Punnett square Guide, Meaning , Facts, Information and Description
A
Punnett square is a tool in
genetics developed by
British geneticist Reginald Punnett, and which
biologists use to this day to predict the
probability of possbile
genotypes of offspring.
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.
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 top and the other on the left side:
- Now follow your finger down the first column at B to the first row b and put both genotype letters in the blank:
- Now do the same for the rest of the square:
- 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 colored rat. In our 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.
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