Which of the following is the correct definition of a dominant allele?

• A. An allele that is only expressed if two copies are present in the genotype.
• B. An allele that is always expressed in the phenotype, even if only one copy is present.
• C. An allele that is only inherited from the father via the Y chromosome.
• D. An allele that is muted in the presence of a recessive allele.

Cystic fibrosis is an inherited condition controlled by a single gene. The dominant allele, $F$, codes for normal mucus production, while the recessive allele, $f$, codes for thick, sticky mucus leading to cystic fibrosis.

Two parents who are both heterozygous for the gene ($Ff$) have a child. Determine the probability that the child will have cystic fibrosis.

Calculate the probability that their next child will be a girl with cystic fibrosis. Use the relation: $probability\; of\; a\; girl\; with\; cystic\; fibrosis=probability\; of\; cystic\; fibrosis\times probability\; of\; being\; a\; girl$