Ans. is 'd' i.e., Point mutationIn sickle cell anemia there is missense type of point mutation.Mutations o A mutation is a permanant change in the DNA.o Mutations that affect germ cells (sperm or ovum) are transmitted to progeny and may give rise to inherited disease. o Mutations that affect somatic cells are not transmitted to progeny but are impoant in the genesis of cancers and congenital malformations.o Mutations may be classified into three categories ?I. Gene mutationso The vast majority of mutations associated with hereditary disease are gene mutations.o These may of different types depending whether it involves complete gene or single base ?(a) Point mutationA single nucleotide base is subustituted by a different base.When a pyrimidine base is subustituted by other pyrimidine base or a purine base is substituted by other purine ---> Transition.When a purine is substituded by a pyrimidine or vice-versa --> Transversion.This may alter the code in a triplet of bases, i.e. in codon and leads to replacement of one aminoacid by another in the gene product.Because these mutations alter the meaning of the genetic code, they are often termed missense mutation. o Example is sickle mutation in which CTC codon in 13-chain of hemoglobin that codes for glutamic acid is changed to CAC codon that codes for valine.Another type of point mutation is nonsense mutation in which a point mutation may change an amino acid codon to a stop codon.o Example is13-thalassemia in which CAG codon in f3-chain of hemoglobin that code for glutamin is changed to stop codon UAG after point mutation.(b) Deletion and inseionsDeletion or inseion of one or two base lead to alterations in the reading frame of the DNA strand frame shift mutation.If the number of base pairs invoved is three or a multiple of three framshift does not occur (because codon is triplet), instead an abnormal protein missing one or more amino acids is synthesized.(c) Trinucleotide repeat mutationo Normally a codon is triplet ie trinucleotide.In this type of mutation a codon, ie trinucleotide sequence undergoes amplification and the same codon is repeated continuously so many times in the chain.o For example in fragile X-syndrome, CGG codon is repeated 250-4000 times, ie. there are 250-4000 tandem repeates of CGG.2.Chromosome mutationo Result from rearrangement of genetic material that give rise to visible structural changes in the chromosome.3.Genome mutationo Involves loss or gain of whole chromosome, e.g. monosomy - Turner syndrome, trisomy - Down syndrome.