Practice question

Unaffected parents lacking visible dominant trait cannot harbor dominant allele under full penetrance model, so they cannot produce affected child via dominant inheritance without new mutation. In recessive mode, unaffected parents can be heterozygous carriers harboring mutant allele silently without clinical signs. By Mendelian segregation, carrier by carrier cross yields twenty-five percent homozygous recessive affected child. This pattern of normal parents with one or more affected offspring within sibship is classic for autosomal recessive inheritance and for X-linked recessive when mother is carrier transmitting to sons.