Blood type is a genetically determined trait. Two siblings donate blood and find out their blood types. The brother is Type A and the sister is Type
B. Which of the following best explains why the siblings have different blood types?
A. The brother has a different number of blood type genes than his sister.
B. The brother's blood type gene is on a different chromosome than his sister's.
c. The brother has different alleles for the blood type gene than his sister does. D. The brother has a different number of chromosomes than his sisted does

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.