Punnett square 529817 225421114 2008-07-13T16:30:27Z Wmasterj 1496855 [[Image:Punnett square (PSF).png|thumb|A Punnett square.]] The '''Punnett square''' is a diagram that is used to predict the outcome of a particular cross or breeding experiment. It is named after [[Reginald C. Punnett]], who devised the approach, and is used by [[biology|biologists]] to determine the [[probability]] of an offspring having a particular [[genotype]]. The Punnett square is a summary of every possible combination of one maternal [[allele]] with one paternal allele for each gene being studied in the cross. ==Typical monohybrid cross== In this example, both organisms have the [[genotype]] '''Bb'''. They can produce [[gamete]]s that contain either the '''B''' or '''b''' alleles. (It is conventional in [[genetics]] to use capital letters to indicate [[dominant]] alleles and lower-case letters to indicate [[recessive]] alleles.) The probability of an individual offspring having the genotype '''BB''' is 25%, '''Bb''' is 50%, and '''bb''' is 25%. {| class="wikitable" style="text-align:center; width:25%" |- !colspan="2" rowspan="2"| !!colspan="2"|Maternal |- ! B !! b |- ! rowspan="2"|Paternal!! B | BB || Bb |- ! b | Bb || bb |} It is important to note that Punnett squares only give probabilities for ''[[genotypes]]'', not ''[[phenotypes]]''. The way in which the '''B''' and '''b''' alleles interact with each other to affect the appearance of the offspring depends on how the gene products ([[proteins]]) interact (see [[Mendelian inheritance]]). For classical dominant/recessive genes, like that which determines whether a rat has black hair ('''B''') or white hair ('''b'''), the dominant allele will mask the recessive one. Thus in the example above 75% of the offspring will be black ('''BB''' or '''Bb''') while only 25% will be white ('''bb'''). The ratio of the phenotypes is 3:1, typical for a [[monohybrid cross]]. ==Typical dihybrid cross== More complicated crosses can be made by looking at two or more genes. The Punnett square only works, however, if the genes are [[Independent assortment|independent]] of each other, which means that having a particular allele of gene X does not imply having a particular allele of gene Y. The following example illustrates a [[dihybrid cross]] between two heterozygous [[pea]] plants. '''R''' represents the dominant allele for shape (round), while '''r''' represents the recessive allele (wrinkled). '''Y''' represents the dominant allele for color (yellow), while '''y''' represents the recessive allele (green). If each plant has the genotype '''Rr Yy''', and since the alleles for shape and color genes are independent, then they can produce four types of gametes with all possible combinations: '''RY''', '''Ry''', '''rY''' and '''ry'''. {| class="wikitable" style="text-align:center; width:50%" |- ! !! RY !! Ry !! rY !! ry |-pudding rocks my socks ! RY | RRYY || RRYy || RrYY || RrYy |- ! Ry | RRYy || RRyy || RrYy || Rryy |- ! rY | RrYY || RrYy || rrYY || rrYy |- ! ry | RrYy || Rryy || rrYy || rryy |} Since dominant traits mask recessive traits, 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 typical for a dihybrid cross. ==Situations where Punnett squares do not apply== The phenotypic ratios of 3:1 and 9:3:3:1 are theoretical predictions based on the assumptions of segregation and independent assortment of alleles (see [[Mendelian inheritance]]). Deviations from the expected ratios can occur if any of the following conditions exists: *the alleles in question are physically [[Genetic linkage|linked]] on the same chromosome *one parent lacks a copy of the gene, e.g. human males have only one [[X chromosome]], from their mother, so only the maternal alleles have an effect on the organism (see [[sex linkage]]) *the survival rate of different genotypes is not the same, e.g. one combination of alleles may be incompatible with life so that the affected offspring expires ''in utero'' *alleles may show incomplete dominance or co-dominance (see [[dominance relationship]]) *there are genetic interactions ([[epistasis]]) between alleles of different genes *the trait is inherited on genetic material from only one parent, e.g. [[mitochondrial DNA]] is only inherited from the mother (see [[maternal effect]]) *the alleles are [[Imprinting (genetics)|imprinted]] ==References== {{Unreferenced|date=September 2007}} ==External links== *[http://www.changbioscience.com/genetics/punnett.html Online Punnett Square Calculator] [[Category:Classical genetics]] [[af:Vierkant van Punnett]] [[de:Punnett-Quadrat]] [[es:Cuadro de Punnett]] [[it:Quadrato di Punnett]] [[nl:Vierkant van Punnett]]