413:
its cycle in the red blood cells and triggers an abnormal drop in oxygen levels in the cell. In carriers, this drop is sufficient to trigger the full sickle-cell reaction, which leads to infected cells being rapidly removed from circulation and strongly limiting the infection's progress. These individuals have a great resistance to infection and have a greater chance of surviving outbreaks. However, those with two alleles for SCA may survive malaria, but will typically die from their genetic disease unless they have access to advanced medical care. Those of the homozygous "normal" or wild-type case will have a greater chance of passing on their genes successfully, in that there is no chance of their offspring's suffering from SCA; yet, they are more susceptible to dying from malarial infection before they have a chance to pass on their genes.
417:
malaria-stricken regions. Other populations in Africa, India, the
Mediterranean and the Middle East have higher allele frequencies, as well. As effective antimalarial treatment becomes increasingly available to malaria-stricken populations, the allele frequency for SCA is expected to decrease, so long as SCA treatments are unavailable or only partially effective. If effective sickle-cell anemia treatments become available to the same degree, allele frequencies should remain at their present levels in these populations. In this context, 'treatment effectiveness' refers to the reproductive fitness it grants, rather than the degree of suffering alleviation.
390:
405:, whereby some of the red blood cells undergo benign effects of SCA, but nothing severe enough to be harmful. Those afflicted with sickle-cell trait are also known as carriers: If two carriers have a child, there is a 25% chance their child will have SCA, a 50% chance their child will be a carrier, and a 25% chance that the child will neither have SCA nor be a carrier. Were the presence of the SCA allele to confer only negative traits, its allele frequency would be expected to decrease generation after generation, until its presence were eliminated by selection and by chance.
266:, or "hybrid vigor", which is the improved or increased function of any biological quality in a hybrid offspring. Previous research, comparing measures of dominance, overdominance and epistasis (mostly in plants), found that the majority of cases of heterozygote advantage were due to complementation (or dominance), the masking of deleterious recessive alleles by wild-type alleles, as discussed in the articles
290:. It is highly unlikely, however, that the same deleterious mutations will be common in both populations after a long period of separation. Since loss-of-function mutations tend to be recessive (given that dominant mutations of this type generally prevent the organism from reproducing and thereby passing the gene on to the next generation), the result of any cross between the two populations will be
25:
127:
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functional supertypes (P = 1.05 × 10) than those represented as low-resolution genotypes (P = 1.99 × 10). These findings constitute evidence that heterozygosity provides an advantage among carriers of different supertype HLA-DRB1 alleles against HCV infection progression to end-stage liver disease in a large-scale, long-term study population.
539:(one defective and one normal copy). However, the frequency of heterozygous null alleles is much higher than expected, indicating a heterozygous advantage for TPI null alleles. The reason is unknown; however, new scientific results are suggesting cells having reduced TPI activity are more resistant against
499:
As of 2016, the selective pressure for the high gene prevalence of CF mutations is still uncertain, and may be due to an unbiased genetic drift rather than a selective advantage. Approximately one in 25 persons of
European descent is a carrier of the disease, and one in 2500 to 3000 children born is
373:
conditions, the cells lose their healthy round shape and become sickle-shaped. This deformation of the cells can cause them to become lodged in capillaries, depriving other parts of the body of sufficient oxygen. When untreated, a person with SCA may suffer from painful periodic bouts, often causing
567:
loci. The reasons proposed for these findings are speculative; however, it has been argued that heterozygosity at MHC loci results in more alleles to fight against a wider variety of diseases, possibly increasing survival rates against a wider range of infectious diseases. The latter claim has been
412:
outbreaks, individuals with the heterozygous state have a distinct advantage (and this is why individuals with heterozygous alleles are far more common in these areas). Those with the benign sickle trait possess a resistance to malarial infection. The pathogen that causes the disease spends part of
452:
ions, which is vital to maintaining equilibrium of water in the body. The malfunctioning protein causes viscous mucus to form in the lungs and intestinal tract. Before modern times, children born with CF would have a life expectancy of only a few years, but modern medicine has made it possible for
338:
This mutation, which at first glance appeared to be harmful, conferred enough of an advantage to heterozygotes to make it beneficial, so that it remained at dynamic equilibrium in the gene pool. Kalmus introduced flies with the ebony mutation to a wild-type population. The ebony allele persisted
554:
There is evidence that genetic heterozygosity in humans provides increased resistance to certain viral infections. A significantly lower proportion of HLA-DRB1 heterozygosity exists among HCV-infected cases than uninfected cases. The differences were more pronounced with alleles represented as
330:
If weakness were the only effect of the mutant allele, so it conveyed only disadvantages, natural selection would weed out this version of the gene until it became extinct from the population. However, the same mutation also conveyed advantages, providing improved viability for heterozygous
416:
This resistance to infection is the main reason the SCA allele and SCA disease still exist. It is found in greatest frequency in populations where malaria was and often still is a serious problem. Approximately one in 10 African
Americans is a carrier, as their recent ancestry is from
339:
through many generations of flies in the study, at genotype frequencies that varied from 8% to 30%. In experimental populations, the ebony allele was more prevalent and therefore advantageous when flies were raised at low, dry temperatures, but less so in warm, moist environments.
