Genetics 2104 Midterm 2.docx

Genetics 2104 Midterm 2.docx - Genetics Midterm 2 Lecture 9...

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Genetics Midterm 2 Lecture 9 – Sex determination and sex chromosomes Different strategies in the natural world for determining sex The one that we are most familiar with is the case of heteromorphic (dissimilar) chromosomes Example: Sex chromosomes X and Y Life cycle depends on sexual differentiation Primary sexual differentiation – involved only gonads where gametes are produced Secondary sexual differentiation – involved overall appearance of organism, including differences in external genitalia. We can distinguish between: Unisexual, dioecious and gonochoric – have only male or female reproductive organs. Bisexual, monoecious and hermaphroditic – have both male and female reproductive organs. Common in plants and animals. Can produce sperm and egg. Many organisms spend the majority of their life cycles as haploid cells, and can reproduce themselves asexually through mitosis. They can transiently adopt a diploid state in order to undergo meiosis to produce gametes. We have seen this with: Yeast and fungi Chlamydomonas (algae) Normally haploid: produces vegetative colonies Under unfavorable nutrient conditions, daughter cells function as gametes. Two gametes fuse together during mating. Gametes not usually morphologically distinguishable (isogametes) Two mating types of Chlamydomonas haploid gametes: o Mt- and mt+ o Mt- cells can mate only with mt+ cells and vice versa. There are chemical differences between these mating types Maize (Zea mays) Plant life cycles alternative between gametophyte and diploid sporophyte stages. o Diploid sporophyte stage predominates o Both males and female structures are present on adult plant o Indicated sex determination occurs differently in different tissues of same plant Lecture 10 – sex determination and sex chromosomes continued C. elegans Major model organisms for development processes – has exactly 959 somatic cells. Nematode work Caenorhabditis elegans has two sexual phenotypes Male have only testes Hermaphrodites have both testes and ovaries. Out of 1090 somatic cells generated during hermaphrodite development, 131 undergo programmed cell death at characteristic times: 959 somatic nuclei, 302 of which are neurons and 95 are body wall muscle cells. The adult male, on the other hand, has 1031 somatic nuclei and 381 of these neurons. The extra neurons in males are mostly dedicated to male mating behavior. Self-fertilization Eggs are fertilized by stored sperm Vast majority produced are hermaphrodites Less than 1% of offspring are males Adults males mate with hermaphrodites. o Produce half male and half hermaphrodite offspring 5 pairs of autosomes and XX self-fertilization: meiosis produces haploid gamete with one of each autosome and 1 X (5X, 1X) o fertilization gives rise to hermaphrodite (2,(5A), XX) rare loss of one X during meiosis produces a gamete with 5 autosomes and 0 X (5A, 0X) o fertilization of 5A, 1X with 5A, 0X produces male (2(5A), X) cross hermaphrodites (5A, X with male (5A, X or 5A, 0X) produces 50% hermaphrodite and 50% male.
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