Heredity and inheritance

• • A genetic disease arises due to a change in the DNA sequence of a gene. DNA in chromosomes is passed from parent to child, and thus genetic disease is also passed from parent to child.
• • A change or alteration in the DNA sequence of a gene is called a mutation. There are various examples of gene mutations. A change or deletion in one of the bases of a gene can change an amino acid in the protein or can shorten the protein, affecting the function of the protein or the production of mRNA. De-novo mutations may arise in a fetus that were not present in the parents. Frameshift mutation, in which there is addition or deletion of a number of base pairs, may result in a transcription error. Missense mutations arise when a single-base substitution mutation results in a codon that specifies a different amino acid.
• • Inherited differences between people reflect inheritance of many different polymorphisms or genetic variations in protein function.
• • Examples of single-gene diseases include mutations of one gene, such as in Huntington's disease.
• • In single-gene disorders, mutation of one gene copy may cause disease, and this is an example of dominant inheritance.
• • Mutation in both gene copies causes a recessive pattern of inheritance, such as in cystic fibrosis.
• • Multi-gene diseases are caused by a combination of mutations in several genes and the influence of environmental factors, such as in asthma and heart disease.
• • 'Anticipation' is the term used when a disease appears at an earlier age or with increasing severity in successive generations.
• • A person is affected if someone has a genetic disease which could be dominant or recessive.
• • A carrier is an individual who has a gene mutation for a recessive disease in only one gene copy, or who does not have disease symptoms but may pass on the mutation to their children.

Correlation between genotype and phenotype

• • Genotype refers to the genetic information that is carried by a pair of alleles that determines a particular characteristic.
• • Penetrance refers to the relationship between the genotype and the pheno^typ^e*
• • Huntington's disease is due to a single highly penetrant autosomal dominant gene.
• • Multifactorial diseases such as hypertension and asthma reflect the lack of highly penetrant genes and possible environmental influences.
• • The relationship between lifetime risk, prevalence data and genotype frequencies and penetrance enables the calculation of relative risks of genotypes.

• Polymorphisms or common genetic variants in DNA coding exist and may differ from low-penetrance mutations, depending on the prevalence.