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Mutation acquired during an organism's lifespan
A postzygotic mutation (or post zygotic mutation) is a change in an organism's genome that is acquired during its lifespan, instead of being inherited from its parent(s) through fusion of two haploid gametes. Mutations that occur after the zygote has formed can be caused by a variety of sources that fall under two classes: spontaneous mutations and induced mutations. How detrimental a mutation is to an organism is dependent on what the mutation is, where it occurred in the genome and when it occurred.

Postzygotic changes to a genome can be caused by small mutations that affect a single base pair, or large mutations that affect entire chromosomes and are divided into two classes, spontaneous mutations and induced mutations.

Most spontaneous mutations are the result of naturally occurring lesions to DNA and errors during DNA replication without direct exposure to an agent. Deamination – The amine group on a base is changed to a keto group. This results in cytosine being changed to uracil and adenine being changed to hypoxanthine which can result in incorrect DNA replication and repair.

Induced mutations are any lesions in DNA caused by an agent or mutagen. Mutagens often demonstrate mutational specificity, meaning they cause predictable changes in the DNA sequence. A few common mutagens that induce mutations are:
Ultraviolet light (UV) – Causes pyrimidine (T or C) nucleotide bases on the same strand to covalent join forming a pyrimidine dimer. Thymine thymine dimers are the most common mutation caused by UV light. Since dimers cause a disruptive kink in DNA structure, polymerases often have trouble reading the region, slowing down DNA replication. Alkylating agents – Compounds that attach an alkyl group to the four bases. When an alkyl group is added to guanine, it can lead to the incorrect pairing with thymine and disrupt the accuracy of replication.

When an individual has inherited an abnormality it is usually present in all of their cells. However some mutations like DNA code change, epigenetic alterations and chromosomal abnormalities, can occur later in development. This would result in one progeny cell line to be normal while the other cell line(s) to be abnormal. As a result, the individual is considered to be a mosaic of normal and abnormal cells. Men with LOY have a higher all cause mortality and cancer mortality compared with unaffected males. Additionally, LOY is associated with greater risk for Alzheimer's disease and cardiovascular disease. Smoking increases the risk of inducing LOY more than three times and has a dose dependent effect on LOY status.

Trisomy 21 (Down syndrome) is one of the most prevalent chromosomal abnormalities amongst live births. Of all trisomy 21 pregnancies, approximately 80% end in spontaneous abortions or still births. 1–5% of people diagnosed with having Down Syndrome are actually in fact "high grade" trisomy 21 mosaics. The rest of trisomy 21 mosaics are marked as "low grade" mosaics, meaning the chromosomal mutation occurs in less than 3–5% of respective tissue. While high grade trisomy 21 mosaics, demonstrate similar features to full Down Syndrome, low grade mosaics have a tendency to show milder features; however, the effects are quite variable depending on the distribution of the trisomic cells.

Somatic mutations are the result of a change in the genetic structure after fertilization. This type of mutation also involves cells outside of the reproductive group and thus is not transmitted to future descendants.

Germ line mutations are the result of a change in the genetic structure of germ cells. These mutations are able to be transmitted to the offspring and give rise to a constitutional mutation. Constitutional mutations is a mutation that when present in one cell, is also present in all other cells associated with the organism.