476:
in 1994 (Gabriel, et al.). The heterozygote (carrier) mouse had less secretory diarrhea than normal, noncarrier mice. Thus, it appeared for a time that resistance to cholera explained the selective advantage to being a carrier for CF and why the carrier state was so frequent.
480:
This theory has been called into question. Hogenauer, et al. have challenged this popular theory with a human study. Prior data were based solely on mouse experiments. These authors found the heterozygote state was indistinguishable from the noncarrier state.
232:. A common example is the case where the heterozygote conveys both advantages and disadvantages, while both homozygotes convey a disadvantage. A well-established case of heterozygote advantage is that of the gene involved in
491:
The most recent hypothesis, published in the
Journal of Theoretical Biology, proposed having a single CF mutation granted respiratory advantage for early Europeans migrating north into the dusty wasteland left by the
1065:
Kalmus, H. (1945). "Adaptive and selective responses of a population of
Drosophila melanogaster containing e and e+ to differences in temperature, humidity, and to selection for development speed".
1404:
Borzan V, Tomašević B, Kurbel S (2014). "Hypothesis: Possible respiratory advantages for heterozygote carriers of cystic fibrosis linked mutations during dusty climate of last glaciation".
1342:"Active intestinal chloride secretion in human carriers of cystic fibrosis mutations: an evaluation of the hypothesis that heterozygotes have subnormal active intestinal chloride secretion"
313:
Cases of both homozygote and heterozygote advantage have been demonstrated in several organisms, including humans. The first experimental confirmation of heterozygote advantage was with
255:), which can simultaneously convey separate advantageous traits and disadvantageous traits upon the same organism. In this instance, the state of the organism's environment will provide
1102:"Lifespan extension conferred by mitogen-activated protein kinase kinase kinase 5 (MAP3K5) longevity-associated gene variation is confined to at-risk men with a cardiometabolic disease"
464:
The presence of a single CF mutation may influence survival of people affected by diseases involving loss of body fluid, typically due to diarrhea. The most common of these maladies is
286:
When two populations of any sexual organism are separated and kept isolated from each other, the frequencies of deleterious mutations in the two populations will differ over time, by
603:
are associated with lifespan. In a study among
Japanese-Americans, homozygous individuals have a longer lifespan than heterozygous individuals, indicating a homozygote advantage.
472:. Those with cholera would often die of dehydration due to intestinal water losses. A mouse model of CF was used to study resistance to cholera, and the results were published in
488:. Tuberculosis was responsible for 20% of all European deaths between 1600 and 1900, so even partial protection against the disease could account for the current gene frequency.
335:
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.
468:, which only began killing Europeans millennia after the CF mutation frequency was already established in the population. Another such disease that CF may protect against is
441:
145:
579:(BAFF) is a cytokine encoded by the TNFSF13B gene. A variant of the gene containing a deletion (GCTGT—>A) renders a shorter mRNA transcript that escapes degradation by
1248:
Lazarin G. A.; Haque I. S.; Nazareth S.; Iori K.; Patterson A. S.; Jacobson J. L.; Marshall J. R.; Seltzer W. K.; Patrizio P.; Evans E. A.; Srinivasan B. S. (2013).
