information. this is because dna is made of two strands that pair together like the two sides of a zipper. the nucleotides are in the center, like the teeth in the zipper, and pair up to hold the two strands together. importantly, the four different sorts of nucleotides are different shapes, so for the strands to close up properly, an a nucleotide must go opposite a t nucleotide, and a g opposite a c. this exact pairing is called base pairing. when dna is copied, the two strands of the old dna are pulled apart by enzymes; then they pair up with new nucleotides and then close. this produces two new pieces of dna, each containing one strand from the old dna and one newly made strand. this process is not predictably perfect as proteins attach to a nucleotide while they are building and cause a change in the sequence of that gene. these changes in the dna sequence are called mutations. mutations produce new alleles of genes. sometimes these changes stop the functioning of that gene or make it serve another advantageous function, such as the melanin genes discussed above. these mutations and their effects on the traits of organisms are one of the causes of evolution. genes and evolution mice with different coat colors a population of organisms evolves when an inherited trait becomes more common or less common over time. for instance, all the mice living on an island would be a single population of mice: some with white fur, some gray. if over generations, white mice became more frequent and gray mice less frequent, then the color of the fur in this population of mice would be evolving. in terms of genetics, this is called an increase in allele frequency. alleles become more or less common either by chance in a process called genetic drift or by natural selection. in natural selection, if an allele makes it more likely for an organism to survive and reproduce, then over time this allele becomes more common. but if an allele is harmful, natural selection makes it less common. in the above example, if the island were getting colder each year and snow became present for much of the time, then the allele for white fur would favor survival since predators would be less likely to see them against the snow, and more likely to see the gray mice. over time white mice would become more and more frequent, while gray mice less and less. mutations create new alleles. these alleles have new dna sequences and can produce proteins with new properties. so if an island was populated entirely by black mice, mutations could happen creating alleles for white fur. the combination of mutations creating new alleles at random, and natural selection picking out those that are useful, causes an adaptation. this is when organisms change in ways that help them to survive and reproduce. many such changes, studied in evolutionary developmental biology, affect the way the embryo develops into an adult body. inherited diseases some diseases are hereditary and run in families; others, such as infectious diseases, are caused by the environment. other diseases come from a combination of genes and the environment. genetic disorders are diseases that are caused by a single allele of a gene and are inherited in families. these include huntington's disease, cystic fibrosis or duchenne muscular dystrophy. cystic fibrosis, for example, is caused by mutations in a single gene called cftr and is inherited as a recessive trait. other diseases are influenced by genetics, but the genes a person gets from their parents only change their risk of getting a disease. most of these diseases are inherited in a complex way, with either multiple genes involved, or coming from both genes and the environment. as an example, the risk of breast cancer is 50 times higher in the families most at risk, compared to the families least at risk. this variation is probably due to a large number of alleles, each changing the risk a little bit. several of the genes have been identified, such as brca1 and brca2, but not all of them. however, although some of the risks are genetic, the risk of this cancer is also increased by being overweight, heavy alcohol consumption and not exercising. a woman's risk of breast cancer, therefore, comes from a large number of alleles interacting with her environment, so it is very hard to predict. genetic engineering since traits come from the genes in a cell, putting a new piece of dna into a cell can produce a new trait. this is how genetic engineering works. for example, rice can be given genes from a maize and a soil bacteria so the rice produces beta-carotene, which the body converts to vitamin a. this can help children with vitamin a deficiency. another gene being put into some crops comes from the bacterium bacillus thuringiensis; the gene makes a protein that is an insecticide. the insecticide kills insects that eat the plants but is harmless to people. in these plants, the new genes are put into the plant before it is grown, so the genes are in every part of the plant, including its seeds. the plant's offspring inherit the new genes, which has led to concern about the spread of new traits into wild plants. the kind of technology used in genetic engineering is also being developed to treat people with genetic disorders in an experimental medical technique called gene therapy. however, here the new, properly working gene is put in targeted cells, not altering the chance of future children inheriting the disease causing alleles.