Page 18 - An Identity Crisis

STR allele from each parent, and that receiving could also have four children with the same
any given allele in no way influences the other genotype, however, they cannot have a child with
alleles received. Additionally, STRs may be used a new allele unless there was a mutation, which is
for paternity testing because half of the offspring’s very rare.
alleles are inherited from one parent and the other
half from the other parent. Electrophoresis is used to separate DNA fragments
by size. Electrophoresis works based upon the
The figure at the bottom of the page shows a principle that there are electrical charges carried
mother with the 2 and 3 repeat alleles of an STR by the phosphate backbone of the DNA molecule.
locus, while the father has the 3 and 6 repeat Since DNA fragments are negatively charged, they
alleles. The Punnett square in the figure predicts migrate toward the positive pole when placed in an
the possible allelic combinations for their children. electric field. When DNA is forced to move through
The father is capable of contributing either the a separation gel matrix, fragments are sorted
3or the 6-repeat allele to his offspring, while the based upon size because the gel prevents larger
mother may contribute either the 2or the 3-repeat fragments from travelling as quickly as smaller
allele. Therefore, as the square indicates, they could fragments. The most common separation matrices
potentially have four children with completely used are agarose and acrylamide gels. Agarose
different STR genotypes. By chance alone they gels, made from a component extracted from

18 THE MYSTERY OF LYLE AND LOUISE