Consider the pedigree below. The circles represent females and the boxes, males. White color indicates a normal phenotype while black, an individual with the disorder. Cystic fibrosis is an autosomal recessive disorder. The couple in generation 3 had two children with cystic fibrosis. Tracing family history revealed that the great grandmother in generation 1 had Cystic fibrosis. The normal allele is represented by C and the disorder is represented by c. Determine all possible genotypes for the different individuals in the four generations.

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Answer:

Generation 1: cc - CC
Generation 2: Cc-Cc-Cc - CC
Generation 3: Cc - Cc - cc
Generation 4: cc - CC - cc - CC

Explanation:

As seen in the question above, dark colored circles or squares develop the cystic fibrosis disorder that is represented by the recessive "c" allele. While light colored circles and quads did not develop the disorder, but can carry the gene, if they present the heterozygous allele pair "Cc".

As we know, homozygous alleles are only phenotypically expressed if they are homozygous, that is, if they present the "cc" pair.

Thus, we can say that in the family shown in the figure below, generation 1 presents a person with the "cc" alleles and another with the "CC" alleles. The interbreeding between these two people, will generate an offspring totally "Cc", that although it has the gene responsible for the disorder, they will not present the disease because they are heterozygous.

According to the result of Generation 3, we can assume that a member of generation 2 (who possessed the Cc alleles) had a cross with an individual with the "CC 'alleles thus generating, generation 3, with two individuals of" Cc alleles. "or an individual from the" CC "allele and one from the" Cc "allele.

The generation 3 individual with the "Cc" allele also crossed with a "Cc" individual. As each member of the offspring receives an allele from each parent this cross would be:

Cc * Cc = CC - Cc- Cc - cc

Based on this, we can say that each member of the offspring would have a 25% chance of presenting the disease. As we can see in the figure below, two members of this offspring have inherited the "cc" alleles and have the disease.