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 biologists to determine the probability of an offspring having a particular genotype. Professor Reginald Crundall Punnett FRS ( June 20 1875 &ndash January 3 1967) was a British geneticist who Foundations of modern biology There are five unifying principles Probability is the likelihood or chance that something is the case or will happen The genotype is the genetic constitution of a cell an organism or an individual (i 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. The genotype is the genetic constitution of a cell an organism or an individual (i They can produce gametes that contain either the B or b alleles. A gamete (from Ancient Greek γαμέτης; translated gamete = wife gametes = husband is a cell that fuses with another gamete (It is conventional in genetics to use capital letters to indicate dominant alleles and lower-case letters to indicate recessive alleles. Genetics (from Ancient Greek grc-Latn genetikos, “genitive” and that from grc-Latn genesis, “origin” a discipline of Biology, is ) The probability of an individual offspring having the genotype BB is 25%, Bb is 50%, and bb is 25%.

It is important to note that Punnett squares only give probabilities for genotypes, not phenotypes. The genotype is the genetic constitution of a cell an organism or an individual (i A phenotype is any observable characteristic of an Organism, such as its morphology, Development, biochemical or physiological properties 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). Proteins are large Organic compounds made of Amino acids arranged in a linear chain and joined together by Peptide bonds between the Carboxyl Mendelian inheritance (or Mendelian genetics or Mendelism) is a set of primary tenets relating to the transmission of hereditary characteristics from parent 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. A Monohybrid cross is a cross between parents who are Heterozygous at one locus for example Bb x Bb (see the Punnett square below

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 of each other, which means that having a particular allele of gene X does not imply having a particular allele of gene Y. Mendelian inheritance (or Mendelian genetics or Mendelism) is a set of primary tenets relating to the transmission of hereditary characteristics from parent

The following example illustrates a dihybrid cross between two heterozygous pea plants. A dihybrid cross is a cross between two F1 offspring of two Individuals that differ in two Traits. A pea (inaccurately called a '''sweet pea''' by food distubutors is most commonly the small spherical Seed or the seed-pod of the Legume Pisum 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.

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). Mendelian inheritance (or Mendelian genetics or Mendelism) is a set of primary tenets relating to the transmission of hereditary characteristics from parent Deviations from the expected ratios can occur if any of the following conditions exists:

• the alleles in question are physically linked on the same chromosome
• one parent lacks a copy of the gene, e. Genetic linkage occurs when particular genetic loci or Alleles for genes are inherited jointly 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. The X chromosome is one of the two sex-determining Chromosomes in many animal species including mammals (the other is the Y chromosome) Sex linkage is the phenotypic expression of an Allele that is related to the chromosomal sex of the individual 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. Epistasis is the Interaction between genes Epistasis takes place when the action of one Gene is modified by one or several other genes which are sometimes called g. mitochondrial DNA is only inherited from the mother (see maternal effect)
• the alleles are imprinted

References

External links

• Online Punnett Square Calculator