Evolution of sexual reproduction Anisogamy Isogamy Germ cell Meiosis Gametogenesis Spermatogenesis Oogenesis Gamete spermatozoon ovum Fertilization External fertilization Internal fertilization Sexual selection Plant reproduction Fungal reproduction Sexual reproduction in animals Sexual intercourse Copulation Human reproduction Lordosis behavior Pelvic thrust.
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He described his discovery of the spermatic animalcules in a letter to the Royal Society in London in Nettie Stevens and Edmund Beecher Wilson are credited with independently discovering, inthe chromosomal XY sex-determination system, i.
Philosophers, naturalists and, more recently, scientists have debated this question for more than 3, years. It would be of great interest to compare the morphology and activity of mitochondria in these sex-reversed mouse gonads with those in normal ones. Sex determination was discovered in the mealworm by the American geneticist Nettie Stevens in This system is observed in a number of insects, including the grasshoppers and crickets of order Orthoptera and in cockroaches order Blattodea.
It should perhaps be admitted that during the years of intensive research into genes and genomes, the cytoplasm has been unduly neglected. The XY system is also found in some insects and plants.
The details of some sex-determination systems are not yet fully understood. It looks almost as if the ancient Pythagoreans knew something that today's scientists still need to discover. Birds have a system that works the other way round: It is called ZW sex-determination system.
The X chromosomes of A. Even though such organisms may be called hermaphrodites, this is not correct, because in intersex individuals either the male or the female aspect is sterile.
Testosterone formation and metabolism during male sexual differentiation in human embryo. Over a long period sex determination, and specifically male sex determination, has been correlated to the presence of the Y chromosome, which in turn has been the karyotype signal of the testes.
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