Cystic fibrosis is a devastating autosomal recessive disorder that results in an overproduction of very thick mucus, which can accumulate in (and clog the) digestive tract, sweat glands, and lungs. A husband and wife both undergo genetic testing and discover that they are both heterozygous for the cystic fibrosis gene. What is the probability that their first child will inherit a copy of the cystic fibrosis gene?
Cystic fibrosis is caused bu a recessive allele. Alleles are versions of a gene and can be either dominant (the stronger version) or recessive (the weaker version). Combinations of alleles make the genotype which can be either homozygous dominant (two dominant alleles), heterozygous (one dominant alleles and one recessive), or homozygous recessive (two recessive alleles).
Let's have "C" represent no cystic fibrosis and "c" represent the cystic fibrosis allele. The, a Punnett Square can be made for the two heterozygous parents (Cc xCc):
C CC Cc
c Cc cc
The genotype ratios for this parent cross are:
25% homozygous dominant (CC)
50% heterozygous (Cc)
25% homozygous recessive (cc)
Thus, their children have a 75% chance of carrying the cystic fibrosis gene (25% from homozygous recessive genotype and 50% from heterozygous).