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Identical Twins, Identical Genes

You probably can tell by their close resemblance that these two individuals are identical twins. Identical twins develop from the same fertilized egg, so they inherited copies of the same chromosomes and have all the same genes. Unless you have an identical twin, no one else in the world has exactly the same genes as you. What are genes? How are they related to chromosomes? And how do genes make you the person you are?

Figure (PageIndex1): Identical twins

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Human Chromosomes

Each species is characterized by a set number of chromosomes. The human number is 23. Human cells normally have two sets of chromosomes in each of their cells, one set inherited from each parent. There are 23 chromosomes in each set, for a total of 46 chromosomes per cell. Each chromosome in one set is matched by a chromosome of the same type in the other set, so there are actually 23 pairs of chromosomes per cell. Each pair consists of chromosomes of the same size and shape, and they also contain the same genes. The chromosomes in a pair are known as homologous chromosomes. As see in Figure (PageIndex2), there are two types of chromosomes, autosomal and sex chromosomes. Read more about this in the genetics chapter.

api/deki/files/15798/f-d_c6ceab13b80d183e913de855dbf46cbd22e2cd094fd84bbefe3dc0ce%252BIMAGE_THUMB_POSTCARD%252BIMAGE_THUMB_POSTCARD.jpg?revision=1&size=bestfit&width=268&height=395" />Figure (PageIndex3): Linkage Map for the Human X Chromosome. This linkage map shows the locations of several genes on the X chromosome. Some of the genes code for normal proteins. Others code for abnormal proteins that lead to genetic disorders.