Question

Do siblings have the same blood group ? How is it determined andwhy is it different from the parent ?

Answer 1

Blood group of an individual is decided by the expression of I gene in humans. It is having more than two alleles, such as I^A, I^B,I⁰ . An individuals blood group is decided by the allelic combination present in its genotype.
The alleles can randomly have the following combinations:

1. I^A I^A - A blood group
2. I^A,I^{0 _} A blood group
3. I^A I^B - AB blood group
4. I^B I^B​ - B blood group
5. I^BI⁰​ - B blood group
6. I⁰I⁰​ - O blood group

​​Hence , the gametes of parents containing a single allele may be IA or IB or I0 which may combine in either of the above manner and may result in similar or dissimilar combinations in siblings.