Introduction to genetics
Other sorts of traits are not easily seen and include blood types or resistance to diseases.
Other traits come from interactions between genes and the environment, so a child who inherited the tendency of being tall will still be short if poorly nourished.
For example, the chances of somebody dying of cancer or heart disease seems to depend on both their genes and their lifestyle.
Genes are made from a long molecule called DNA, which is copied and inherited across generations.
DNA is made of simple units that line up in a particular order within it, carrying genetic information.
The language used by DNA is called genetic code, which lets organisms read the information in the genes.
Genes are pieces of DNA that contain information for the synthesis of ribonucleic acids (RNAs) or polypeptides.
An egg and sperm join to form a zygote with a complete set of genes.
In this generation, there is, therefore, a chance of the recessive allele showing itself in the phenotype of the children—some of them may have red hair like their grandfather.
This is because of the large number of genes involved; this makes the trait very variable and people are of many different heights.
[5] The function of genes is to provide the information needed to make molecules called proteins in cells.
Similarly, if a cell needs to do something faster or slower than before, it makes more or less of the protein responsible.
Proteins are made of a chain of 20 different types of amino acid molecules.
The RNA copy made from a gene is then fed through a structure called a ribosome, which translates the sequence of nucleotides in the RNA into the correct sequence of amino acids and joins these amino acids together to make a complete protein chain.
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.
Alleles become more or less common either by chance in a process called genetic drift or by natural selection.
[14] 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.
Many such changes, studied in evolutionary developmental biology, affect the way the embryo develops into an adult body.
[15] Genetic disorders are diseases that are caused by a single allele of a gene and are inherited in families.
Cystic fibrosis, for example, is caused by mutations in a single gene called CFTR and is inherited as a recessive trait.
This variation is probably due to a large number of alleles, each changing the risk a little bit.
[18] 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.
[23] However, here the new, properly working gene is put in targeted cells, not altering the chance of future children inheriting the disease causing alleles.
