of the homozygotes.
	www.u.arizona.edu /~ajgulbis/MedLinks/Genetics/GeneticsType/9.doc (677 words)

	Evolution as a Process
		Reproductive advantage cannot be assessed until the offspring in turn, reach reproductive age and reproduce.
		Another possibility is that heterozygotes may exhibit superior phenotypes and compensate for liabilities experienced by homozygotes of the same alleles.
		If the heterozygote were less fit than either homozygote, the allele with an initially higher frequency would continue to increase until the other allele(s) was eliminated.
	uts.cc.utexas.edu /~bramblet/ant301/four.html (8701 words)

	Balancing selection Summary
		This is called "heterozygote advantage." As a result, the beta-globin sickle-cell allele will be maintained along with the wild-type allele in populations exposed to malaria—an example of balancing selection.
		The two most well studied are heterozygote advantage (overdominance) and frequency dependent selection.
		In heterozygote advantage, an individual who is heterozygous at a particular gene locus has a greater fitness than a homozygous individual.
	www.bookrags.com /Balancing_selection (1050 words)

	Faculty - Molecular Biology Program - Bioscience Graduate Studies - University of Utah (Site not responding. Last check: 2007-11-04)
		This in turn favors the evolution of MHC-based disassortative mating preferences because such matings produce progeny that are high fitness because either they are MHC heterozygotes or they have an advantage in the molecular arms race with pathogens.
		Heterozygotes have no advantage when infected individually with either pathogen.
		This is the first experimental confirmation that MHC heterozygote advantage may emerge because heterozygotes have greater overall resistance during multiple infections.
	www.bioscience.utah.edu /mb/mbFaculty/potts/potts.html (767 words)

	Natural Selection Lecture 4 (Site not responding. Last check: 2007-11-04)
		Since each of the two homozygous types is inferior to the heterozygote, natural selection keeps both alleles in the population in intermediate frequencies.
		This may happen when a genotype has a net advantage when it is rare.
		The survival advantage of the gray morph is its camouflage.
	home.earthlink.net /~dayvdanls/evol18_4.html (525 words)

	Population Genetics Made Simple
		If this "selective advantage" is.01, then that means that the chance this mutation will be passed on to an offspring is about 1.01 times higher than the chance that DNA without this mutation will be passed on to an offspring.
		The selective advantages of mutations are assumed to be independent, meaning roughly that the number of copies of DNA passed on from an individual with several mutations is the product of the number passed on for each individual mutation.
		The only advantage of a large population is that if there are say 100 mutations with selective advantages of.01, they can spread through the population in parallel, leading to a rate of evolution similar to that of one mutation with a selective advantage of about 1.
	www.cs.unc.edu /~plaisted/ce/genetics.html (9332 words)

	Gene Expression: Rare male advantage?
		Some of this might be due to heterozygote advantage in the MHC complex (an example of balancing selection).
		But, once the child is born the one with the novel HLA profile might be at an advantage in the event of pathogenic sweeps through the local population.
		For example, if a woman lives among her kin in a tribe, call them A, and she is one of the few to marry someone from tribe C, she might have all the skills and accumulated "wisdom" of her own people, while her mate could bring information over from his own tribe.
	www.gnxp.com /MT2/archives/002536.html (851 words)

	Changes in Gene Frequencies (Site not responding. Last check: 2007-11-04)
		Selection occurs much more rapidly at the haploid stage, because deleterious recessive genes (with w=0) are eliminated and cannot be hidden in the heterozygote.
		More heterozygotes of any one generation are produced than is either homozygote, so both genes will be maintained at stable (equilibrium) frequencies through "storage" in the heterozygotes, even if both homozygotes are lethal.
		Advantageous for the mimic, disadvantageous for the species being mimicked.
	www.bios.niu.edu /Parrish/224.htm (1169 words)

	Untitled
		The recessive sickle-cell allele is preserved because the heterozygotes have an advantage that outweighs the lethality of the homozygous condition.
		There are many more heterozygotes in a population than recessive homozygotes.
		The reason for heterozygote advantage has to do with covering up deleterious recessives and the general heterozygote advantage (two different alleles is better that two of the same allele for various reasons).
	www.botany.uga.edu /~darley/btny1210/Nov23Mon.htm (2034 words)

	Evidence for a Heterozygote Advantage in Congenital Adrenal Hyperplasia due to 21-Hydroxylase Deficiency -- Witchel et ...
		Evidence for a Heterozygote Advantage in Congenital Adrenal Hyperplasia due to 21-Hydroxylase Deficiency -- Witchel et al.
		Evidence for a Heterozygote Advantage in Congenital Adrenal Hyperplasia due to 21-Hydroxylase Deficiency
		1994 Cystic fibrosis heterozygote resistance to cholera toxin in the cystic fibrosis mouse model.
	jcem.endojournals.org /cgi/content/full/82/7/2097 (2956 words)

	answers (Site not responding. Last check: 2007-11-04)
		The heterozygote advantage is a circumstance where the heterozygote for a particular trait has a better chance to survive and reproduce than either of the two homozygotes.
		An example of heterozygote advantage is the gene for b-hemoglobin.
		The normal allele for this gene is the HbA allele; the mutant allele is known as HbS.
	www.wiley.com /college/bioinquiry/contents/answers/ans7.html (654 words)

	Zoology 304, Evolution
		The load decreases as the advantageous allele increases in frequency, until the new allele is fixed.
		If genetic variation were maintained by heterozygote advantage, genetic load would result from selection against the homozygotes.
		If all of the many numerous molecular polymorphisms that can be observed were independently maintained by heterozygote advantage, the total mortality resulting from this segretional load would be impossibly high.
	www.science.siu.edu /zoology/king/304/neutral.htm (2603 words)

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