Blood Type Genetic Basis:
In genetics, blood type gene has two alleles, each allele has genotype A, B or O.
The A and B are dominant, and O is recessive. So allele A combined with allele O is type A.
Similarly, BO is type B, AA is type A, BB is type B, OO is type O, and AB is type AB.
If both parents have type A blood, then the alleles could be AA or AO, thus the allele A frequency is 75%, allele O frequency is 25% for both parents.
So the chance of alleles OO is 25% × 25% = 6.25%,
alleles AA is 75% × 75% = 56.25%,
alleles AO is 75% × 25% = 18.75%,
alleles OA is 25% × 75% = 18.75%.
Since AA, AO and OA are blood type A, and OO is blood type O, thus their child has 6.25% chance to be blood type O and 93.75% chance to be blood type A.
The +/- is called the rhesus factor, with + being dominant, and - being recessive.
So if both parents are -, the kids are always -, otherwise the kids might be + or -.
| Father's Blood Type | |||||
| A | B | AB | O | ||
| Mother's Blood Type | |||||
| A | A/O | A/B/AB/O | A/B/AB | A/O | |
| B | A/B/AB/O | B/O | A/B/AB | B/O | |
| AB | A/B/AB | A/B/AB | A/B/AB | A/B | |
| O | A/O | B/O | A/B | O | |
| Child's Blood Type | |||||
| A | B | AB | O | ||
| One Parent's Blood Type | |||||
| A | A/B/AB/O | B/AB | B/AB | A/B/O | |
| B | A/AB/O | A/B/AB/O | A/AB | A/B/O | |
| AB | A/B/AB/O | A/B/AB/O | A/B/AB | Impossible | |
| O | A/AB | B/AB | Impossible | A/B/O | |
