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Punnett square

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ThePunnett squareis a square diagram that is used to predict the genotypes of a particular cross or breeding experiment. It is named afterReginald C. Punnett,who devised the approach in 1905.[3][4][5][6][7][8]The diagram is used bybiologiststo determine theprobabilityof an offspring having a particulargenotype.The Punnett square is a tabular summary of possible combinations of maternalalleleswith paternal alleles.[9]These tables can be used to examine the genotypical outcome probabilities of the offspring of a single trait (allele), or when crossing multiple traits from the parents.

A Punnett square showing a typicaltest cross.(green pod color is dominant over yellow for pea pods[1]in contrast to pea seeds, where yellowcotyledoncolor is dominant over green[2]).
Punnett squares for each combination of parents' colour vision status giving probabilities of their offsprings' status, each cell having 25% probability in theory.

The Punnett square is a visual representation ofMendelian inheritance,a fundamental concept in genetics discovered byGregor Mendel.[10]For multiple traits, using the "forked-line method" is typically much easier than the Punnett square. Phenotypes may be predicted with at least better-than-chance accuracy using a Punnett square, but the phenotype that may appear in the presence of a given genotype can in some instances be influenced by many other factors, as whenpolygenic inheritanceand/orepigeneticsare at work.

Zygosity

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Main article:Zygosity

Zygosityrefers to the grade of similarity between theallelesthat determine one specifictraitin anorganism.In its simplest form, a pair of alleles can be eitherhomozygousorheterozygous.Homozygosity, withhomorelating tosamewhilezygouspertains to azygote,is seen when a combination of either twodominantor tworecessivealleles code for the same trait. Recessive are always lowercase letters. For example, using 'A' as the representative character for each allele, ahomozygous dominantpair'sgenotypewould be depicted as 'AA', whilehomozygous recessiveis shown as 'aa'. Heterozygosity, withheteroassociated withdifferent,can only be 'Aa' (the capital letter is always presented first by convention). Thephenotypeof a homozygous dominant pair is 'A', ordominant,while the opposite is true for homozygousrecessive.Heterozygous pairs always have a dominant phenotype.[11]To a lesser degree,hemizygosity[12]and nullizygosity[13]can also be seen in gene pairs.

Monohybrid cross

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Main article:Monohybrid cross

"Mono-" means "one"; this cross indicates that the examination of a single trait. This could mean (for example) eye color. Each genetic locus is always represented by two letters. So in the case of eye color, say "B = Brown eyes" and "b = green eyes". In this example, both parents have thegenotypeBb.For the example of eye color, this would mean they both have brown eyes. They can producegametesthat contain either theBor theballele. (It is conventional ingeneticsto use capital letters to indicatedominantalleles and lower-case letters to indicaterecessivealleles.) The probability of an individual offspring's having the genotypeBBis 25%,Bbis 50%, andbbis 25%. The ratio of the phenotypes is 3:1, typical for amonohybrid cross.When assessing phenotype from this, "3" of the offspring have "Brown" eyes and only one offspring has "green" eyes. (3 are "B?" and 1 is "bb" )

Paternal

Maternal
 B b
B BB Bb
b Bb bb

The way in which theBandballeles interact with each other to affect the appearance of the offspring depends on how the gene products (proteins) interact (seeMendelian inheritance). This can include lethal effects andepistasis(where one allele masks another, regardless of dominant or recessive status).

Dihybrid cross

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Main article:Dihybrid cross

More complicated crosses can be made by looking at two or more genes. The Punnett square works, however, only if the genes areindependentof each other, which means that having a particular allele of gene "A" does not alter the probability of possessing an allele of gene "B". This is equivalent to stating that the genes are notlinked,so that the two genes do not tend to sort together during meiosis.

The following example illustrates adihybrid crossbetween two double-heterozygote pea plants.Rrepresents the dominant allele for shape (round), whilerrepresents the recessive allele (wrinkled).Arepresents the dominant allele for color (yellow), whilearepresents the recessive allele (green). If each plant has the genotypeRrAa,and since the alleles for shape and color genes are independent, then they can produce four types of gametes with all possible combinations:RA,Ra,rA,andra.

