How do you do question?

A woman's sister has cystic fibrosis, a disease caused by recessive genes. Neither of her parents has the disease. What chance is there that her mother is a carrier (heterozygous) for the trait? What chance is there that the woman herself is a carrier for the trait?

 

There is 100% chance that her mother is a carrier (heterozygous) for the trait.

There is 50% chance that the woman herself is a carrier for the trait.

Step-by-step explanation

• Cystic fibrosis is a genetic disorder that is inherited in an Autosomal Recessive pattern, which means that the disease is inherited when both the parents have one copy of mutated gene (Mutated CFTR gene).
• Hence followed by fertilization, total two copies of mutated gene received from each parent gives the disease phenotype.
• Here, it is mentioned that neither of the patient's parents has the disease. But since the patient has cystic fibroids, it means that she must have inherited one copy of mutated gene from each parent respectively making it two copies of mutated CFTR gene.
• Thus, there is 100% chance that patient's mother is a carrier (heterozygous) for Cystic fibrosis because she contributed one defective gene.
• Also, it is clear that the patient's father is also a carrier of Cystic fibrosis because he would have contributed for other copy of defective CFTR gene.
• When both parents are carrier of Cystic fibroids, there is 50% chance (2 in 4 offsprings will be carriers) that the offspring will be carrier of disease. Hence the chance that the woman herself is a carrier for the trait is 50%.