Copyright © 2007 Dorling Kindersley
Genetics is the study of the instructions, contained in cells, for how our bodies develop and function. These instructions are called GENES and are carried by tiny objects called chromosomes in the nuclei (centres) of cells. Packed into the chromosomes is a chemical called deoxyribonucleic acid (DNA), which holds the instructions in coded form. People’s genes come from their parents. The passing on of biological characteristics from parents to children via their genes is called INHERITANCE.
Chromosomes are tiny structures in the nucleus at the centre of each body cell. Each cell carries 46 chromosomes. Just before a cell divides, each chromosome makes a copy of itself. In this way, a complete set of chromosomes can pass into each of the two new cells that form during division.
When a chromosome copies itself just before cell division, for a short time the chromosome and its copy are joined at a point called the centromere. This gives each chromosome an X shape, with four arms extending from the centromere. Each arm consists of a long threadlike molecule of DNA, coiled up on itself. A gene is a section of the DNA molecule.
If the DNA molecule could be unravelled, it would reveal a structure like a twisted ladder, called the double helix. The sides of this ladder consist of a chainlike substance called sugar-phosphate. The rungs of the ladder are formed from chemicals called nucleotide bases. There are four different types of base. Their sequence within a gene (a long section of DNA) is a code that holds the instructions carried by that gene.
A full chromosome set consists of 23 pairs of chromosomes. Twenty-two of these chromosome pairs are the same in girls and boys. The last pair, called sex chromosomes, differ between the sexes. Girls have two sex chromosomes called X chromosomes. Boys have just one X chromosome and a smaller partner, called the Y chromosome. This Y chromosome contains genes that give boys their male characteristics.
A gene is a section of DNA in a chromosome. People receive half their genes from their mothers and half from their fathers, via the egg and sperm cells from which they grew. Every gene plays a part in determining how a person looks and functions. Genes influence the body through complex mechanisms in cells that translate the coded messages in DNA into the activities of each cell. The differences between people result partly from tiny variations in their genetic makeup – that is, in the sequence of bases in their DNA.
Different genes on a single human chromosome may hold the instructions for a wide range of body activities. Scientists are gradually building up maps that show the locations of specific genes on each chromosome. As they work out what each gene does, they add this information to the maps. Altogether, there are probably between 30,000 and 40,000 genes on the human chromosomes.
Everyone’s DNA sequence is amazingly similar. The numbers below indicate in percentage terms exactly how similar. Human DNA is also very like that of chimpanzees, our closest animal relatives.
Family members often resemble each other in traits (characteristics) controlled by genes, such as eye colour. Some traits are determined by the combined action of just two genes, one from each parent. Each member of this gene pair exists as one of two or more forms, with differing effects. One of these forms, called the dominant form, may mask the action of others and only has to be inherited in a single dose to produce its specific effect. Other gene forms, called recessive, have no effect unless they are inherited in a double dose, one from each parent.
An example of a simply inherited trait is the ability to taste phenylthiocarbamide (PTC), a bitter substance found in some fruits and vegetables. Only about two-thirds of us can taste PTC. To be a PTC taster depends on inheriting at least one copy of the dominant “taster” gene for this characteristic. People are nontasters only if they inherit an alternative “nontaster” gene from both parents.