Is it possible to have different genes in different parts of our body?

Question

I want to understand genetic mutation specially in the context of multicellular organisms like humans. I studied biology only till high school and I can’t fully understand wikipedia pages on this subject.

I know that genetic material is in chromosomes inside the nucleus of a cell. When that cell multiplies, the chromosomes too generates a copy of themselves and both child cells get one set of the chromosomes.

I would like to know:

1. Does genetic mutation mean an error in chromosome generation at fission stage or will it occur later in the life of the cell?

2. Multicellular organisms begin their life from a single cell (zygote). Under normal circumstances, every cell that grows from this single cell should have the same genetic material. Can genetic mutation eventually result in us having different genetic material in cells of different parts of our body? For example, when only a part of our body gets exposed to radiation.

3. If answer to question 2 is yes, which genetic material will control the behaviour of the cells? Let’s say, for example, that the genetic material of cells in our stomach favour high growth rate of hair but the cells in the scalp themselves do not have that particular gene. What will be the result?

Answer 1

The general answer is that no, it's not possible to have different genes in different parts of the body. There are two interesting classes of exceptions, but they are exceptions.

The first is that mutations can and do occur in cells during their growth. Most of these are probably neutral - they occur in the vast wasteland of genetic junk in the chromosome, and nothing happens. But some of them are harmful, and this is what puts cells on the pathway toward cancer. Outright cancers, that we can detect, are the result of at least a half-dozen mutations occurring in the same cell lineage, that cumulatively allow the cells in that lineage to grow in uncontrolled manner, to avoid immune clearance, to encourage their own blood vessel growth, and so on. (In fact, cancer cells generally have far more than a half-dozen mutations, because a route toward getting the mutations they need is to turn down DNA repair, allowing mutations to accumulate with minimal control. For example, Genomic landscape of DNA repair genes in cancer, Genomic profiling of pediatric acute myeloid leukemia reveals a changing mutational landscape from disease diagnosis to relapse.)

The second set of exceptions is in the immune system. Both B cells and T cells specifically modify their chromosomes during their development, making huge numbers (trillions) or new genes. The catch is that these are all in the same region, the antibody or T cell receptor genes. It's part of the mechanism by which the adaptive immune system "remembers" which pathogens it's interacted with in the past. The wikipedia article on V(D)J recombination is a starting point.

Answer 2

1. Genetic mutation usually refers to any errors made while copying DNA. This can certainly occur in the lead-up to cell division, when chromosomes are copied, although they can also occur when repairing DNA that has been damaged.

2. I'm reading your use of the term genetic material as referring to DNA. Genetic mutation can change the sequence of DAN, but it doesn't really change what genetic material we have in any given cell. I think that it's important to understand that all of the cell in our bodies contain the same genome, or set of genes. What differs from one cell type to another is which genes are active (or "expressed" as biologists say). Mutations can alter gene activity. Say, for instance, that the deletion of a single nucleotide causes a frameshift; this can lead to the loss of that gene product in the affected cell and its daughters.

3. Understanding that all genes are present, but only some genes are active/expressed should help clear up your third question.