568:
tested in an experiment, which showed outbreeding mice to exhibit MHC heterozygosity enhanced their health and survival rates against multiple-strain infections.
1250:"An empirical estimate of carrier frequencies for 400+ causal Mendelian variants: results from an ethnically diverse clinical sample of 23,453 individuals"
345:
occurs when "a heterozygote has a lower overall fitness than either homozygote." Heterozygote disadvantage occurs in mammals, birds, and insects.
42:
1159:"Biased sex-ratio and sex-biased heterozygote disadvantage affect the maintenance of a genetic polymorphism and the properties of hybrid zones"
1100:
Morris, Brian J.; Chen, Randi; Donlon, Timothy A.; Masaki, Kamal H.; Willcox, D. Craig; Allsopp, Richard C.; Willcox, Bradley J. (2021-03-19).
1581:
524:
274:, but there were also findings of overdominance, especially in rice. More recent research, however, has established that there is also an
202:
genotype. Loci exhibiting heterozygote advantage are a small minority of loci. The specific case of heterozygote advantage due to a single
393:
Possible advantage of being heterozygous for sickle cell anemia disease (A) vs. normal blood cell response (B) when infected with malaria.
397:
Because the genetic disorder is incompletely recessive, a person with only one SCA allele and one unaffected allele will have a "mixed"
1390:
259:, with a net effect either favoring or working in opposition to the gene, until an environmentally determined equilibrium is reached.
89:
163:
108:
583:, thus increasing expression of BAFF, which consequently up-regulates the humoral immune response. This variant is associated with
61:
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of the heterozygote lies outside of the phenotypical range of both homozygote parents, and heterozygous individuals have a higher
564:
389:
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individuals. The heterozygote expressed none of the disadvantages of homozygotes, yet gained improved viability. The homozygote
1207:
563:
Multiple studies have shown, in double-blind experiments, females prefer the scent of males who are heterozygous at all three
68:
46:
1229:
Bunn, H. Franklin (November 1, 2012). "The triumph of good over evil: protection by the sickle cell gene against malaria".
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Often, the advantages and disadvantages conveyed are rather complicated, because more than one gene may influence a given
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75:
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35:
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contribution to heterozygote advantage, primarily as determined in plants, though also reported in mice.
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1413:
493:
195:
82:
358:
256:
233:
1539:"Major histocompatibility complex genes, symmetry, and body scent attractiveness in men and women"
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these people to live into adulthood. However, even in these individuals, CF typically causes male
1716:
1449:"Evidence for human leukocyte antigen heterozygote advantage against hepatitis C virus infection"
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1082:
1047:
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favoring the heterozygote, and this mechanism is used to explain the occurrence of some kinds of
591:, but heterozygote carriers of the variant have decreased susceptibility to malaria infection.
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401:: The sufferer will not experience the ill effects of the disease, yet will still possess a
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240:
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Thornhill R, Gangestad S, Miller R, Scheyd G, McCollough J, Franklin M (March–April 2013).
989:
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for genetic research. In a classic study on the ebony mutation, Kalmus demonstrated how
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1134:
965:
941:"Hybrid vigor and transgenerational epigenetic effects on early mouse embryo phenotype"
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Another theory for the prevalence of the CF mutation is that it provides resistance to
366:
342:
320:
199:
1633:
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1599:"MHC heterozygosity confers a selective advantage against multiple-strain infections"
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298:
287:
207:
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Charlesworth D, Willis JH (November 2009). "The genetics of inbreeding depression".
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523:, which affect the dimerisation of this protein, are causal for a rare disease,
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433:
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caused by the presence of two incompletely recessive alleles. When a sufferer's
275:
24:
1425:
1310:
1297:
Josefson, Deborah (May 16, 1998). "CF Gene May
Protect against Typhoid Fever".
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669:
512:
301:, where the fitness advantage of the cross is caused by being heterozygous at
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192:
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1004:
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747:"Recent approaches into the genetic basis of inbreeding depression in plants"
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Case in which having two different versions of a gene provides an advantage
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898:
Baranwal VK, Mikkilineni V, Zehr UB, Tyagi AK, Kapoor S (November 2012).
