Gene Abnormalities

Genetic disorders can also be caused by much smaller changes in genetic material.  These changes can occur at the gene level, or even as small as a single nucleotide. Single or multiple DNA bases can be deleted, inserted, or substituted for other “incorrect” bases. A change in a single nucleotide is called a point mutation. The DNA sequence is read by our body in 3 letter segments (called codons). Therefore, a mutation where the number of nucleotides inserted or deleted is not a multiple of 3 often causes a much more severe effect because it causes the reading frame to get off track for the remainder of the gene sequence. This type of mutation is commonly referred to as frameshift mutation.

Examples of mutations in a DNA sequence

You can think of a DNA sequence as a sentence of three letter words. Below are some examples of mutations in a DNA segment and in a simple sentence. DNA nucleotide bases are represented by the letters C, A, T, and G. C = cytosine, A = adenine, T = thymine, and G = guanine.

Normal Sequences

DNA: CAC TGG GAT AGG GTC TCA

Sentence: THE FAT CAT ATE THE RAT

Sequence deletion: (the nucleotide/letter between the two red nucleotides/letters was deleted in the sequences)

CAC TGG ATA GGG TCT CA

THE FAC ATA TET HER AT

Sequence insertion: (the red nucleotide/letter was inserted and not present in the original (normal) sequence)

CAC TGG GAT ACG GGT CTC A

THE FAT CAT AMT ETH ERA T

Sequence point mutation: (the red nucleotide/letter was switched for an incorrect one, no frame shifting occurs)

CAC TAG GAT AGG GTC TCA

THE FST CAT ATE THE RAT

Sequence inversion: (the red nucleotides/letters were inverted; a portion of the sequence is oriented backwards)

CAC TGG GAC TGG GAT TCA

THE FAT CAE HTE TAT RAT