What is a Dihybrid Punnett square?
Hey, I just learned that my blood type is AB+. I know where the AB came from due to one of your previous epic lectures, but where does the plus come from?
The positive and negative blood phenotypes represent the Rh antigen.
How is that different from the A and B antigens?
The ABO blood types are the result of a single gene, while the positive and negative blood types are coded by different genes.
So two genes code for blood types.
Yes, for the 8 blood types that are the most important to know before doing a blood transfusion.
So there are more antigens on my red blood cells?
Yes, there are many antigens on your red blood cells, but the ABO and Rh antigens are the most important.
Do I use a Punnett square to solve both the ABO and Rh antigens?
Yes, but the Punnett square is usually a 4×4 instead of a 2×2.
Wait, there are 16 boxes?
Yep. It’s called a dihybrid Punnett square
It sounds. . .
Fun?
No, the opposite of fun.
Like
No. More like
How to Solve Dihybrid Crosses
A dihybrid cross is used to find the probability of phenotypic and genotypic outcomes of two genes that are on separate chromosomes. The 8 common blood types are the product of two genes. One gene codes for the A, B, O, and AB blood types. The other gene codes for the Rh factor, which is the + or – of each blood type. Therefore, the blood type AB+ is produced by two different genes.
In blood typing:
- The A and B alleles are codominant and the O allele is recessive to A and B alleles
- AA, AO = Type A blood
- BB, BO = Type B Blood
- AB = Type AB Blood
- OO = Type O Blood
- The + (R allele) blood type is completely dominant to – (r allele) blood type
- RR, Rr = + blood type
- rr = – blood type
Example #1
An A+ female with the genotype AORr mates with a B+ male with the genotype BORr. What is the probability that this couple will have a child with the O- blood type?
Step #1: Write out the Parental Genotypes
AORr x BORr
Step #2: Find the Possible Gametes
To find the gametes, you will need to do a 2×2 Punnett Square for each parent. The Punnett squares are slightly different because they show meiosis (making of the gametes) rather than fertilization (offspring probabilities).
Step #3: Solve the Dihybrid Punnett Square
Now that you know the possible gametes (sperm and eggs), you can now put the eggs on the top of the Punnett square and the sperm on the left side (or you can put the sperm on top and eggs on the left side).
Step #4: List the possible offspring phenotype outcomes
- What is the probability that the couple will have a child with blood type A+? 18.75%
- What is the probability that the couple will have a child with blood type B+? 18.75%
- What is the probability that the couple will have a child with blood type O+? 18.75%
- What is the probability that the couple will have a child with blood type AB+? 18.75%
- What is the probability that the couple will have a child with blood type A-? 6.25%
- What is the probability that the couple will have a child with blood type B-? 6.25%
- What is the probability that the couple will have a child with blood type AB-? 6.25%
- What is the probability that the couple will have a child with blood type O-? 6.25%
Example #2
For some dihybrid Punnett squares, you do not need to use a 4×4 Punnett Square to solve the offspring genotype and phenotype ratios. A 2×4 Punnett square will work if some of the gametes produced are the same. For example, an A+ female with the genotype AORR mates with a B+ male with the genotype BORr.
Step #1: Write Out the Parental Genotypes
AORR x BORr
Step 2: Find the Possible Gametes
Step #3: Solve the Punnett Square and Calculate the Probabilities
