Thanks for this question. This is my favourite topic! Gene, the molecules of life.
Gene are the basic unit of inheritance. Genes are made up of a chemical compound DNA (deoxyribonucleic acid). DNa can make copies of itself (replicate) and can carry information. Actually your DNA has all the instructions on how to make up your body and sustain it. The information in DNA is written in a biological code made up of 4 chemical bases adenine, guanine, cytosine and thiamine (AGCT). A chain of DNA will look like ATTCCAGGT. This information is read by cells to make the proteins necessary to build a whole organism. This information is also carried to the next generation of the organism. DNA is stored wrapped around proteins in the nucleus and this forms Chromosomes. Humans have 23 pairs of chromosomes (46 in total).
Gene mutation happens when a change in one or more bases of the DNA occurs, e.g. ATTCCAGGT to ATTACAGGT or ATTAAACCAGGT. Cahnges in DNA lead to different proteins and therefore different appearance of an organism. This sometimes leads to disorders like sickle cell.
Chromosomal mutation occurs when there is a change in structure or number of chromosomes. This could cause disorders like Down syndrome which is the result of an extra number of chromosomes (47 chromosomes instead of 46).