Punnett square
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A Punnett square is a grid used in genetics to predict the possible genotypes of offspring, and how likely each is, from the alleles of two parents.
Also: Punnett squares, Punnett grid
Definition
What is a Punnett square?
A Punnett square is a simple diagram for working out the likely outcome of a genetic cross. The possible gametes of one parent, each carrying one allele per gene, are written along the top, and those of the other parent down the side. Each box is filled with the combination formed when those two gametes meet, giving one possible offspring genotype. Because every box is assumed to be equally likely, counting the boxes gives the expected ratios. The grid is named after Reginald Punnett, a British geneticist who worked at Cambridge in the early 1900s.
For a cross between two heterozygotes (Aa × Aa), a four-box square gives AA, Aa and aa in a 1:2:1 ratio. If A is dominant, that means about three offspring with the dominant trait for every one with the recessive trait. Following two genes at once, a dihybrid cross, needs 16 boxes and, for genes that are not linked, gives the 9:3:3:1 ratio Mendel saw in peas.
A Punnett square gives probabilities, not guarantees. Two carriers of a recessive condition have a one-in-four chance in each pregnancy of a child with two copies, whatever earlier children inherited.
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Terms that go with it
- Mendelian inheritanceMendelian inheritance is the pattern by which single genes pass from parents to offspring, following the laws of segregation and independent assortment.
- GenotypeA genotype is the genetic makeup of an organism, either at one locus, such as Aa, or across its whole genome, including alleles it does not express.
- AlleleAn allele is any of the alternative forms of a gene or DNA sequence that can occupy the same position, or locus, on a chromosome.
- HeterozygousHeterozygous describes an organism that carries two different alleles of a gene at a given locus, such as one dominant and one recessive allele.
- Dominant alleleA dominant allele is a version of a gene whose effect shows in the phenotype even when only one copy is present, masking a recessive allele.
- GameteA gamete is a haploid reproductive cell, such as a sperm or an egg, that fuses with a gamete of the opposite type at fertilization to form a zygote.
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Written for biofideX in plain words, after checking these references:
- OpenStax Biology 2e, 12.2 Characteristics and Traits
- DNA from the Beginning (Cold Spring Harbor Laboratory): Reginald Crundall Punnett
- LibreTexts, Introductory Biology (CK-12), 3.6 Punnett Squares
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