An unaffected man whose mother had sickle-cell anemia marries an unaffected woman whose father had sickle-cell anemia. which of the following statements about their offspring is not true?
a. about 75% can be expected to have the sickle-cell allele.
b. they must all have the sickle-cell trait.
c. about 50% can be expected to be heterozygous for the sickle-cell allele.
c. they may not have either sickle-cell trait or sickle-cell anemia.

The correct answer is b.  They must all have the sickle-cell trait.

Explanation:

Sickle cell anemia is an autosomal recessive disorder that can be inherited from parents to offspring. A person can only be affected when both the allele came from parents are mutated alleles.

So unaffected male must be heterozygous(Aa) and unaffected women also must be heterozygous(Aa). So when cross between both the heterozygous condition is will happen then the four offspring type will be AA, Aa, Aa, and aa.

This shows that 25% of trait will be homozygous recessive(aa) and will not have any sickle-cell trait. Therefore the correct answer is b. They must all have the sickle-cell trait.