huelsenbeck7-8notes - Lecture 7 Friday, March 13, 2009...

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Lecture 7 Friday, March 13, 2009 Recombination Diploid organisms : The first step in sexual reproduction is the production of gametes, each of which has half the chromosomes of the parent. Chromosomes assort independent into gametes (Mendel’s second law). Crossing over occurs within chromosomes, creating chromosomes that differ from chromosomes in the parents. Fertilization creates individuals that have genotypes different from either of their parents. Siblings differ in genotype from one another. Bacteria and viruses: Although sexual reproduction occurs only in eukaryotes, bacteria and viruses have other mechanisms that result in new combinations of alleles. Sexual reproduction is an evolved trait There is great variety in modes of sexual reproduction Isogamy vs. aniogamy: isogamous species produce gametes of the same size and form; anisogamous species produce gametes that differ in size and form. Some fungi and algae are isogamous. All other sexually reproducing species are anisogamous. The sex that produces the larger gamete is defined to be the female. Hermaphroditic individuals vs. separate males and females : most plants are hermaphroditic; most animals have separate sexes. Some hermaphrodites, for example peas, can self-fertilize; some, for example slugs, cannot. Genetic vs. environmental sex determination : in humans, as in many other species with separate sexes, sex is determined by genotype. In mammals, XX individuals are female; XY individuals are male. In birds, males are ZZ and females are ZW. Other genetic mechanisms are found in other groups. For example, in bees, ants and wasps, males are haploid and females are diploid. In some species of plants and animals, sex is determined by the environment. For example, in most species of turtles and in all crocodilians, the sex of an individual depends on the temperature of the egg during a critical period of development. Usually males are produced when eggs are incubated at low temperatures and females are produced when eggs are incubated at higher temperatures. Capable or incapable of asexual reproduction : Some plants and animals can produce new genetically identical individuals by budding or fission. Some plants can produce seeds asexually (apomixis). Some animals can produce diploid eggs asexually (parthenogenesis). Both apomixis and parthenogenesis result in offspring that are genetically identical to their mother. Some species never engage in sexual reproduction. For example, several species of whiptail lizards, including the endangered California species, Cnemidophorus hyperythrus , are comprised of females that reproduce only by parthenogenesis. Many species, including aphids, can reproduce both sexually and asexually
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This note was uploaded on 05/13/2009 for the course BIO 1B taught by Professor Carlson,mischel,power during the Spring '07 term at University of California, Berkeley.

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huelsenbeck7-8notes - Lecture 7 Friday, March 13, 2009...

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