Visual Encyclopedia

Punnett square

The Punnett square is a diagram that is used to predict an outcome of a particular cross or breeding experiment. It is named after Reginald C. Punnett, who devised the approach. The diagram is used by biologists to determine the probability of an offspring having a particular genotype. The Punnett square is a tabular summary of possible combinations of maternal alleles with paternal alleles. These tables can be used to examine the genotypic outcome probabilities of the offspring of a single trait (allele), or when crossing multiple traits from the parents. The Punnett Square is a visual representation of Mendelian inheritance. It is important to understand the terms "heterozygous", "homozygous", "double heterozygote" (or homozygote), "dominant allele" and "recessive allele" when using the Punnett square method. For multiple traits, using the "forked-line method" is typically much easier than the Punnett square. Phenotypes may be predicted with at least better-than-chance accuracy using a Punnett square, but the phenotype that may appear in the presence of a given genotype can in some instances be influenced by many other factors, as when polygenic inheritance and/or epigenetics are at work.

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correction to EX2. There is a 100% chance the child will have brown eyes. BB is brown eyes and Bb is brown eyes. The only difference it that the child has 50% chance of caring the recessive allele and passing it to their child. Sorry

Contributed by Jessica Mallard

Consider a dad with brown eyes and a mom with brown eyes. Brown dominant, blue recessive. 1) B B B BB BB B BB BB or 2) B B B BB BB b Bb bB or 3) B b B BB Bb b Bb bb EX1) Both parents have 2 dominant alleles. 100% chance their child has brown eyes with 2 dominant alleles. EX2) One parent has a recessive allele, but the dominant allele still makes brown eyes. 50% chance the child has brown eyes, 50% chance the child has blue eyes. EX3) Both parents have 1 recessive allele. 75% chance the kid has brown eyes, 25% chance blue eyes. If both parents have blue eyes, 0% chance the kid has brown eyes. Chances are per child. In #3, it doesn't mean 1 in 4 offspring have blue eyes and 3 in 4 have brown.

Contributed by Jessica Mallard

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