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 missing genotype?
In this question, we have been given a Punnett square, which is a visual diagram used to predict the probability of phenotypes resulting from a genetic cross. This Punnett square represents the inheritance of a gene associated with cystic fibrosis. The gene has two possible alleles or alternative versions. The dominant allele is represented by uppercase F, and the recessive allele is represented by lowercase f. As we’re told in the question, it’s this recessive allele that causes cystic fibrosis.
Punnett squares can be used to figure out whether the child of two parents could be at risk of developing genetic disorders like cystic fibrosis. In human reproduction, a child inherits two alleles for each gene: one from their biological mother and the other from their biological father. In the Punnett square, the mother’s alleles are found in the top row and the father’s alleles are found on the left-hand side. Each cell in the Punnett square is filled in with the allele from the corresponding column and row. And we can see that all three cells have been filled in this way.
Each cell now represents a possible combination of alleles that a child of these parents could have. And that combination of alleles is called a genotype. As long as at least one copy of the dominant allele is present in the genotype, the child will not display cystic fibrosis. However, if two copies of the recessive allele are present in a genotype, the child will display cystic fibrosis.
Now let’s try filling in the blank cell. We have one dominant allele from the mother and one recessive allele from the father. The missing genotype in this Punnett square is therefore uppercase F lowercase f, and this child won’t have cystic fibrosis.
