Video Transcript
Cystic fibrosis is an inherited disorder caused by a recessive allele. Which of the following best explains what this means? A person will express the disease in their phenotype even if only one allele is in their genotype. A person must have three copies of the allele for cystic fibrosis to be expressed in the phenotype. A person can carry more than one copy of the allele and not have cystic fibrosis. Cystic fibrosis only affects offspring once every two generations. Or a person must have two copies of the allele for cystic fibrosis to be expressed in the phenotype.
Let’s start by defining some of the key terms in the question to better understand how to answer it. Alleles are different versions of the same gene. Alleles determine many of our inherited characteristics, such as the eye color and our natural hair color. There is a gene for eye color. And the different alleles that we might have can make our eye color possibly blue or possibly brown, as you can see in these diagrams. Some other alleles, however, can also result in the inheritance of genetic disorders. Cystic fibrosis is an inherited disorder. It is caused by a mutation in the CFTR gene. It can lead to problems such as a buildup of excess mucus in both the respiratory system and in the digestive system.
For each gene, such as the CFTR gene, a person will have two alleles in their genetic makeup or their genotype. Their phenotype is the observable characteristic, in this case whether or not they will express cystic fibrosis. Recessive alleles are only expressed in a person’s characteristics or their phenotype if there are two copies present, or if the dominant allele is absent. Dominant alleles will always be expressed in the phenotype even if only one copy is present in the person’s genotype.
For the CFTR gene, a dominant allele, which allows the gene to function correctly, is represented by an uppercase F. The recessive allele, which could potentially cause cystic fibrosis, is represented by a lowercase f. As cystic fibrosis is a recessive disorder, it will only be expressed if two copies of this recessive allele are present in the genotype.
Let’s summarize the potential genotypes for the CFTR gene. If a person has both dominant alleles, they will not express cystic fibrosis. If a person has both recessive alleles, they will have cystic fibrosis. If a person has both the dominant allele and the recessive allele, the recessive allele is not expressed as it’s masked by the dominant allele. This person does not express cystic fibrosis, but they are known as a carrier. This is because they have the potential to pass on the allele for cystic fibrosis to their offspring.
Let’s review our answer choices and eliminate those that we know are wrong. Diseases caused by recessive alleles will not necessarily skip generations. This diagram shows how the offspring of a person affected by cystic fibrosis can inherit the alleles and therefore the disease. If the child inherits one cystic fibrosis allele from their father who has cystic fibrosis and one allele from their mother who is a carrier, they will have two recessive alleles in their genotype and will therefore also be affected by the disorder. Therefore, the correct answer is that a person must have two copies of the allele for cystic fibrosis to be expressed in the phenotype.
