The Punnett square is a diagram designed by Reginald Punnett and used by biologists to determine the probability of an offspring having a particular genotype. It is made by comparing all the possible combinations of alleles from the mother with those from the father. Image:Reg Punnet. ... This article or section does not adequately cite its references or sources. ... Probability is the chance that something is likely to happen or be the case. ... This article does not cite its references or sources. ... An allele is any one of a number of alternative forms of the same gene occupying a given locus (position) on a chromosome. ...

Typical monohybrid cross

The Punnett square shows every possible combination when combining one maternal allele with one paternal allele. In this example, both organisms have the genotype Bb. They can produce gametes 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 probablility of an individual offspring having the genotype BB is 25%, Bb is 50%, and bb is 25%. This article does not cite its references or sources. ... A gamete is a specialized germ cell that fuses with another gamete during fertilization (conception) in organisms that reproduce sexually. ... For a non-technical introduction to the topic, please see Introduction to genetics. ... The word dominant has several possible meanings: In music theory, the dominant or dominant note (second most important) of a key is that which is a perfect fifth above the tonic; in just intonation the note whose pitch is 1. ... In genetics, the term recessive gene refers to an allele that causes a phenotype (visible or detectable characteristic) that is only seen in a homozygous genotype (an organism that has two copies of the same allele). ...

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. The phenotype of an individual organism is either its total physical appearance and constitution, or a specific manifestation of a trait, such as size or eye color, that varies between individuals. ... A representation of the 3D structure of myoglobin, showing coloured alpha helices. ... Mendelian inheritance (or Mendelian genetics or Mendelism) is a set of primary tenets relating to the transmission of hereditary characteristics from parent organisms to their children; it underlies much of genetics. ... A monohybrid cross, in genetics, is the mating between two heterozygous individuals. ...

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.

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). Each plant has the genotype Rr Yy, and since the alleles for shape and color genes are independent, they can produce four types of gametes with all possible combinations: RY, Ry, rY and ry. A dihybrid cross is a cross in which two dihybrids are mated and to test for dominant genes and recessive genes in two separate characteristics, and such a cross has a variety of uses of in Mendelian genetics and genetic linkage experiments. ... Binomial name Pisum sativum L. A pea is the small, edible round green bean which grows in a pod on the leguminous vine Pisum sativum, or in some cases to the immature pods. ...

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: Mendelian inheritance (or Mendelian genetics or Mendelism) is a set of primary tenets relating to the transmission of hereditary characteristics from parent organisms to their children; it underlies much of genetics. ...

• the alleles in question are physically 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 lethal so that the affected offspring dies before birth
• 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 imprinted

Genetic linkage occurs when particular alleles are inherited jointly. ... The X chromosome is one of the two sex-determining chromosomes in many animal species, including mammals (the other is the Y chromosome). ... It has been suggested that sex chromosome be merged into this article or section. ... It has been suggested that dominant allele be merged into this article or section. ... Epistasis takes place when the action of one gene is modified by one or more others that assort somewhat independently. ... // Mitochondrial DNA (mtDNA) is DNA that is located in mitochondria. ... A maternal effect, in genetics, is the phenomena where the genotype of a mother is expressed in the phenotype of its offspring. ... Genomic imprinting is the phenomenon whereby a small subset of all the genes in the genome are expressed according to their parent of origin. ...

External links

• Online Punnett Square Calculator