Cystic Fibrosis (CF) Is Caused by a Recessive Allele. a Child Has CF, Even Though Neither of His Parents……
Thursday, August 25th, 2011 at
5:37 pm
Cystic Fibrosis
A question asked by Nikki:Cystic fibrosis (CF) is caused by a recessive allele. A child has Cystic Fibrosis, even though neither of his parents……
…….has CF. If this couple has another child, what is the probability he or she will NOT have Cystic Fibrosis?
a. 3/4
b. 2/3
c. 2/4
d. 1/3
e. 1/4
Top answer:
Answer by Dsf S
3/4
How about adding your own answer to the comments below!
Tagged with: allele • caused • child • cystic • even • fibrosis • neither • Parents • Recessive • though
Filed under: Cystic Fibrosis
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a. 3/4
The parents are heterozygous for the CF allele – their genotypes will both be Cc (with ‘c’ denoting the recessive disease causing allele).
Genotypes Cc Cc
Gametes C c C c
Offspring CC Cc cC cc
The theoretical outcome of a cross bewteen two heterozygotes for any recessive allele is 3:1 unaffected:affected
Since the parents are not affected, but had an affected, child, then the parents are both carriers (Cfcf). So, their possible offspring are:
Cfcf x Cfcf
offspring
CfCf, normal, non-carrier
Cfcf, normal, carrier
Cfcf, normal, carrier
cfcf, cystic fibrosis
So, as stated by the others, the probability of them having a child that does not have cystic fibrosis is 3/4.
3/4
If two carriers of the mutated CF gene have children then there is:
• A one in four chance that their baby will have CF
• A one in four chance that their baby will not have CF or carry a CFTR mutated gene
• A two in four chance that the baby will not have CF but will carry one CFTR mutated gene
The risk to other relatives depend on who is a carrier eg the risk to the offspring of an unaffected sibling of an affected individual is (2/3 x 1/25 x 1/4 = 1/150). 2/3 is the risk of an unaffected sibling being a carrier, 1/25 is the population carrier risk and a ¼ chance of parental carriers both handing on their mutated CFTR gene.