How to Calculate the Ratio of the Blood Type of Children

It's possible to figure out the probability of a child having a certain blood type with a few pieces of information. Blood type is determined by two copies of a gene: one each from the mother and father. The combination, called alleles, form the gene that codes the blood type. There are four blood types: A, B, AB and O. Someone with type A blood can either receive two A's or an A from one parent and an O from the other parent; the same is true of B blood type because both A and B are dominant over O type. Someone with AB type blood received an A from one parent and a B from the other because A and B are co-dominant and their combination creates its own blood type. Type O blood can only be inherited if both parents donate an O allele. You can calculate the probability of a blood type using the parents' blood type information.

Instructions

• 1

  Draw a square on a piece of paper. Divide the square into four boxes by drawing a vertical line down the middle and a horizontal line from left to right. This is called a Punnett square.

• 2

  Write the mother's blood type above the square, placing the first allele above the top left box and the second allele above the top right box. For example, if the mother's blood type is A or B, you would have to know her exact genotype -- that is, the underlying combination of alleles that make up the blood type. Type A could either be composed of AO or AA alleles; similarly, type B could be composed of either BO or BB alleles. For this reason, if you know that someone's blood type is either A or B, you need further information to find out if it's AO or AA, or BO or BB. For this example, we will assume that the mother has type B blood with BO alleles. Write B above the top left box and O above the top right box.

• 3

  Write the father's blood type down the left side outside the square. For this example, we will assume the father's blood type is AB. You would place the A to the left of the top left box and the B to the left of the bottom-left box. AB type blood is made up of co-dominant alleles, so someone with this blood type can only have an A allele and a B allele. Similarly, because blood type O is recessive, it is always made up of two O alleles.

• 4

  Fill in each box with the two alleles that cross for that box. Using our example, in the first box, you would write AB since the father's first allele is A and the mother's is B. In the top right box, you would write AO since the father's A allele is matched with the mother's O allele. Since A is dominant over O, the blood type is simply written as A. Fill out the bottom two boxes in the same way.

• 5

  Count the number of boxes with the same blood type combination and convert to percentages of the whole. In our example, we would end up with AB, AO, AB and BO. Since half the squares are type AB, this means there is a 50 percent chance that a child will inherit the AB blood type. There is a 25 percent chance that a child will inherit type A and another 25 percent chance that a child will inherit type B.