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Haplodiploidy Males are haploid (1N) and develop by parthenogenesis. Females are diploid (2N). Ex. Honey bee (order Hymenoptera) Workers and queens are 2N, whereas drones are 1N. Parthenogenesis: When unfertilized eggs may become activated and develop into viable individuals as part of the normal life cycle. • Parthenogenesis may occur in a number of invertebrate groups (i.e. insects) as well as in scattered vertebrate species (some fishes, a few lizards, and even turkeys). • Ex. Beltsville small white turkeys Parthenogenesis: Development of an ovum into a new individual without fertilization. • In turkey eggs, parthenogenetic development can be initiated spontaneously. Early embryonic development in parthenogens is often delayed. Only 20 % of the parthenogens that complete embryonic development survive to produce offspring. In parthenogenetically developing blastoderms, approx. 40.3 % haploid and 48.9 % diploid cells are found. The haploid cell complement of parthenogens declines to 1.5 % at 29 d of incubation, 1.4 % at 4 wk posthatch, and 1.3 % in adulthood. The semen volume of male parthenogens is reduced. However, their germ cells are capable of normal meiosis and sperm maturation leading to normal fertility. The mechanism through which diploidy is obtained has not yet been elucidated, however, the diploid number could be the result of the union of the haploid egg nucleus and the second polar body or the lack of meiosis II. Sex of Parthenogens in Birds and Mammals Mammal Bird XY 1st polar body Y XX XX X 2nd polar ovum X X body recombine XX Combine (artificially induced) X 2º oocyte X 2º oocyte XX (embryo dead by day 10 of gestation) Protenor sp. X and Y chromosomes are found in most insects, all mammals (i.e. humans), in many fish and amphibia, and in a few plants. Male Female Chromosome constitutions (karyotypes) of males and females in Drosophila melanogaster. Calvin Bridges model for sex determination in Drosophila melanogaster • “A” = a set of 3 autosomes • Therefore, a normal fertile female having 8 total chromosomes is designated as 2A2X, and a normal fertile male having 8 total chromosomes is designated as 2AXY. P AAXX A AXX x AAXY AX AY Result of nondisjunction in Anaphase I (AAXXY) 2:2 XA Normal Female (AAXXX) 3:2 XA Meta Female (AAXO) 1:2 XA Sterile Male (AAOY) 0:2 XA Dead Calvin Bridges model for sex determination in Drosophila melanogaster X > 1 = metafemale A X = 1 = fertile female A 0.5 < X < 1 = intersex A X = 0.5 = male; + Y = fertile male A 0 < X < 0.5 = metamale A