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Instructions
Draw a three by three grid that resembles a tic-tac-toe board.
Skip the top left square or color it in to remind yourself not to write in it, as it is not part of the Punnett square.
Look at the genetic characteristics you were given; these genotypes, or genetic blueprints, determine the phenotypes -- visible expressions of genetic characteristics -- of the parents. For example, a person with the phenotype of blue eyes has the genotype bb; the two lowercase letters denote that blue eyes are a recessive trait, so scientists write them as lowercase letters. A person with the brown-eyed phenotype could have either Bb or BB as a genotype, as the gene for brown eyes (B) is dominant over the gene for blue eyes (b).
Place the two letters that describe the male parent's genotype into the top middle and top right squares, one letter per square. Write the two letters that denote the female parent's genotype into the middle left and bottom left square, again placing one letter into each square.
Carry each letter down or across into its respective row or column. For example, if a father has a Bb genotype and a mother has a bb genotype, the middle and bottom middle squares will contain an uppercase B from the father and a lowercase b from the mother. The right middle and right bottom squares will each contain one lowercase b from the father and one lowercase b from the mother.
Count each pair of alleles in the boxes as a 25 percent probability of a given set of genotypes and add identical types together to arrive at overall probabilities of genotypes and phenotypes in offspring. Using the example of a Bb father with brown eyes and a bb mother with blue eyes, each child has a 50 percent chance of inheriting the B allele from their father and having a genotype of Bb, giving these offspring the brown-eyed phenotype. There is a 50 percent chance of acquiring the recessive b allele from the father that pairs with the mother's recessive b allele, which would result in blue eyes.
Check your work to ensure that your answer is logical. If you have negative probabilities or numbers larger than 100, you have made a mathematical error. Each box must have two letters, so ensure that each of the four boxes in the Punnett square contains a pair of letters, one from each parent.