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Forensic Science and Allele Repeates

A group of 6 bodies is found buried in the woods, three adult males, one adult female, and two children. They are thought to include a missing family (2 parents and 2 children). DNA is extracted from the skeletal remains and analyzed for two STR markers using PCR. In the results below, the numbers indicate the number of tandem repeats in an allele (e.g., at STR 1 Child 1 has one allele with 7 repeats and one with 8).

STR1 STR2
Male 1 8/9 5/7
Male 2 6/8 5/5
Male 3 7/10 7/7
Female 8/8 3/5
Child 1 7/8 5/7
Child 2 8/8 3/7

A) Could the female have been the mother of both the children? Why or why not?

B) Which male, if any, is the father of Child 1?

C) Does Child 2 have the same father? Why or why not?

Solution Preview

A group of 6 bodies is found buried in the woods, three adult males, one adult female, and two children. They are thought to include a missing family (2 parents and 2 children). DNA is extracted from the skeletal remains and analyzed for two STR markers using PCR. In the results below, the numbers indicate the number of tandem repeats in an allele (e.g., at STR 1 Child 1 has one allele with 7 repeats and one with 8).

STR1 STR2
Male 1 8/9 5/7
Male 2 6/8 5/5
Male 3 7/10 7/7
Female 8/8 3/5
Child 1 7/8 5/7
Child 2 8/8 3/7

A) Could the female have been the mother of both the children? Why or why not?

Every individual has two copies of each gene (excluding the genes on the X and Y chromosomes, of course). Therefore, what these numbers represent are the two copies of each gene. If we look at the female, we see that she has one copy of STR1 that contains 8 repeats, and she has another copy of the same gene that also contains 8 repeats. You could say that she's homozygous for the STR1 gene. With respect to ...

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