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Ipsilandis, C. G.; Tokatlidis, I. S.; Vafias, B.; Stefanis, D. (2006).
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535:(two defective copies of the TPI gene), but have no obvious effect in
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383:
379:
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811:
712:
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1340:
Högenauer C, Santa Ana CA, Porter JL, et al. (December 2000).
428:(CF) is an autosomal recessive hereditary monogenic disease of the
939:
Han Z, Mtango NR, Patel BG, Sapienza C, Latham KE (October 2008).
516:
429:
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However, convincing evidence indicates, in areas with persistent
248:
515:, the main pathway for cells to obtain energy by metabolizing
120:
18:
1212:
Information Center for Sickle Cell and
Thalassemic Disorders
654:"What is the evidence for heterozygote advantage selection?"
327:
can persist in a population through heterozygote advantage.
1658:"Overexpression of the Cytokine BAFF and Autoimmunity Risk"
1018:
Gillespie, Rosemary G; Roderick, George K (February 2000).
262:
Heterozygote advantage is a major underlying mechanism for
210:. Overdominance is a rare condition in genetics where the
794:
Charlesworth, Deborah; Willis, John H. (November 2009).
457:. It is the most common genetic disease among people of
990:"Criteria for Developing Second-cycle Hybrids in Maize"
141:
386:. Typically, the disease results in premature death.
851:"Molecular mechanisms of polyploidy and hybrid vigor"
297:
This article deals with the specific case of fitness
1391:"Cystic fibrosis gene protects against tuberculosis"
440:. The disorder is caused by the malfunction of the
136:
may be too technical for most readers to understand
49:. Unsourced material may be challenged and removed.
527:. Other mutations, which inactivate the enzyme (=
1597:Penn DJ, Damjanovich K, Potts WK (August 2002).
1490:"Human body odour, symmetry and attractiveness"
900:"Heterosis: emerging ideas about hybrid vigour"
559:MHC heterozygosity and human scent preferences
1447:Hraber P, Kuiken C, Yusim K (December 2007).
8:
1157:Nouvellet, P.; Gourbière, S. (August 2013).
519:. In humans, certain mutations within this
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550:Resistance to hepatitis C virus infection
164:Learn how and when to remove this message
148:, without removing the technical details.
109:Learn how and when to remove this message
1208:"Malaria and the Sickle Hemoglobin Gene"
1020:"Endless Forms – Species and Speciation"
751:Philos. Trans. R. Soc. Lond. B Biol. Sci
1573:The Handbook of Evolutionary Psychology
796:"The genetics of inbreeding depression"
644:
1576:. John Wiley & Sons. p. 357.
740:
738:
251:almost always have multiple effects (
146:make it understandable to non-experts
7:
525:triosephosphate isomerase deficiency
47:adding citations to reliable sources
1656:Steri M, et al. (April 2017).
1488:Rikowski A, Grammer K (May 1999).
14:
1206:Bridges, Kenneth (2 April 2002).
658:Trends in Ecology & Evolution
652:Hedrick, Philip W. (2012-12-01).
1389:MacKenzie, Debora (2006-09-07).
1036:10.1046/j.1365-2540.2000.0704b.x
745:Carr DE, Dudash MR (June 2003).
180:describes the case in which the
125:
23:
1662:New England Journal of Medicine
1163:Journal of Evolutionary Biology
997:Asian Journal of Plant Sciences
599:Variants of the kinase-related
34:needs additional citations for
319:, a fruit fly that has been a
1:
957:10.1095/biolreprod.108.069096
867:10.1016/j.tplants.2009.12.003
511:(TPI) is a central enzyme of
500:affected by Cystic fibrosis.
444:protein, which controls inter
218:than homozygous individuals.
1603:Proc. Natl. Acad. Sci. U.S.A
585:systemic lupus erythematosus
531:) are lethal when inherited
572:BAFF and autoimmune disease
1733:
1426:10.1016/j.jtbi.2014.08.015
1311:10.1136/bmj.316.7143.1477j
670:10.1016/j.tree.2012.08.012
272:Complementation (genetics)
849:Chen ZJ (February 2010).
