In pedigree B, how can unaffected parents have an affected child?

The unaffected mother, who is marrying in, does not carry an allele for the disease; so the affected child inherits an allele only from the affected father. No child could be affected by a single autosomal recessive allele, or X-linked recessive allele, so the trait is dominant

The childs mother could be a carrier without knowing it which would create the couple to have a child thats affected.

Explanation:

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