In This Chapter

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Chapter Summary

  1. At almost the same time that Charles Darwin was publishing On the Origin of Species, Augustinian monk Gregor Mendel was breeding and scoring tens of thousands of pea plants. This work gave birth to the field of genetics, including transmission genetics.
  2. Mendel’s laws are (a) the law of segregation, which states that each individual has two gene copies at each locus and these gene copies segregate during gamete production, so that only one gene copy goes into each gamete, and (b) the law of independent assortment, which states that which of the two gene copies is passed down to the next generation at one locus is independent of which gene copy is passed down to the next generation at the other loci. The second law holds true only for unlinked loci.
  3. Mendel’s work provided empirical evidence that traits from the two parents were not irreversibly blended in the offspring. He demonstrated that the heritable "factors" were particulate.
  4. For most of the last 4 billion years, nucleic acids have been the chemical underpinning of life on Earth. Changes in DNA, as well as the ways these changes are expressed, create the variation on which evolutionary processes act.
  5. DNA is packed into chromosomes. Diploid organisms have two copies of each chromosome. Organisms with a single copy of each chromosome are known as haploids. In cells of eukaryotic organisms, most chromosomes are threadlike structures composed of tightly coiled DNA that is wrapped around proteins called histones.
  6. Proteins are long strings of amino acids that are essential building blocks of life and serve many different functions within cells. Some proteins act as enzymes that initiate and regulate chemical reactions. Other proteins are chemical signals that are used in communication within and between cells.
  7. Some proteins bind to DNA and help to regulate when and how DNA is expressed; others serve structural functions, forming the cytoskeleton or elements of the extracellular matrix.
  8. Most amino acids can be encoded by more than one nucleotide triplet (codon). Proteins are constructed using 20 different amino acids. The correspondence between codons and amino acids is known as the genetic code.
  9. A gene is a sequence of DNA that specifies a functional product. In eukaryotes, protein-coding genes are composed of exons and introns.
  10. Regulatory elements control the rate at which RNA molecules are transcribed from the DNA. This process is known as transcriptional regulation. Regulatory elements that increase the rate of transcription are called enhancers, while those that decrease the rate of transcription are known as silencers.
  11. Genetic variation enters a population from one of four sources: recombination, mutation, migration, or lateral gene transfer.
  12. Mutation rates can be measured in many ways and differ across species and even across different tissue types in the same species.
  13. With respect to changes in relative fitness, mutations can be beneficial, deleterious, or neutral, but they are always undirected. Estimating the relative frequency of beneficial, deleterious, and neutral mutations is an active area of research within the science of evolutionary biology.