# How to determine your baby's blood type using a Punnett square

Each person's blood cells contain different combinations of antigens that determine their blood type. The types of blood that exist are A, B, AB and O, and any of them can be positive or negative for the Rh factor (another antigen). When the blood type of both parents is known, the possible blood types of a baby can be determined. First, you must determine which genes each parent can pass on, then draw a Punnett square, and finally calculate the percentage probability of each blood type.

## Steps

### Part 1 of 3: Determine Which Alleles Can Be Transmitted

#### Step 1. See a professional to test your blood type

Healthcare professionals have the ability to test your blood type. Get a test done at your next appointment. Alternatively, you can donate blood to an organization (eg Red Cross) and they will test your blood type and report the results to you. There are four probabilities:

• TO
• B
• AB
• OR

#### Step 2. Get a blood type test at home

An alternative to having your blood type tested by professionals is to order a blood type test kit and do it yourself. With these kits, you will expose the blood to a series of antibodies and obtain the results by analyzing which antibodies affect your blood.

#### Step 3. Determine the possible allele combinations for the blood types of both parents

Knowing the blood type of the father and mother will allow you to determine which genes may be present in each. There are different combinations of genes that can express the same blood type because some genes are dominant and some are recessive. For instance:

• The O blood type means that the person has two O alleles. This is because O is recessive and is only expressed when both alleles match.
• Blood type A means that the person has two A alleles or one A and one O. This is because A is dominant and will be expressed as long as it is present.
• Blood type B means that the person has two B alleles or one B and one O. This is because B is dominant and will be expressed as long as it is present.
• The AB blood type requires an A allele and a B allele. Both are dominant and expressed equally.

#### Step 4. Include the Rh factor

Testing for the major blood antigens (A and B) will determine whether you are A, B, AB, or O. However, the Rh factor is another important antigen that can affect the blood's reaction to a transfusion and can be vital for pregnant women. The Rh factor test is similar to the test for the other antigens. Blood is exposed to antibodies, if it reacts, you are Rh positive. If he doesn't react, you are Rh negative. Although this factor is important for medical purposes, it is not necessary for simple blood type predictions. Considering the Rh factor makes the prediction more complicated, since it doubles the number of possible blood types:

• TO+
• TO-
• B+
• B-
• AB+
• AB-
• OR+
• OR-

### Part 2 of 3: Draw the Punnett square

#### Step 1. Make a column for the possible alleles of the father

The Punnett square is used to determine the genotype. Begin the chart by writing down all the possible alleles of the father in a column on the left side. This represents the first half of the baby's genetics.

### For example, if the father has type "A" blood, you would write down the allele combinations "A, A" and "A, O." That means you have four alleles from top to bottom in the left column (three A's and one O's)

#### Step 2. Put the possible alleles of the mother in the top row

Then write the possible alleles of the mother in the top row. These alleles represent the other half of the baby's genetics. This will allow you to predict which alleles the baby might inherit from the mother and father.

• For example, if the mother has type “AB” blood, there is only one combination of alleles that can produce it. This means that you would write two alleles in the first row (one A and one B).
• The Punnett square can also be used to determine other traits. For example, to predict the baby's eye color.

#### Step 3. Draw a row corresponding to each allele in the first column

The number of rows in a Punnett square is determined by the number of possible alleles on the father's side and will vary depending on the father's blood type. If the father has a homozygous blood type, there will be fewer rows. If your blood type could be heterozygous, there will be more rows.

### Homozygous means that the father only carries one type of allele (O, O). Heterozygous means that the father carries two different alleles (A, O)

#### Step 4. Draw a column under each allele in the first row

Just as the number of rows depends on the number of alleles from the father, the number of columns depends on the alleles from the mother. You must draw a column for each allele that the mother might possess. Any alleles you have can be transferred to the baby.

### Part 3 of 3: Assessing Your Baby's Blood Type Odds

#### Step 1. Write the possible blood types

Each box in the Punnett square represents a blood type that the baby might have. Enter in each box the allele of the father and the allele of the mother corresponding to the row and column of that box.

### For example, you would fill in the first box with the "A" allele from the father and the "A" allele from the mother

#### Step 2. Count the number of squares present

The number of boxes present will tell you how many possible gene combinations they could pass on to the baby. This is important in determining the chances of transmission of any blood type. Just count the boxes to determine the number of possibilities.

### In case one parent is type “A” and one parent is type “AB”, there will be eight boxes

#### Step 3. Add the number of repetitions of each blood type

Once you've counted the number of cells, count the number of times each blood type repeats. There are likely to be duplicates in some boxes. Duplicates mean that the baby is more likely to have that blood type.

### For example, you might have eight cells in total, three AA, three AB, one AO, and one BO

#### Step 4. Calculate the percentage probability of each blood type

To do this, you just need to take the number of cells occupied by a certain blood type and divide it by the total number of cells. Starting from the previous example, the possibilities for each blood type would be:

• AA = ⅜ = 37.5% probability
• AB = ⅜ = 37.5% probability
• AO = ⅛ = 12, 5% probability
• BO = ⅛ = 12, 5% probability
• In this case, there is a 50% chance that the blood type is A, since both AO and AA will be expressed as type A blood.