# Genetics questions – Essaylink

We identified your fingerprint at one locus, D1S80, with 25 known alleles. Within the human genome, many short tandem repeat (STR) regions have been identified. Tens of thousands of STR regions exist within exons and introns of various genes. About half of these are highly polymorphic, that is, the number of STRs is highly variable. STRs consist of a three to seven base pair motif that is repeated a variable number of times in a tandem array.

Consider also the following three STRs, used most frequently by crime labs, and answer the following questions about your experiment and about crime lab DNA fingerprinting. The first one (CSF1PO) is located on chromosome arm 5q and the number of repeats varies between 6 and 15 (i.e. there are ten known alleles). These alleles lie within a proto-oncogene. A second STR (TPOX) lies within the thyroid peroxidase gene on chromosome arm 2p and the repeat number varies between 6 and 12 repeats (i.e. there are seven known alleles). Finally, a third STR site (THO1) is located on 11p and the number of repeats varies between 5 and 11 (but also includes a 9.3 repeat allele caused by a truncated repeat, for 8 known alleles).

How many different allelic combinations (i.e. genotypes) could exist in the human population for the D1S80 locus? (Consider the number of different homozygous and heterozygous genotypes that could occur with the 25 known alleles.) Show your calculations here and on the remaining problems!

How many different allelic combinations could exist in the human populations for the

CS1PO locus?

TPOX locus?

THO1 locus?

If each combination occurred with equal frequency, what is the probability that a randomly-chosen person would carry the same genotype as you at

CS1PO locus?

TPOX locus?

THO1 locus?

For DNA fingerprinting, crime labs identify a person’s DNA fingerprint at each of the 3 loci: CSF1PO, TPOX and THO1. If each combination occurred with equal frequency, what is the probability that a randomly-chosen person would carry the same genotype as you at all three loci?

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