Amy has cystic fibrosis which is caused by a recessive allele. Her partner is heterozygous for cystic fibrosis. What is the chance that her future child has cystic fibrosis?

Alleles are different forms of a gene which can become mutated to cause different characteristics or diseases. They can be dominant (usually denonimated by a captial letter H) or recessive (usually denominated by a lower case letter f). Dominant alleles only require one copy to be present for the gene to be displayed where as a recessive allele requires two copies of the same allele to be present for the gene to be displayed. 

Cystic fibrosis is recessive. As Amy has cystic fibrosis, she must have two copies of the same allele (ff). Her partner is heterozygous, this means he has one normal allele and one abnormal allele (Ff). During reproduction, a combination of the parent's allele produces the genes of the offspring. Using a punnet square, we can illustrate the different possibilities and combinations that the parents alleles can give. The mother's alleles, ff and the father's alleles Ff, give rise to the following combinations: Ff, Ff, ff, ff. The Ff combination denotes a carrier of the faulty allele where ff denotes the condition, cystic fibrosis. Looking at the combinations, we can see that there is a 50% chance of the child being a carrier (Ff) and a 50% chance or 1 in 2 chance of the child having cystic fibrosis.