Mutations. Classification of mutations. Modifications and aberrations of chromosomes.

Date: 2015-10-07; view: 457.

New combination of genes.

New genotypes arise in 3 ways

1. Independent assortment of chromosomes during gamete formation.

2. Reciprocal recombination of linked genes in chromosomes by crossing over in the prophase of meioses 1.

3. Random fertilization (theoretically each spermatozoon can fertilize each ovum).

4. Random combination of parent's pairs.

These processes simply reshuffle the already existing genes (and, consequently, trains), but result in the distribution of different sets of traits to different members of a population. It gives new material for activity of natural selection.

The term mutation was introduced by Hugo De Vries in 1901.

Mutation is defined as a sudden relatively stable, discontinuous change in the genetic materiel (DNA) of a cell or organism.

Mutations may be in somatic cells and in gamete cells. Mutations occurring in somatic cell called as somatic mutations. They can be inherited by daughter cells produced by mitosis. Thereby they don't pass from one generation of organisms that reproduce by sexual way to other generation.

Mutations, that take place in gamete cells are named as generative mutation. These mutations pass from one generation to other generation of organisms using sexual way of reproduction.

There are two main types of mutations:

1. Chromosomal mutations (modifications).

a. Chromosomal mutations caused by changing number of chromosomes.

b. Chromosomal mutations caused by changing number of chromosomes (chromosomal aberrations).

2. Gene (point) mutations.

Chromosomal aberrations take a region of DNA large than five nucleotides pairs. The big forms of these mutations may be seen under the microscope.

Gene (point) mutations are changes of nucleotides sequence.

Chromosomal mutations, or changes in the number of chromosomes, are usually the result of errors occurring during meiosis but they can also occur during mitosis. These changes may involve the loss or gain of single chromosomes, a condition called aneuploidy, or the increase in entire haploid sets of chromosomes, a condition called euploidy (poliploidy). Sometimes there is haploidy – the decrease of number chromosomes in entire haploid set.

Aneuploidy can arise from the failure of a pair, or pairs, of homologous chromosomes to separate during anaphase 1 of meiosis. If this occurs, both sets of chromosomes pass to the same pole of the cell and separation to the formation of gamete cells containing either one or more chromosomes, too many or too few. Fusion of either of these gametes with normal haploid gamete produces a zygote with an odd number of chromosomes. For example Down's syndrome forms due to presence of 47 chromosomes in cells of a person.