Punnett square
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A Punnett square is a tool in genetics developed by British geneticist Reginald Punnett, and which biologists still use to predict the probability of possible genotypes of offspring.
| R | R | |
| R | RR | RR | r | Rr | Rr |
The above example Punnett square represents the possible genotypes resulting from a dominant homozygous parent and a heterozygous parent. It demonstrates that the possible genotypes of offspring are RR (dominant homozygous) or Rr (heterozygous). There is a 50% probability that the offspring will have have a dominant homozygous genotype, and a 50% probability that the genotype will be heterozygous, and a 100% chance that the phenotype for R will be expressed, because the dominant allele R is present in all four possible genotypes.
How to interpret a Punnett square
- Start out with two of the same types of organisms, for this example we'll use two rats with varying hair color: BB for homozygous black, meaning both genes have black phenotypes and a bb for the homozygous white, meaning it is dominant recessive and has a white colored phenotype. Typically the more predominant color is denoted by capital letters and the recessive by small cases. Homo- means the same and in this case means that both genes are either BB or bb. If the rat were heterozygous it would read Bb meaning it has one gene for black and the other for white.
- With the two homozygous rats, make a table putting one rat genotype on the top row and the other down the left column:
B B b _ _ _ _ b _ _ _ _ - Now go down the first column at B to the first row b and put both genotype letters in the blank:
B B b Bb _ _ b _ _ _ _ - Now do the same for the rest of the square:
B B b Bb Bb b Bb Bb - When the genotype is paired up as Bb, the dominant gene is shown in the phenotype, meaning the rat with genotype Bb will be a black rat. In the mating of a homozygous black rat and a homozygous white rat, its offspring will have a 100% chance of having a Bb genotype and a black phenotype. However, if this 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. If it mates with a white rat, there is a 1/2 chance the offspring will be white. 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.
| B | b | |
| B | BB | Bb | b | Bb | bb |