RA Ra rA ra
RA RRAA RRAa RrAA RrAa
Ra RRAa RRaa RrAa Rraa
rA RrAA RrAa rrAA rrAa
ra RrAa Rraa rrAa rraa

Since dominant traits mask recessive traits (assuming no epistasis), there are nine combinations that have the phenotype round yellow, three that are round green, three that are wrinkled yellow, and one that is wrinkled green. The ratio 9:3:3:1 is the expected outcome when crossing two double-heterozygous parents with unlinked genes. Any other ratio indicates that something else has occurred (such as lethal alleles, epistasis, linked genes, etc.).

Forked-line method

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The forked-line method (also known as the tree method and the branching system) can also solve dihybrid and multi-hybrid crosses. A problem is converted to a series of monohybrid crosses, and the results are combined in a tree. However, a tree produces the same result as a Punnett square in less time and with more clarity. The example below assesses another double-heterozygote cross using RrYy x RrYy. As stated above, the phenotypic ratio is expected to be 9:3:3:1 if crossing unlinked genes from two double-heterozygotes. The genotypic ratio was obtained in the diagram below, this diagram will have more branches than if only analyzing for phenotypic ratio.

There are also Punnett squares forepistasis.In these cases the genotype epistatic over the other genes hinders their expression in the phenotype.

