Role of mutation in natural selection: of mutation in natural selection: Mutations may lead to formation of multiple alleles at a given locus. New alleles change the gene frequency in a population that upsets genetic equilibrium, causing microevolution. In diploid organisms mutations are recessive and therefore remain hidden for a long time but keep spreading and accumulating in the population causing hybrid vigour (Heterosis) and variations, which help in natural selection. In haploid organisms they affect the character immediately and expose it to natural selection. Recombination of mutant genes takes place during crossing over, which makes the population heterozygous and much fitter when exposed to natural selection. Due to the genetic drift, an advantageous mutation may be quickly fixed helping in natural selection or sometimes harmful mutations may be fixed, causing extinction of the population. 5. Chromosomal aberrations: Chromosomal aberrations: This involves breaking and rejoining of a segment of chromosome during the prophase of meiosis, by deletion, duplication, inversion or translocation. 1. Deletion: Deletion: When a small part of the chromosome breaks apart, generally during crossing over by the action of endonuclease and is lost. In man, a terminal deletion in chromosome 21 causes granulocytic 5th leukemia and deletion of one arm of chromosome causes Cri-du-chat syndrome in children. In Drosophila, deletion in X chromosome causes notched wings. Deletion causes abnormality in crossing over during pachytene stage of meiosis. 2. Duplication: Duplication: When one gene is represented more than once, if deleted portion of one chromosome gets attached with another chromosome. The zygote will have 3 doses of genes and crossing over will be unequal. Bar-eye in Drosophila arises due to duplication of a small section of X chromosome. 3. Inversion: Inversion: When a piece of chromosome breaks and joins at the same place after rotation, the sequence of genes is altered. It is most likely to occur in meiosis in germ cells if chromosomes form a loop. Chromosome breaks at centre and gets reattached after reversing. This will cause abnormal synopsis and crossing over. There is not loss or gain of genes but sometimes the effect may be due to new position of the genes. 4. Translocation: Translocation: This involves transfer of one gene block from one linkage group to another in non- homologous chromosomes. Most of the translocations do not produce any abnormality in carriers as deletion in one is balanced by the other. 50% of the offspring of carriers (heterozygotes) will be grossly abnormal. Translocations, like inversions, do not produce immediate effects but accumulate over long periods to introduce reproductive isolation in allopatric populations. In Drosophila 6 species have been produced by translocation from the ancestral Drosophila virilis, which has 6 pairs of chromosomes. 6. Aneuploidy: When the entire chromosome is either lost or duplicated, it is called Aneuploidy. In Drosophila the gene for white eye is X chromosome. When two X chromosomes fail to separate during oogenesis (non-disjunction), female offspring with 3X chromosomes will be superfemales, which have low viability and are sterile, while some eggs will have no X chromosome at all. OY individuals do not survive because of the upset in gene balance. In man non-disjunction in sex chromosomes influences secondary sexual characters. For example, trisomic XXX produces superfemales, while XXY causes Klinefelter’s syndrome. Monosomic (2n-1) XO causes Turner’s syndrome, while YO is non-viable. Down’s syndrome or Mongolism is caused due to duplication of chromosome No. 21 and it causes mental retardation, malformed ears, no sexual