Three independently assorting STR markers (A, B, and C) are used to assess the paternity of a colt recently born to a quarter horse mare. Blood samples are drawn from the mare, her colt, and three possible male sires (S₁, S₂, and S₃). DNA at each marker locus is amplified by PCR, and a DNA electrophoresis gel is run for each marker. Amplified DNA bands are visualized in each gel by ethidium bromide staining. Gel results are shown below for each marker. Calculate the PI and CPI based on these STR markers, using the following population frequencies: A₁₂ = 0.12, A₁₀ = 0.18; B₁₈ = 0.08, B₁₂ = 0.17; C₁₆ = 0.11, C₁₄ = 0.20.

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Hi, everyone. Welcome back. Let's look at our next question. It says the combined paternity index C P I serves as the basis for determining the percentage probability of paternity. Suppose we have the following P values gene 15.34, gene 24.21 G 32.13 gene 43.54. Which of the following gives the correct C P I value choice. A 15.22 B 40. C 312.45 or D 169.51. So in the problem, we're given P I values paternity index values at four different genes. And we need to calculate AC P I value or combined paternity index. So we need to recall what each of these things mean. So the P I values at each individual gene refer to a probability. And it's essentially a measure of how many times more likely is it that the observed phenotype in terms of the, all you see at the alley you see of this particular gene will be seen if a given man is the father. So the man being tested versus a random man selected from the population. And that value is calculated by dividing the probability that the observed phenotype will be seen if the tested man is the father. By the probability, it will be seen if a random man is the father. So each of these is a probability at each gene locus that the particular man being tested is the father. When we look at combined paternity index, we're getting a greater certainty in our test results by looking at multiple gene loci and saying, OK, if all of these gene loci have these particular alleys, what does that mean about the probability that the tested man is the father? And because we're talking about independently assorting jeans, that means that you multiply the probabilities at the different low to obtain that overall probability. Because again, the probability of a given multiple given events occurring if they don't have any dependence on each other, as in this case, where they sort independently is the product of the probabilities of each individual event occurring. So the math here is very simple. All we have to do is multiply together each of the P I values. So the C P I or combined paternity index is the product of all the P I values. So we're going to multiply the result at gene 15.34, multiplied by 4.21 gene two 2.13 and 3.54. So we've multiplied together all of the individual P I values. And that result gives us 169.51 as our C P I value of note here is that any C P I value over 100 tells you that you have a greater than 99% probability that the tested man is the father. So that's why you use that C P I. Because by combining multiple gene los high, you get a very high value of likelihood to rely on in these paternity tests. So our answer will be choice D corresponding to our calculated result of 169.51. See you in the next video.

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Key Concepts

Short Tandem Repeat (STR) Markers

Paternity Index (PI) and Combined Paternity Index (CPI)

Allele Frequency and Probability Calculations in Forensic Genetics