Basic Genetic Principles Statistics And Probability

We balance probabilities and choose the most likely. It is the scientific use of the imagination.

(Sherlock Holmes, The Hound of the Baskervilles)

You can... never foretell what any one man will do, but you can say with precision what an average number will be up to. Individuals vary, but percentages remain constant. So says the statistician.

(Sherlock Holmes, The Sign of Four)

The genetics section of this book (Chapters 19-23) discusses basic principles that are important when considering forensic DNA profiles. These principles are based on the DNA Advisory Board Recommendations on Statistics (see Appendix V). Example equations are examined and discussed using population allele frequency information from Appendix II. The goal is not to perform an in-depth examination of each genetic and statistical principle but rather to keep things in an understandable format for beginners in the field. Those readers desiring more extensive information on the topics discussed within this book may refer to additional references listed at the end of this chapter.

Statistical genetic information is often more difficult for DNA analysts to grasp than the technology and biology issues addressed earlier in this book because of its heavy use of mathematics particularly algebra. The concepts of probabilities can be challenging to forensic scientists schooled in biology rather than mathematics. In the course of the next few chapters, we endeavor to explain and expand on equations used to calculate random match probabilities.

In this chapter we introduce the basic concepts of probability, statistics, and population genetics. Chapter 20 covers the analysis of DNA population data for allele and locus independence using tests for Hardy-Weinberg equilibrium and linkage equilibrium. Chapter 21 describes calculations for random match probability estimates and introduces the concept of likelihood ratios. Chapter 22 delves into various approaches for interpreting mixtures and results from degraded DNA profiles including probabilities of exclusion. Finally, Chapter 23

concludes the genetics section with the unique challenges of kinship analysis and paternity testing.