Dominant and Recessive Genetic Disorders

Cystic fibrosis is a recessive genetic disorder. The faulty allele is f, and the normal allele is F.

For a child to have cystic fibrosis, they must have two copies of the faulty allele. Therefore the child's genotype is ff.

Since the child is ff, they must have received one f allele from each parent. Because the parents are unaffected, they each have one normal allele masking the faulty one.

Each parent must be Ff — carrying one normal allele (F) and one faulty allele (f). These individuals are called carriers: phenotypically healthy but able to pass the recessive allele to their children.

We need to find the parent genotypes when two unaffected parents have a child with cystic fibrosis (a recessive disorder).

If both parents were FF, they could only pass F alleles to their children. No child could ever be ff, so this option is impossible.

If both parents were ff, they would have cystic fibrosis themselves — but the question states neither parent has the disease. This option is eliminated.

If one parent were FF, they could only contribute an F allele. An ff child would be impossible unless both parents contribute f alleles. Since both parents must be carriers, 'Ff or FF' is not the precise answer.

Both parents must be Ff (carriers) — this is the only genotype that is unaffected yet can produce an ff child when two carriers have children together.

A dominant allele is expressed in the phenotype even when only one copy is present. This means any individual who inherits the allele — whether homozygous dominant or heterozygous — will show the disorder.

Because there are no silent carriers for dominant disorders, the faulty allele cannot be hidden in an unaffected individual and passed on without being noticed. Every individual who inherits the allele is affected.

The disorder appears in every generation because anyone with the dominant allele shows the disorder — there is no way for the allele to 'skip' through an unaffected individual.

The question asks why dominant disorders don't skip generations — we need the correct mechanistic explanation.

Dominant does not mean more common. The notes explicitly warn against this misconception. Frequency in the population is a separate concept from dominance.

While Huntington's disease has late onset, this is a feature of one specific dominant disorder. It is not the reason why dominant disorders in general don't skip generations — in fact, late onset means some affected individuals have children before knowing they're affected.

Autosomal dominant disorders like Huntington's disease affect both males and females equally. This option describes X-linked inheritance, not autosomal dominant.

The correct answer is that anyone who carries the dominant allele shows the disorder — because one copy is sufficient for expression, there are no hidden carriers to cause a skipped generation.