Genetics and Heredity

Background to Genetics and Inheritance

Bodies are made up of thousands of cells. Almost all cells contain a complete set of genetic material (genome) comprising the set of instructions (the genes) for building and maintaining the body. There are approximately 30,000 genes in a genome and most occur in pairs. Genes are made of a chemical called deoxyribonucleic acid (DNA) and packaged into structures called chromosomes. Usually people have 23 pairs of chromosomes (46 in total). One of each chromosome pair has been inherited from each parent. When a man or woman has a child, each passes on one of each pair of genes and chromosomes in the egg or sperm. Twenty-two chromosome pairs are the same in men and women (the autosomes) and are numbered from 1 (the largest) to 22 (the smallest). The 23rd pair are called sex chromosomes because they determine gender: usually, women have two X chromosomes and men have one X chromosome and a smaller Y chromosome. (See Figure 1.)

The relationship between cells, chromosomes, genes, and DNA can be imagined as a library of information (the nucleus) at the center of each cell. The chromosomes are equivalent to shelves and each gene equivalent to a recipe or instruction book, giving information on how to make one particular protein or substance. The DNA is effectively the paper and ink from which the books are made.

Variations in genes cause differences between individuals and, ultimately, are the basis of evolution and differences between species. Depending on its role, if a gene is altered it can cause a genetic trait or a genetic disease. A gene alteration that stops the gene functioning is known technically as a mutation. Everyone carries five or six altered genes on average. Gene alterations are rather like a word spelled differently somewhere in the book or perhaps a whole page missing or duplicated: any change that means that the instruction is not followed as usual. This is not as simplistic as it seems: there are four DNA building blocks (adenine, thymine, cytosine, and guanine—shortened to A, T, C, and G) which spell out a gene’s code: effectively, there is a four-letter genetic alphabet.

The proteins coded for by genes fulfill many different functions, e.g., structural building blocks, enzymes driving chemical reactions, or proteins involved in controlling whether other genes are switched on. Genes are involved in every aspect of the functioning of the body, e.g., the brain and cognition, other major organs like the heart and kidneys, the eyes and seeing, height, hair, and eye coloring—and the ears and hearing. The different functions of proteins mean that an alteration in a single gene will not always show the same sort of effect and this means that altered genes show different patterns of inheritance. A genetic change may have been inherited from further back in the family, but may occur, by chance, in one individual for the first time. This means that conditions can be genetic without necessarily being inherited. (See Figure 2.)

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