 Genome, DNA, chromosome, homologous, diploid, haploid, genes, alleles and chromatids, the field of genetics uses a whole bunch of words specific to this area of biology. Are you overwhelmed? Well, let's take a guided tour to understand what they all mean and how they fit together. First, the genome. This is all of the genetic material of an organism. The human genome holds many clues as to how we look and behave, but there is so much information there that we still don't understand most of it. It's a huge area of ongoing research. For most organisms, this genetic information is made up of DNA. DNA, or deoxyribonucleic acid, is the storage structure for our biological information. It is made of smaller units called bases. These are the famous ATC and Gs. Three billion of them strung together on a backbone of sugar and phosphate. The DNA is organized in two strands that fit together in a double helix. The genome of humans is divided into several very long DNA double helices, each of which is called chromosome. The DNA of different humans are incredibly similar but not identical, except for identical twins. And these differences make each of us unique. Each living organism has a characteristic number of chromosomes. Humans, for example, have 22 pairs of autosomal chromosomes, each of them unique in size and sequence, and two types of sex chromosomes, X and Y. Most of our cells, apart from sperm or eggs, have pairs of similar chromosomes. This makes us overall deployed organisms. These pairs of chromosomes, which are called homologous chromosomes, are very similar but not identical, as one is from mom and one is from dad. The human, sperm, and egg, which are called gametes, are different in that they only have one copy of each autosomal chromosome, and carry only one sex chromosome. Having just one copy makes gametes haploid. When a sperm fertilizes an egg, there are now 23 pairs of chromosomes, or 46 in total. The fertilized egg then divides, making the many cells of the human body. Now let's look more closely at this process of cell division, which is also called mitosis. As a cell prepares to divide, each DNA molecule is replicated, resulting in two identical copies of each chromosome. This means that the 46 chromosomes temporarily double to 92 chromosomes in each cell. The two exact copies of each chromosome are called sister chromatids. Initially, the sister chromatids are connected to each other at a central point, which is often depicted as the X shaped drawing of a chromosome. At the end of mitosis, the two sister chromatids detach and are pulled apart in opposite directions. Two daughter cells are created, each with 46 chromosomes in the exact same genome. So let's look closer at the structure of a chromosome. Within the long DNA strand, there are regions called genes that contain the instructions to make proteins. There are around 22,000 protein coding genes in humans. The proteins that genes code for make us who we are. They give us ginger hair or B blood type O. These traits are also called phenotypes in the field of genetics. Zooming back out to the chromosome level once again, we can start to understand where variation in human phenotypes begins. As humans have pairs of homologous chromosomes, it follows that they also have pairs of genes. A pair of genes may be identical to one another, or they may differ subtly in their DNA sequences, in which case they are called alleles. Sometimes different alleles still give rise to the same phenotype. If the two genes on homologous chromosomes produce the same phenotype, for example both causing blue eyes, the person is homozygous for that phenotype. However, sometimes the two alleles produce different phenotypes, for example blue eyes and brown eyes. This makes a person heterozygous. The actual outcome in the person, for example what eye color they end up with, depends on whether one allele is dominant over the other. For example, the allele for brown eyes is dominant over blue, so the person will have brown eyes. The blue allele is recessive, because it has no effect on the phenotype of the heterozygote. Most phenotypes, however, are not as simply explained and understood as eye color. Height, intelligence, and susceptibility to different diseases, for example, are complex blends of numerous genes and their many alleles. Phew! Now that we've covered all of these different terms involving DNA, we can delve into all of the amazing things that DNA does and understand how the study of genetics is revolutionizing medicine and advancing our knowledge of life.