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This mode of reproduction is called asexual, and it is still used by many species, particularly unicellular, but it is also very common in multicellular organisms.In sexual reproduction, the genetic material of the offspring comes from two different individuals.As sexual reproduction developed by way of a long process of evolution, intermediates exist.

Typically, prior to an asexual division, a cell duplicates its genetic information content, and then divides. In sexual reproduction, there are special kinds of cells that divide without prior duplication of its genetic material, in a process named meiosis.Some of the many plausible theories include: that sex creates variation among offspring, sex helps in the spread of advantageous traits, that sex helps in the removal of disadvantageous traits, and that sex facilitates repair of germ-line DNA.Sexual reproduction is a process specific to eukaryotes, organisms whose cells contain a nucleus and mitochondria.XY sex determination is used by most mammals, and from a comparison between chicken and human, the Z chromosome appeared similar to the autosomal chromosome 9 in human, rather than X or Y, suggesting that the ZW and XY sex-determination systems do not share an origin, but that the sex chromosomes are derived from autosomal chromosomes of the common ancestor of birds and mammals.A paper from 2004 compared the chicken Z chromosome with platypus X chromosomes and suggested that the two systems are related.

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