Cystic Fibrosis is a recessive genetic disease If a man and

Cystic Fibrosis is a recessive genetic disease. If a man and woman who do not have cystic fibrosis but are both carriers (heterozygous) for cystic fibrosis have children together then what is the chance that their first child will have this syndrome? __________

What percentage of the woman’s eggs will carry the cf allele?__________

What percentage of the man’s sperm will carry the cf allele?____________

Show a Punnett square with the results

Solution

Cystic fibrosis is an autosomal recessive condition. In the given example, the unaffected parents had an affected children Assume that the gene coding for the normal allele is C and the gene coding for the cystic fibrosis is c and C is dominant over c. Now, cross between two heterozygotes will have the offspring with the following genotypes.

Cc* Cc = CC (normal), Cc (carrier), Cc (carrier), cc (affected).

Means, 1/4th children are affected and 3/4th children are normal.

C

c

C

CC

Cc

c

Cc

cc

Thus, the probability for the first child to be affected is = ¼

The percentage of the woman’s eggs will carry the cf allele is, 50% (women is carrier)

The percentage of the man’s sperm will carry the cf allele is, 50% (man is also a carrier)