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Inheritance of physical traits Inheritance of physical traits Genetics is the study of how genes are passed on from parent to offspring (child)—how living things inherit characteristics. Genes, coded instructions located inside the nucleus of a living thing's cells, determine most of its characteristics (also called “traits”), for example, the shape of a plant’s seeds, the colour of a cat’s fur or the height of a human. Genes do not determine quite all characteristics, because living things are affected by their environment. For example, the weather affects a plant’s growth, and environment affects a human’s behaviour. Humans have about 23,000 genes. The complete set of genetic information, acting as the organism’s instruction manual for developing, growing and maintaining itself through life, is known as the genome.

The 23 pairs of human chromosomes The 23 pairs of human chromosomes

Chromosomes, DNA and genes

Almost all cells in the human body carry their own, unique set of coded instructions. The instructions, called genes, are found in two sets of 23 chromosomes located inside the nucleus of the cell. A human inherits one set of 23 chromosomes from its mother and one set from its father. Together they make up the 46 paired chromosomes. Chromosomes are made primarily of a substance called DNA. The genes are “written” in the chemical structure of the long DNA molecule. Particular genes are located in particular regions of each chromosome. For example, the gene that gives a human red hair is located on chromosome 16, towards the end of its longer arm.

The genomes of more than 180 species have been sequenced, including the house mouse, African lion, elephant shark, monarch butterfly and many bacteria.

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