Final answer:
Cystic fibrosis demonstrates that the observation of phenotype affects the type of dominance because the disease manifests only when two recessive alleles are inherited, signifying the importance of allele type in phenotypic expression. In contrast, with autosomal dominant diseases like neurofibromatosis type I, a single dominant allele is enough to express the disorder.
Step-by-step explanation:
In biology, especially in genetics, we often discuss inheritance patterns like autosomal dominant inheritance, as seen in conditions such as neurofibromatosis type I, and autosomal recessive inheritance, as illustrated by cystic fibrosis. Dominance is the phenomenon where one allele masks the expression of another in the heterozygous state. In cystic fibrosis, two recessive alleles need to be present (as in genotype ff) for the disease phenotype to be expressed. This is because the production of normal CFTR proteins by a single dominant allele (F) is sufficient to prevent the disease symptoms, making the normal phenotype dominant over cystic fibrosis.
Conversely, for a disorder like neurofibromatosis type I, which follows an autosomal dominant pattern, inheriting one mutated allele (N) is enough to express the disease phenotype because a single copy of the altered gene can cause the disorder. In a heterozygous condition (Nn), the dominant allele N leads to the disease, overshadowing the normal allele n. This demonstrates that the phenotypic expression depends greatly on whether an allele is dominant or recessive and influences the type of dominance observed.
Mendelian genetics also explains that the likelihood of inheriting certain traits, including diseases, follows predictable patterns as shown by Punnett squares. However, the probability may not always correspond with the actual outcomes in small family sizes, contrasting with Mendel's large sample size that confirmed predicted phenotypic ratios. The occurrence of a specific phenotype is ultimately dependent upon the particular combination of alleles inherited from the parents.