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Cystic fibrosis is an inherited disorder that creates a thick, sticky mucus. It mainly affects the lungs and pancreas. It is caused by a recessive allele. In a genetic diagram:

• the recessive allele can be shown as f
• the dominant allele can be shown as F

In example one, both parents are heterozygous Ff – they are carriers of the disease. This means they have the cystic fibrosis allele and might pass it on to their children. But they also have a dominant, normal allele, so they do not suffer from the disease themselves. The chance of them producing a child with cystic fibrosis is 1 in 4, or 25%.

In example two, only one parent (the father) has a copy of the recessive allele. There is no chance of them producing a child with cystic fibrosis. However, half the possible offspring will be homozygous, FF, and half will be heterozygous, Ff. The ratio of FF to Ff is 1:1, or 50%.

Family trees

Cystic fibrosis is an inherited disorder. It is passed down through generations and can be tracked in family trees.

If a male without cystic fibrosis (FF) and a carrier female (Ff) reproduce, they can produce offspring who either don’t carry the cystic fibrosis allele (FF), or are carriers (Ff). This means they show no symptoms of the disease but are able to pass along an allele for the disease to their offspring.

If the carrier offspring reproduced with another carrier, then it is possible that their children will either not carry the allele at all (FF), be a carrier (Ff), or have the disease cystic fibrosis (ff).