 |
| Genetic Variation |
|
| |
|
| The variations at the level of the gene which form the basis of the differences between related organism as well as the diversity among organisms is called gene mutation. Gene mutation is defined as a sudden discrete change in the genetic material of gene which is heritable. |
| |
| Mutations when they arise suddenly in nature are called spontaneous mutations. However, many agents such a radiations and some chemicals called mutagens can be applied to create induced mutations. |
| |
| Occurrence of mutations in gametic cells can be transmitted to the next generation but those in somatic cells are confined only to that individual. |
| |
|
 |
| |
| fig. 19.14 Deletion and Inversion Mutations in Chromosomes |
| |
 |
| |
| fig. 19.15 Translocation and Duplication Mutation in Chromosomes |
| |
| Four different types of structural alterations deletion, duplication, inversion and translocation have been identified. |
| |
| Deletion |
| |
| Sometimes a part of the chromosome is lost. This loss can be from one end or from chromosome parts between the ends. Accordingly, the former is known as terminal and the latter as interstitial deletion. |
| |
| Example: |
| |
| uniduchat syndrome: loss of half of the short arm of chromosome 5 in human. |
| |
| Duplication |
| |
| Duplication occurs when a portion of a chromosome is repeated. The duplication can either be in tandem sequence or in reverse order. |
| |
| Inversion |
| |
| When a segment of chromosomes breaks but later rejoins after rotating by 180o it result in inversion. |
| |
| If the centromere is included in the inverted segment, it is known as pericentric but of inversion occurs only in one arm and the centromere is not involved, it is referred as paracentric inversion. |
| |
| Translocation |
| |
| Sometimes a segment of chromosome breaks and relocates within a non- homologous chromosome. This leads to interstitial translocation. |
| |
| In another type, Two non-homologous chromosomes exchange a segment; this creates a reciprocal translocation. |
| |
|
| Change in chromosome number can also bring about visible effects on the phenotype. |
| |
| Each species has a characteristic number of chromosomes which is a reflection of a basic set or monoploid number (n). |
| |
| Euploids |
| |
| Those having multiples of monoploid number. Euploids which have more than twice the number of monoploids are called polyploids. It is also called genomic mutation. |
| |
| Ployploidy |
| |
| Polyploidy is of 2 types |
| |
| a. auto-polyploidy |
| |
| b. allo-ployploidy |
| |
| Autopolyploidy |
| |
| It arises by increase in the number of chromosome sets of the same species. |
| |
| Allopolyploids |
| |
| It develops in a hybrid between two species by doubling of chromosome sets. Example: AB to AABB. |
| |
| Aneuploidy |
| |
| It involves the addition or deletion of one or a few chromosomes from the usual diploid set of chromosomes. |
| |
| Monogamy |
| |
| The presence of a single copy of a particular chromosome (2n 1). |
| |
| Trisomy |
| |
| Presence of three copies of a particular chromosome. (2n+1). |
| |
| Tetrasomic |
| |
| An aneuploid having four copies of one chromosome is known as tetrasomic (2n+2). |
| |
| Nullisomic |
| |
| An aneuploid lacking a pair of homologous chromosomes is said to be nullisomic (2n-2). |
| |
|
| Prokaryotic cells lack nuclear membrane and the genetic material is found in a compact region called nucleoid. The chromosome is made up of DNA and has associated proteins. It is about 1.2 mm in length in the bacterium Escherichia coli. The chromosome is highly coiled and super coiled with several loops. |
| |
