The Punnett square is a genetic diagram designed by Reginald Punnett that biologists continue to use in order to determine the probability of an offspring expressing a particular genotype.

	R	R
R	RR	RR
r	Rr	Rr

In the Punnett square example above, 50 % of the offspring will have a dominant homozygous genotype (RR), and a 50% probability that the genotype will be heterozygous (Rr). However, there is a 100% chance that the phenotype (the actual trait) for R will be expressed, because the dominant allele R is present in all four possible genotypes.

Interpreting a Punnett square

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1. As an example to demonstrate how to use a Punnett square, two rats with varying hair color will be crossed: A homozygous black rat with alleles BB, meaning the organism carries both dominant alleles and is black in color, and a homozygous white with a genotype of bb, meaning it carries both recessive alleles and is white in color. (Typically, the dominant trait is denoted by capital letters and the recessive by small cases.) Homozygous means that both alleles (different possible genes for a trait) are the same. Heterozygous means both alleles are different. So, if the rat were heterozygous its genotype would be Bb, meaning it has one gene for black and the other for white.

2. Using the genotypes of the two rats, a table is made putting one of the rat's genotype on the top row and the other rat's genotype down the left column. As a rule, the male's genotype is usually written down the side of the square, while the female's is listed on the top:

   	B	B
   b	_ _	_ _
   b	_ _	_ _

3. Now go down the first column at B to the first row b and put both genotype letters in the blank (remember that in a genotype the dominant gene is written first):

   	B	B
   b	Bb	_ _
   b	_ _	_ _

4. The same is done for the rest of the square:

   	B	B
   b	Bb	Bb
   b	Bb	Bb

5. When the genotype is paired up as Bb, the dominant allele is shown in the phenotype, meaning the rat with genotype Bb will be a black rat. All of the offspring from this cross demonstrate the phenotype for the dominant allele, though they are all heterozygous.

Results of crossing

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In the mating of a homozygous black rat and a homozygous white rat (as shown above), its offspring will have a 100% chance of having a Bb genotype and a black phenotype.

	B	b
B	BB	Bb
b	Bb	bb

However, if one of the offspring of this cross (known as the F₁ generation), where the Bb black rat mates with another black rat carrying the recessive white genotype Bb, there is a 1/4 chance one of their children will have the white phenotype (bb as the genotype). There is a 1/2 chance of a Bb genotype and 1/4 chance of a BB genotype. That indicates a 3/4 chance of having a black phenotype. Therefore, the ratio of the phenotypes is 3:1. This is a monohybrid cross.

If a heterozygous rat mates with a white rat, there is a 1/2 chance the offspring will be white and 1/2 chance the offspring will be black.

The transmission of some diseases is explained by showing how two parents, each with a recessive genotype of the disease, have a 1/4 chance of having a child with the disease, a 1/4 chance of having a child with no disease whatsoever, and 1/2 chance of having a child with the recessive genotype of the disease.

Complicated crosses

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The previous example used only one trait, with four possible outcomes. In reality, crossing is more complicated due to the fact that there is usually more than one trait being crossed. As an example, the pea plant has traits including pea shape and pea color. The dominant gene for pea shape is round, while the recessive trait is wrinkled. The dominant pea color is yellow, while the recessive color is green. In a dihybrid cross, two heterozygous pea plants are crossed, where the gametes for each pea plant will either be RY, Ry, rY, or ry:

	RY	Ry	rY	ry
RY	RRYY	RRYy	RrYY	RrYy
Ry	RRYy	RRyy	RrYy	Rryy
rY	RrYY	RrYy	rrYY	rrYy
ry	RrYy	Rryy	rrYy	rryy

The result is a 9:3:3:1 phenotypic ratio.

Notes and references

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• Note that the majority of dihybrid crosses with heterozygous gametes will result in this ratio when governed by the Law of Complete Dominance).

External links

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• University Of Toronto Mendelian Genetics Lecture
• Online Punnett Square Calculator

Classical genetics

Vierkant van Punnett | Punnett-Quadrat | Vierkant van Punnett