See also

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References

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  1. ^Mendel, Gregor Johann(1866) [1865].Versuche über Pflanzen-Hybriden.Verhandlungen des naturforschenden Vereins (in German and English). Vol. IV (Separate ed.). Brno: Verlag des naturforschender Vereins zu Brünn / Georg Gastl's Buchdruckerei /. p. 14.Archivedfrom the original on 2021-03-29.Retrieved2020-06-01.{{cite book}}:|work=ignored (help)
  2. ^Mendel, Gregor Johann(1866) [1865].Versuche über Pflanzen-Hybriden.Verhandlungen des naturforschenden Vereins (in German and English). Vol. IV (Separate ed.). Brno: Verlag des naturforschender Vereins zu Brünn / Georg Gastl's Buchdruckerei. p. 47.Archivedfrom the original on 2021-03-29.Retrieved2020-06-01.{{cite book}}:|work=ignored (help)
  3. ^Punnett, Reginald Crundall(1907).Mendelism(2 ed.). London, UK:Macmillan.(NB. The 1905 first edition of this book does not contain the Punnett square. In 1911, the third edition gives a more thorough explanation.)
  4. ^Edwards, Anthony William Fairbank(March 2012)."Punnett's square".Studies in History and Philosophy of Science Part C: Studies in History and Philosophy of Biological and Biomedical Sciences.43(1): 219–224.doi:10.1016/j.shpsc.2011.11.011.PMID22326091.Abstract:The origin and development of Punnett's Square for the enumeration and display of genotypes arising in a cross in Mendelian genetics is described. Due to R. C. Punnett, the idea evolved through the work of the 'Cambridge geneticists', including Punnett's colleagues William Bateson, E. R. Saunders and R. H. Lock, soon after the rediscovery of Mendel's paper in 1900. These geneticists were thoroughly familiar with Mendel's paper, which itself contained a similar square diagram. A previously-unpublished three-factor diagram by Sir Francis Galton existing in the Bateson correspondence in Cambridge University Library is then described. Finally the connection between Punnett's Square and Venn Diagrams is emphasized, and it is pointed out that Punnett, Lock and John Venn overlapped as Fellows of Gonville and Caius College, Cambridge.
  5. ^Edwards, Anthony William Fairbank(September 2012)."Reginald Crundall Punnett: First Arthur Balfour Professor of Genetics, Cambridge, 1912".Perspectives.Genetics.192(1). Gonville and Caius College, Cambridge, UK:Genetics Society of America:3–13.doi:10.1534/genetics.112.143552.PMC3430543.PMID22964834.pp. 5–6:[...] Punnett's square seems to have been a development of 1905, too late for the first edition of hisMendelism(May 1905) but much in evidence inReport III to the Evolution Committee of the Royal Society[(Bateson et al. 1906b) "received March 16, 1906" ]. The earliest mention is contained in a letter to Bateson from Francis Galton dated October 1, 1905 (Edwards 2012). We have the testimony of Bateson (1909, p. 57) that "For the introduction of this system [the 'graphic method'], which greatly simplifies difficult cases, I am indebted to Mr. Punnett." [...] The first published diagrams appeared in 1906. [...] when Punnett published the second edition of hisMendelism,he used a slightly different format ([...] Punnett 1907, p. 45) [...] In the third edition (Punnett 1911, p. 34) he reverted to the arrangement [...] with a description of the construction of what he called the "chessboard" method (although in truth it is more like a multiplication table). [...](11 pages)
  6. ^Wimsatt, William C.(2012-05-15), "The analytic geometry of genetics: Part I: the structure, function, and early evolution of Punnett squares",Archive for History of Exact Sciences,66(66): 359–396 [359],doi:10.1007/s00407-012-0096-7,S2CID119557681
  7. ^Edwards, Anthony William Fairbank(June 2016)."Punnett's square: a postscript".Studies in History and Philosophy of Science Part C: Studies in History and Philosophy of Biological and Biomedical Sciences.57.Elsevier Ltd.:69–70.doi:10.1016/j.shpsc.2016.01.001.Retrieved2021-03-29.(2 pages)
  8. ^Müller-Wille, Staffan; Parolini, Giuditta (2020-12-09). "Punnett squares and hybrid crosses: how Mendelians learned their trade by the book".Learning by the Book: Manuals and Handbooks in the History of Science.BJHS Themes. Vol. 5.British Society for the History of Science/Cambridge University Press.pp. 149–165.doi:10.1017/bjt.2020.12.S2CID229344415.Archivedfrom the original on 2021-03-29.Retrieved2021-03-29.[...] Nilsson-Ehle was experimenting with a visual arrangement that would become very popular in Mendelian genetics. The lower half of his notes comes close to what is known as the 'Punnett square' [...] Punnett introduced this square diagram to the literature in 1906 in a paper co-authored with Bateson and Edith R. Saunders, and included it in the second edition of his Mendelism. In the third edition (1911), he added a verbal description of how to construct the diagram, and the Punnett square became a standard feature of Mendelian literature. As a detailed reconstruction by A.W.F. Edwards has shown, the diagram first took shape in an exchange of letters between Bateson and Galton for the more complex case of a trihybrid cross, and may well have been inspired by the way in which Mendel presented a case of trifactorial inheritance of flower colour in beans. [...]
  9. ^Griffiths, Anthony J. F.; Miller, Jeffrey H.; Suzuki, David T.; Lewontin, Richard C.; Gelbart, William M. (2000).An Introduction to Genetic Analysis(7 ed.). New York, USA:W. H. Freeman.
  10. ^Basu, Arkatappa.""The Hindu ePaper | Daily News and Current Affairs"".epaper.thehindu.com.No. 1 April 2024. The Hindu.Archivedfrom the original on 2024-03-13.Retrieved2024-04-01.
  11. ^AthenaMyth (2014-06-16)."Dominant/Recessive vs Hetero/Homozygous".DeviantArt.Archivedfrom the original on 2021-03-29.Retrieved2017-11-19.
  12. ^Shiel Jr., William C. (2018-12-12) [2017]."Medical Definition of Hemizygous".MedicineNet.MedicineNet, Inc.Archivedfrom the original on 2021-03-29.Retrieved2017-11-19.
  13. ^Robles, Ivan Suarez (2010-11-16)."nullizygous".Huntington's Outreach Project for Education, at Stanford (hopes).web.stanford.edu.Archivedfrom the original on 2021-03-29.Retrieved2017-11-19.

Further reading

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