Question

An expectant father is heterozygous (cc) for the mutation known to cause cystic fibrosis. He is confident his children won't have this disease because the mother of his child is confirmed to be homozygous dominant (CC). Under Mendelian rules, their child should be healthy with genotypes of either be CC or Cc.

They are quite surprised to learn their child is born with cystic fibrosis. The baby is karyotyped and confirmed to be a euploid (46 chromosomes and no chromosomal alterations).

From what you learned for this week, what is the most likely explanation as to why the baby has cystic fibrosis?

Answer 1

Father = cC
Mother CC

Parental cross: CC X Cc
Expected progeny = Cc
Observed progeny = cc
The affected progeny contains a normal chromosome number.
This shows that nondisjunction occurred in both parents during gametogenesis.

The CFTR gene is located on chromosome 7
So, nondisjunction in mother produced a gamete with 22 chromosomes (Without 7th chromosome)
Nondisjunction in father produced a gamete with 24 chromosomes (An additional copy of the 7th chromosome)