Population Genetics

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In 2009, Sean Hodgson was released from a British prison after serving 27 years behind bars for the murder of Teresa De Simone, a clerk and part-time barmaid. Hodgson, who suffers from mental illness, initially confessed to the crime but withdrew his confession during the trial. Throughout his years in prison, he maintained his innocence. More than two decades after the crime, the courts analyzed DNA of the assailant found at the crime scene and determined that it did not come from Mr. Hodgson. His conviction was overturned, and the police have now reopened the investigation of Ms. De Simone’s murder. As you will learn in this chapter, the DNA-based analysis used to exonerate Mr. Hodgson and hundreds of other wrongly convicted prisoners was dependent on population genetic analysis.

The principles of population genetics are at the heart of many questions facing society today. What are the risks that a couple will have a child with a genetic disease? Have the practices of plant and animal breeding caused a loss of genetic diversity on the farm, and does this loss of diversity place our food supply at risk? As the human population continues to expand and wildlife retreats into smaller and smaller parts of the earth, will wildlife species be able to avoid inbreeding and survive? The principles of population genetics are also fundamental to understanding many historical and evolutionary questions. How are human populations from different regions of the world related to one another? How has the human genome responded as humans have spread out across the globe and become adapted to different environments and lifestyles? How do populations and species evolve over time?

A population is a group of individuals of the same species. Population genetics analyzes the amount and distribution of genetic variation in populations and the forces that control this variation. It has its roots in the early 1900s, when geneticists began to study how Mendel’s laws could be extended to understand genetic variation within whole populations of organisms. While Mendel’s laws explain how genes are passed from parent to offspring in the cases of controlled crosses and known pedigrees, these laws are insufficient to understand the transmission of genes from one generation to the next in natural populations, in which not all individuals produce offspring and not all offspring survive. Geneticists began developing the principles of population genetics in the early 1900s, but at the time, they had rather limited tools to actually measure genetic variation. With the development of DNA-based technologies over the past three decades, geneticists now have the ability to observe directly differences between the DNA sequences of individuals throughout their genomes, and they can measure these differences in large samples of individuals in many species. The result has been a revolution in our understanding of genetic variation in populations.

In this chapter, we will consider the concept of the gene pool and how geneticists estimate allele and genotype frequencies in populations. Next, we will examine the impact that mating systems have on the frequencies of genotypes in a population. We will also discuss how geneticists measure variation using DNA-based technologies. We will then discuss the forces that modulate the levels of genetic variation within populations. Finally, we will look at some case studies involving the application of population genetics to questions of interest to society.