Acouple meets with a genetic counselor. the woman's father had sickle-cell anemia, and she and her mother were unaffected. neither the man nor his parents or siblings had sickle-cell anemia. this couple is concerned that their unborn child may be affected with this disease. would amniocentesis be necessary to determine whether the fetus is affected

Yes, an amniocentesis is necessary.
While both the woman and the husband may not exhibit sickle cell anemia, they could be carriers of the sickle cell recessive gene. If she is heterozygous for sickle cell allele, because of her father, and the man has no recessive gene, then the child has no chance of exhibiting the disease but has a 0.5 chance of being a carrier. 
However, if the man is also a carrier, the child has a 0.25 probability of having sickle cell anemia.

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