Genetic variation is what makes humans and other multi cellular organisms different from each other, and gives the species the ability for to adapt to suit a particular environment, ensuring the survival of a particular species. The reason that siblings are not identical, unless they are identical twins, is because of four main reasons; independent assortment, recombination, crossing over and mate selection. Independent assortment occurs as each chromosome pair is sorted independently of the other pairs during meiosis. It is this random shuffling process that creates a huge variety of gametes from a single parent. Recombination occurs when pieces of chromosomes are exchanged within their homologous pair. This increases the swapping of genetic information leading to more genetic variation. Crossing over only occurs in the first division of meiosis, this is when whole groups of genes are mutually exchanged between the homologous chromosomes. The other factor allowing genetic variation to take place is mate selection. Variation is furthered by the ability of an individual to choose a mate with which to produce offspring. Different combinations of genes will come together in the offspring during fertilisation, depending on which two individuals mate.
Mutations also cause variation, as they are the source of all new genetic information. Whilst most mutations are harmful, there are others that are described as ‘silent’ this means that these mutations have no known effect on the individual, and some can provide an improvement on the original version of the gene. These beneficial mutations are then past on to offspring aiding the survival of a species. Mutations can be caused by a number of ways for example ionising radiation from X-rays or ultraviolet radiation, Viruses and micro organisms, Alcohol and dietary components such as high levels of fatty foods or alcohol in a diet and Environmental poisons and irritants. Mutations occur in two types Single gene mutations and Chromosome mutations. Single gene mutations cause alterations in the genetic instruction coded in the DNA of chromosomes where as Chromosome mutations is when whole pieces of chromosomes are rearranged or lost completely during meiosis.
Sickle Cell disease is an inherited disorder caused by a gene disorder that causes the red blood cells to deform and become sickle shaped. This creates a number of problems, this is because the sickle cells are rapidly removed from the circulation leading to jaundice and anaemia, other problems include heart defects, brain damage, kidney defects, enlargement of the spleen and skin lesions. This is caused by a point mutation which is located on the HBB gene on chromosome 11 and involves the substitution of one base for another this alters a single amino acid. This new amino acid makes the haemoglobin behave in a different manner as it has different properties. One other inherited disorder is Down syndrome, which is caused by Trisomy. This is when three chromosomes represent one chromosome pair. The mutation for Down syndrome occurs on chromosome 21, the most common form of this condition arises when meiosis fails to separate the pair of chromosome 21 in female ova. One in 800 babies born to mothers aged 30 to31 have Down syndrome, however this increases rapidly with the mothers age. Some of the symptoms include poor muscle tone, congenital heart disease and mental retardation.
