ZOL141 1st Edition Lecture 8 Outline of Last Lecture II meiosis A Meiosis 1 B Meiosis 2 Outline of Current Lecture III Meiosis New Gene Combinations A Random assortment B Crossing over I Formation of Gametes a Males b Females Current Lecture Meiosis New Gene Combinations Two mechanisms produce new combinations Random Assortment o Chromosome pairs can line up differently in metaphase 1 of meiosis o If 2n 6 8 different gametes o Maternal and paternal chromosomes align independently o For 3 chromosome pairs 8 possible gametes 2 3 possibilities 2n Crossing Over o Alleles what is it One variant of a gene Sometimes a different allele can result in a different phenotype Sister chromatids are genetically identical so they will always be the same Homologous chromosomes can differ Gene can have many alleles but each individual has only two alleles One from mom one from dad o Chromosome arms non sister chromatids physically exchange genetic material o Combination of alleles is changed o Increases genetic variation in humans o Chromosome combos of offspring theoretically possible 8x10 23 Formation of Gametes Males spermatogenesis begins at puberty 64 days o One spermatocyte generates four spermatozoa o Spermatogonia in the testes divide by mitosis to produce spermatocytes o Spermatocytes undergo meiosis to form spermatids haploid o Spermatids undergo structural changes to become functional sperm o 2n 2n n n n o seminiferous tubule where it takes place Females oogenesis begins during embryogenesis 12 50 years o Oogenia divide by mitosis to form primary oocytes o Primary oocytes undergo meiosis to form secondary oocytes haploid o Secondary oocyte produced by the first meiotic division o Unequal division of cytoplasm One larger functional gamete ovum two or three smaller polar bodies o Ovum the haploid cell produced by meiosis that becomes the functional gamete egg
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