Answer:
C
Explanation:
The brother has different alleles for the blood type gene than his sister does. Blood type is inherited from parents like many other traits that can be displayed on a punnet square. Depending on what the parents blood type alleles were, the brother and sister could have different blood types.
A different number of chromosomes doesn't lead to different blood types, so D is wrong. And A is incorrect because the number genes also does not affect blood type. And I think the blood type genes are on the same chromosome, so B is wrong. C is correct because blood type is inherited by alleles, and different alleles can cause different traits, such as blood types.
