Introduction to Genetic Analysis 485

Introduction to Genetic Analysis 485 - 44200_15_p481-520...

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484 Chapter 15 Large-Scale Chromosomal Changes MONOPLOIDS Male bees, wasps, and ants are mono- ploid. In the normal life cycles of these insects, males de- velop by parthenogenesis (the development of a special- ized type of unfertilized egg into an embryo without the need for fertilization). In most other species, however, monoploid zygotes fail to develop. The reason is that vir- tually all individuals in a diploid species carry a number of deleterious recessive mutations, together called a “genetic load.” The deleterious recessive alleles are masked by wild-type alleles in the diploid condition, but are automatically expressed in a monoploid derived from a diploid. Monoploids that do develop to advanced stages are abnormal. If they survive to adulthood, their germ cells cannot proceed through meiosis normally be- cause the chromosomes have no pairing partners. Thus, monoploids are characteristically sterile. (Male bees, wasps, and ants bypass meiosis; in these groups, gametes
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