Our bodies are made up of billions of cells. Most of the cells contain a set of around 20,000 genes. Genes act like instructions, directing our growth and development and how our bodies work. Genes are carried on
structures called chromosomes. There are usually 46 chromosomes, 23 inherited from our mother and 23 inherited from our father, so we have two sets of 23 chromosomes in ‘pairs’. Apart from two sex
chromosomes (two Xs for a girl and an X and a Y for a boy) the chromosomes are numbered 1 to 22. Chromosome 1 is the largest chromosome. Each chromosome has a short arm and along arm.
Chromosomes can’t be seen, but if they are stained and magnified under a microscope, each one has a distinctive pattern of light and dark bands. In a 1q21.1 microduplication, the chromosome has broken in two places in band q21.1, and a tiny amount of chromosome material between them is repeated.Looking at chromosomes under a microscope, one can sometimes see where the chromosome has broken. With an extra piece (a duplication) that is large enough, one can sometimes see the pattern of bands that
help to show how big it is.But with a microduplication, the extra piece is so tiny that you can’t see it under even the highest-powered microscope. Only molecular, DNA technology can identify it. The most common technique is known as microarrays. This shows gains and losses of tiny amounts of DNA throughout the chromosomes. Microarrays can show whether particular genes or bits of genes are present once, twice three times or not at all.

Normal genetic variation?
1q21.1 microduplications are found in the general population as well as in people referred for chromosome testing. At first they were thought to be part of the normal genetic variation between individuals. However, they are more common in people referred for genetic testing and they
are now thought to raise susceptibility to a range of developmental problems.