Polymorphisms/Mutations, STR's, Markers and Alleles
Most of our DNA is identical to DNA of others. However, there are inherited regions of our DNA that can vary from person to person. Variations in DNA sequence between individuals are termed " polymorphisms". As we will discover in this activity, sequences with varying degrees of polymorphism are very useful for DNA analysis in genetic genealogy and paternity testing. This activity is based on analyzing the inheritance of a class of DNA polymorphisms known as " Short Tandem Repeats" , or simply STR's
Short Tandem Repeats are multiple repeats of specific base pair combinations. Sequences in these special regions involve multiple repetitions of short combinations of letters, such as "GATA".
1 AATTTCTGTA TTTTATTTAG AGACGGGGTT TCACCATGTT GGTCAGGCTG ACTATGGAGT
61 TATTTTAAGG TTAATATATA TAAAGGGTAT GATAGAACAC TTGTCATAGT TTAGAGCGAA
121 CTAACGATAG ATAGATAGAT AGATAGATAG ATAGATAGAT AGATAGATAG ATAGACAGAT
181 TGATAGTTTT TTTTTATCTC ACTAAATAGT CTATAGTAAA CATTTAATTA CCAATATTTG
241 GTGCAATTCT GTCAATGAGG ATAAATGTGG AATCGTTATA ATTCTTAAGA ATATATGTTC
301 CCTCTGAGTT TTTGATACTT CAGATTTTAA GGCC
Notice the 15 repeats of the four bases string GATA
Markers are segments of DNA in which STR's are tested. Due to the differing lengths of STR's, a larger string of base pairs must be tested so as to include the largest number of STR's possible. These regions are called markers. Most but not all markers associated with genetic string are prefixed with DYS. These coded regions have been assigned marker status because of their unique signatures over time. These signatures (STR's) can relate information that we use to understand our DNA inheritance.
Alleles are repetitive copies of a marker. One of two or more alternative forms of a gene at a given position ( locus) on a chromomsome caused by a difference in the sequence of DNA. A gene which controls eye colour in humans may have two alternative forms – an allele that can produce blue eyes, and an allele that produces brown eyes. In a plant that occurs in tall and short forms, there may be an allele that tends to produce tall plants and an alternative alleles that produces short plants.