Video Transcript
Cystic fibrosis is an inherited disease caused by a recessive allele, lowercase f. The Punnett square shows the genotypes of a male and a female and the predicted genotypes for their offspring. What is the probability, in percent, that a child born to these parents will be a carrier for the cystic fibrosis allele?
The question talks about cystic fibrosis, a genetic disorder in which the body produces thick, sticky mucus, which causes difficulty breathing and frequent infections. As mentioned in the question, this disorder is caused by a recessive allele, which is represented by a lowercase f. You may know that an allele is an alternative form of a gene. The dominant allele of this gene would be represented with an uppercase F.
Recessive alleles are only expressed when there’s no dominant allele present. This means that only people who carry two recessive alleles of the cystic fibrosis gene like this will have cystic fibrosis. If a person has one dominant allele and one recessive allele like this, they are said to be carriers of cystic fibrosis. They won’t actually have the condition themselves, but they do carry one recessive allele for this gene. A Punnett square can be used to figure out whether a child of a couple could be at risk of inheriting or carrying genetic disorders like cystic fibrosis.
The parents’ alleles are written along the top and left-hand side of the square. As you can see, each box represents a possible combination of alleles that a child of this couple could have. Each combination of alleles is called a genotype. Each box in the Punnett square is filled in with the corresponding allele from the biological mother and the corresponding allele from the biological father.
Our question is asking us to figure out the probability that a child born to this couple will be a carrier for the cystic fibrosis allele. Remember, a carrier is a person with one dominant allele and one recessive allele. We can see that out of the four possible genotypes, two of them have this combination. This means that two out of the four possibilities will be carriers of the cystic fibrosis allele. In percent, two out of four would be 50 percent.
