Sex among microbes sexual reproduction allows members

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Sex among Microbes Sexual reproduction allows members of the same species to create new combinations of genes by genetic recombination. But not all organisms reproduce sexu- ally. Bacteria, for example, reproduce asexually by a process called binary fission . In binary fis- sion,each bacterium makes a complete copy of all its genes,then splits,apportioning a complete copy of the genetic blueprint to each of the two new cells.Yet bacteria, too, exhibit genetic va- riety. Are there mechanisms by which organisms like bacteria that reproduce asexually can share genes? Mutations Are Another Source of Genetic Variety Occasionally, when large numbers of plants or animals are grown domestically, an indi- vidual is born with an entirely new characteristic never before seen in that group or any of its ancestors.This phenomenon,which occurs in natural populations as well as domestic ones, was well known to Darwin. He referred to these rare individuals as “sports of nature”; we call them mutants .A long pedigree of red roses, for example, bred for many years to produce only crimson blooms,may suddenly produce a pink rose.Or within a herd of sheep, bred for many years with long legs, a single, short-legged lamb may appear. Darwin was at a loss to explain such phenomena, although he recognized the role these “sports”have in providing variety upon which natural selection can act.(Indeed,knowing nothing of genetic recombination,Darwin wondered if mutations alone could provide all of the necessary variety for natural selection.) Mendel’s laws do not explain the origin of these anomalies, although new features can often be propagated by careful breeding. Once present in a breeding population, a new trait obeys all of Mendel’s laws as if the allele that causes it had been present all along.What, in genetic terms, is a mutation, and how do mutants fit into our Mendelian view of inheritance? A mutation is the sudden appearance of a new allele.Although mutations can occur at any time, they become heritable mutations when genes are copied and partitioned into gametes during sexual reproduction.When males and females make sperm and eggs Exploration
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3-2 Why Aren’t Members of the Same Species Identical? 81 (the gametes that carry one allele for every gene into the next generation), they make copies of every single allele to be passed on. This process of gene replication has been studied extensively in the past several decades and is well understood, even at the level of the molecules involved;we’ll focus more on that in Chapter 6.For now,suffice it to say that the copying process is pretty good, but not perfect. Occasionally a mistake slips in, and a new allele emerges as a result. Many times, errors in copying are fatal. Imagine yourself as a maker of fine watches. Your timepieces are assembled from carefully designed parts that fit together to work harmoniously to keep time, much as the genes of a living thing work together to build a finely tuned organism. Each com- ponent of your watches is made by a different machine. Suddenly, a machine that makes
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