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Cystic fibrosis is an autosomal recessive disorder (c). Consider a cross between two heterozygote parents. Using...

Cystic fibrosis is an autosomal recessive disorder (c). Consider a cross between two heterozygote parents. Using a Punnett square, demonstrate what percentage of their offspring would have the disease? What are the genotypic ratios of all the possible outcomes from this cross?

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Expert Solution

Cystic fibrosis is a genetically inherited disorder caused by a mutation in a gene which is located in chromosomes number 7 (autosomal chromosome). In this disorder, cells producing digestive enzymes, mucus are damaged. As a result, the digestive system and the respiratory system of a person cannot work properly. This disorder may cause death.

Let's suppose allele​ 'c' is mutated for cystic fibrosis, i.e., causes cystic fibrosis and allele 'C' is normal, i.e., doesn't cause cystic fibrosis. As the disorder is recessive, a person with the homozygous recessive condition, i.e., with 'cc' genotype, will have the cystic fibrosis disorder. On the other hand, a person with heterozygous (Cc) or homozygous dominant (CC) condition will not have the disorder.

In the above question, the parents are heterozygous (Cc). Each of them will produce 50% gametes with '​​​​​​C' allele and the rest 50% gametes with '​​​​​​c' allele.

Punnett square -

C c
C CC Cc
c Cc cc

Now we can see that out of four offsprings only one is homozygous recessive (cc) which will have cystic fibrosis disorder.

Hence, the percentage of offsprings with cystic fibrosis disorder = (number of homozygous recessive offspring ÷ total number of offsprings) ×100 = (1÷4)×100 = 25%

The genotypic ratios of the cross are -

CC:Cc:cc = 1:2:1


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