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A Punnett square is used to predict the chances of an offspring to have its parents’ traits. These squares are most commonly divided into four parts, with each part equalling a 25% chance of the offspring receiving that set of genes. More complicated squares may have more than four parts, though the same basic method applies.
The letters surrounding and within each square represent alleles. They are one part of a gene pair occupying a specific part of a chromosome. All dominate alleles have capital letters, while the recessive ones are lowercase. Dominate alleles will always overpower recessive ones in the expression of the gene.
If the alleles for a parent do not match, they are known as heterozygous. In the image above the Gg is heterozygous. This can happen if there is a dominate and a recessive gene in the parent. If the alleles are the same for that expressed gene, it is known as homozygous. This is seen if both alleles are dominate or if both alleles are recessive; e.g., GG or gg. In order for a recessive gene to be expressed, the alleles must be homozygous.
Draw the Punnett square. This is done by drawing a square, followed by a straight line up and down and another from side to side. This will quarter, or create 4 equally sized boxes within the square.
Place the father’s alleles on the top of the Punnett square with one letter above each box. Place the mother’s alleles on the left hand side of the square, with one letter in front of each box. Be sure to use capital letters for the dominate genes and lower case letters for the recessive alleles. For this example, let’s say this square represents the color of a flower. The father has one dominant blue and one recessive orange allele. The mother has two recessive orange alleles.
Drop the father’s alleles down into the squares and bring the mother’s across. This will provide you with all possible combinations of alleles for the offspring. Each square represents a 25% chance of the offspring having that combination. If there are squares with the same cominations in them, the squares can be added together to determine the percentage.
From the completed square above, we can see that 50% of offspring will be blue since any dominant allele paired with a recessive one will win. There are, however, two homozygous combinations in which both genes are recessive, so 50% of the offspring will be orange. This means that half of the offspring will be blue, while the other half will be orange. Easy, right?
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