Mammalian sex determination, development 1 Flashcards
vegetative reproduction
- used by bacteria, plants, some invertebrates
- budding (yeast), fission (bacteria), fragmentation
parthenogenesis
reproduction using eggs but without sex (eggs never fertilized by a male)
advantages/disadvantages of parthenogenesis
- advantages: pass genes on intact (genetic immortality), rapidly populate new environment
- disadvantages: not good long-term
list 2 theories about why sexual reproduction persists
- muller’s ratchet
- the red queen
muller’s ratchet
- theory for why sexual reproduction persists
- most genetic mutations are deleterious –> accumulate
- populations will have rapidly decreasing fitness
- “mutational meltdown of clones”
- sexual reproduction allows genetic mixing and repair
the red queen hypothesis
- theory for why sexual reproduction persists
- biological enemies like bacteria and parasites constantly evolve and reproduce much faster –> genetic mutations become prevalent quickly
- our defense is to be genetically variable, more so with each generation
why are 3+ sexes not seen?
- can exist
- unstable and usually revert to 1 or 2 sex system
types/levels of sex (8)
- genetic (chromosomes)
- gonadal (ovary or testis)
- germ cell (egg or sperm)
- hormonal (estrogen or androgens)
- phenotypic (genetalia, secondary sex characteristics)
- somatic (brain)
- behavioral
- legal
3 steps in mammalian sex determination
- genetic (chromsomal) sex determined at time of fertilization (X or Y sperm)
- sex determination occurs when bipotential/indifferent gonad is put on male or female pathway (Y chromosome present or not)
- sexual differentiation of male/female phenotype from hormone secretions of ovary or testis
what method of genetic sex do mammals utilize
dominant Y, single pair method –> can’t use environmental cues or estrogens due to development of viviparity and a placenta (presence or absence of Y determines sex)
when is genetic sex determined in mammals
at the time of fertilization and depends on whether and X or Y bearing sperm fertilizes the oocyte
what is the testis-determining factor a product of
the gene SRY (sex-determining region Y chromosome) –> tip of Y chromosome
SRY gene info
- single copy gene that encodes SRY protein
- transported to nucleus where it binds DNA to induce a bend
- expressed in urogenital ridge prior to gonadal differentiation
- expression is independent of the presence of primordial germ cells
other functions of Y chromosome
- transgenic XX mice that have SRY insert develop as males with penis/testis but are sterile due to lack of spermatogenesis
- increase in height
- blood pressure and cholesterol
- “genetic junk”
3 sets of kidneys in embryonic development
- pronephros
- mesonephros
- metanephros
pronephros
- degenerates
- duct persists as the mesonephric duct
mesonephros
- contains tubules induced to form by the residual mesonephric duct with which they communicate
- degenerates except for some of the caudal tubules and the mesonephric duct
metanephros
develops into kidney and ureter
where do gonads develop rom
urogenital ridge –> a strip of mesenchyme adjacent to the ventromedial surface of the mesonephric kidney
how are cellular cords of the indifferent gonad formed
mesenchyme (urogenital ridge) is invaded by coelomic mesothelial (epithelial) cells, which along with cells from degenerating mesonephric tubules, form the cellular cords of the indifferent gonad
what are transcription factors acting on
somatic cells –> don’t need germ cells to form the indifferent gonad