509:Triosephosphate isomerase
504:Triosephosphate isomerase
343:Heterozygote disadvantage
309:Experimental confirmation
577:B-cell activating factor
58:"Heterozygote advantage"
1570:Buss, David M. (2005).
1299:British Medical Journal
800:Nature Reviews Genetics
595:Cardiometabolic disease
545:PlosOne, December 2006
316:Drosophila melanogaster
1624:10.1073/pnas.162006499
1506:10.1098/rspb.1999.0717
763:10.1098/rstb.2003.1295
633:Polymorphism (biology)
394:
187:has a higher relative
178:heterozygote advantage
1674:10.1056/NEJMoa1610528
1556:10.1093/beheco/arg043
1118:10.18632/aging.202844
613:Balanced polymorphism
392:
224:can be maintained by
1266:10.1038/gim.2012.114
1218:on 27 November 2011.
494:Last Glacial Maximum
43:improve this article
1712:Population genetics
1615:2002PNAS...9911260P
1418:2014JThBi.363..164B
1067:Journal of Genetics
369:are exposed to low-
234:sickle cell anaemia
230:genetic variability
1543:Behavioral Ecology
1079:10.1007/BF02989038
917:10.1093/jxb/ers291
589:multiple sclerosis
446:membrane transport
395:
359:Sickle-cell anemia
354:Sickle-cell anemia
1583:978-0-471-72722-4
1466:10.1002/hep.21889
1346:Am. J. Hum. Genet
1176:10.1111/jeb.12180
757:(1434): 1071–84.
403:sickle cell trait
349:In human genetics
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529:null alleles
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303:one specific
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253:pleiotropism
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222:Polymorphism
220:
206:is known as
182:heterozygous
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41:Please help
36:verification
33:
1412:: 164–168.
904:J. Exp. Bot
455:infertility
361:(SCA) is a
154:August 2022
1706:Categories
1453:Hepatology
1254:Genet. Med
513:glycolysis
374:damage to
276:epigenetic
193:homozygous
69:newspapers
1717:Selection
1126:1945-4589
1073:: 58–63.
1044:0018-067X
1005:1812-5697
820:1471-0064
678:0169-5347
461:descent.
399:phenotype
333:wild type
282:In theory
268:Heterosis
264:heterosis
257:selection
226:selection
212:phenotype
200:recessive
1692:28445677
1643:12177415
1524:10380676
1475:17935228
1434:25150458
1376:11055897
1327:27062771
1284:22975760
1193:29634107
1185:23837902
1144:33739303
1087:27175926
1052:36932749
1024:Heredity
975:18562704
926:23095992
885:20080432
828:19834483
781:12831473
721:19834483
686:22975220
607:See also
581:microRNA
459:European
450:chloride
247:. Major
196:dominant
185:genotype
1683:5605835
1611:Bibcode
1515:1689917
1414:Bibcode
1367:1287919
1319:9616022
1275:3908551
1135:8034933
966:2844494
876:2821985
772:1693197
474:Science
470:typhoid
466:cholera
410:malaria
380:strokes
216:fitness
189:fitness
140:Please
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521:enzyme
517:sugars
384:anemia
371:oxygen
292:fitter
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1323:S2CID
1231:Blood
1189:S2CID
1106:Aging
1083:S2CID
1048:S2CID
993:(PDF)
832:S2CID
725:S2CID
639:Notes
430:lungs
382:, or
249:genes
245:morph
241:trait
204:locus
90:JSTOR
76:books
1688:PMID
1639:PMID
1578:ISBN
1520:PMID
1471:PMID
1430:PMID
1372:PMID
1315:PMID
1280:PMID
1181:PMID
1140:PMID
1122:ISSN
1040:ISSN
1001:ISSN
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881:PMID
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816:ISSN
777:PMID
717:PMID
682:PMID
674:ISSN
587:and
442:CFTR
436:and
270:and
62:news
1678:PMC
1670:doi
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1422:doi
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1362:PMC
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871:PMC
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808:doi
767:PMC
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666:doi
565:MHC
448:of
243:or
144:to
45:by
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