Cystic Fibrosis Inheritance

Cystic Fibrosis Inheritance


Genes and chromosome pairs:

  • Gene; a length of DNA that codes for a protein
  • Every cell (except sex cells) contain two copies of each gene, one from each parent
  • Both copies have the same locus on each of the paired chromosomes
  • 23 pairs of chromosomes; the chromosomes in each pair are called homologous chromosomes
  • Cystic fibrosis is caused by a gene mutation that is passed from parent to offspring

Genotypes, Phenotypes and alleles:

  • Mutation in the CF gene (the length of DNA that codes for the CFTR protein)
  • CF gene occurs in two alternative forms or alleles
  • Healthy gene = F / Non-functional protein = f


  1. FF – a person with two identical copies of the normal allele does have cystic fibrosis
  2. ff – a person with two copies of the mutated allele has cystic fibrosis
  3. Ff – A person with one normal allele and one mutated allele does not have cystic fibrosis but is a carrier and coud pass on the disease


  • The alleles that a person has make up their genotype
  • Homozygous genotype; both the same (ff/FF)
  • Heterozygous genotype; different alleles (Ff)
  • Characteristic caused by the genotype is the phenotype


Predicting the genotype of offspring:

  • When gametes are produced, each egg or sperm has only one allele, f or F
  • Punnett square; illustrates all the possible ways in which the two types of allele can combine, showing the possible genotypes of children
  • Carriers (Ff) have some protection against typhoid


Inheritance of genetic diseases:

  • Cystic fibrosis is a monohybrid inheritance; characteristics controlled by one gene


  • Thalassemia; genetic disease cause by recessive alleles of a gene on chromosome 11
  • Gene is involved in the manufacture of haemoglobin which carries oxygen round body
  • Someone who is homozygous makes no haemoglobin, or non-functional haemoglobin – homozygous condition is often lethal
  • Heterozygous conditions (advance) – have no symptoms but have protection against malaria


  • Albinism, phenylketonuria and sickle cell anaemia are also called by single recessive alleles


  • Achondroplasia is caused by a dominant allele
  • A homozygote always dies
  • Heterozygote – restricted growth


  • Huntington’s diseae and the ability to taset PTC are also caused by dominant alleles


The work of Mendel:

  • Carried out breeding experiments
  • He established that a number of characteristics of the garden pea were determined by separate genes