Earth Science: Human – Genetically induced Diseases
Mutations are random permanent alterations of genetic information. They may occur spontaneously or due to triggers such as ultraviolet light. In many cases a mutation does not affect an organism’s functions.
However, some mutations may result in serious disabilities or even death, as they may cause cancer or genetic diseases.
Biology – Genetics and heredity – Mutations
Mutations may alter single genetic segments (gene mutation). Sometimes an entire chromosome is affected by a mutation (chromosome mutation) or even entire chromosome sets (genome mutation).
There are different types of mutations: a gene mutation alters a single gene, which results in the development of a new allele (gene expression); a point mutation affects only one base on a DNA-nucleotide chain. The deletion or addition of individual bases results in a frameshift mutation. In this case none of the nucleotide base triplets following the altered section of the nucleotide strand can be read correctly. All original information is lost. A base exchange may also result in no changes to the information.
This may be the case if, for example, the altered triplet happens to encode the same amino acid as before. Since amino acids are mainly determined by the first and second position of a codon, point mutations of the third position on a triplet are usually less dramatic. Chromosome mutations change the structure of individual chromosomes. This may be the result of chromosomes breaking up during division and fragments getting lost, known as deletion, or attaching to one of the sister chromatids in a process called duplication.
Other potential mechanisms are an exchange of fragments between non-homologous chromosomes (translocation) or the inverted reattachment of fragments onto the chromosome (inversion). A genome mutation is an alteration in the number of chromosomes. Sometimes individual chromo-some pairs or chromosomes do not separate during mitosis or meiosis, which is called a nondisjunction.
This results in aneuploidy, a condition in which there is an increased or decreased number of chromosomes in the daughter cells. If one chromosome is missing, it is known as a monosomy and if one extra chromosome is present, it is a trisomy. A change that affects entire sets of chromosomes is called euploidy. Most organisms have a double set of chromosomes, which is known as diploidy; if instead three or more chromosome sets are present, then this is called polyploidy.
LACTOSE INTOLERANCE
Geneticists have found that earlier humans tolerated lactose only during childhood (lactose tolerance). About 9,000 years ago, a mutation emerged in humans of lighter skin that allowed them to tolerate lactose throughout their lives.
Thus, many of their descendants are able to tolerate milk consumption after the age of weaning, while adults of Asian and African descent may suffer from digestive problems and discomfort.