Rm i I

Restriction Digest Result

Allele 1 contains 3 repeat units, 250 bp restriction product Allele 2 contains 6 repeat units, 400 bp restriction product

Fig. 4. Schematic diagram describing length polymorphisms of a VNTR sequence. Allele 1 contains three VNTR repeats (open boxes), whereas allele 2 only contains six repeats. These result in different size fragments, 250 bp or 400 bp, being detected by gel electrophoresis.

of approx 1 per 10 kb of sequence (12,13). Polymorphic DNA sequences can be used as markers for determining allelic inheritance of disease-causing genes and for identity testing.

Mutations represent permanent alterations to a DNA sequence. Common mutations, although representing sequence variants, are not usually considered polymorphisms, by virtue of the fact that they represent a rare genetic event (occurring with a frequency of less than 0.1%). Two general categories of point mutations include transitions and transversions. These mutations involve specific substitutions of the organic bases that make up a DNA sequence. The bases adenine and guanine are purine bases, whereas thymine and cytosine are pyrimidine bases. A transition mutation involves a substitution of a purine with another purine, or a pyrimidine with another pyrimidine; a transversion mutation involves the substitution of a purine for a pyrimidine or vice versa. The clinical significance of a mutation is determined by the type of mutation; its location within the gene; and the tissue involved, as not all tissues express all genes. Germline mutations are those that occur in germ cells— cells that give rise to gametes—and are, thus, heritable; these mutations will be present in every cell of the body in subsequent generations. In contrast, an acquired mutation by a cell other than a germ cell is referred to as a somatic mutation. These mutations are only passed on to daughter cells within an individual and are not heritable.

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