Life has evolved from simple stages to more complex ones, and so have the reproduction mechanisms. The penis contains a tube through which semen a fluid containing sperm travels. Sexual reproduction is a process specific to eukaryotes , organisms whose cells contain a nucleus and mitochondria. XY sex determination is used by most mammals,  but also some insects,  and plants Silene latifolia. Environmental sex determination For many species, sex is not determined by inherited traits, but instead by environmental factors experienced during development or later in life. In birds, which have a ZW sex-determination system , the opposite is true: 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, including many of those with sexual reproduction. In most birds, both excretion and reproduction is done through a single posterior opening, called the cloaca —male and female birds touch cloaca to transfer sperm, a process called "cloacal kissing".
Genetic sex-determination usually depends on asymmetrically inherited sex chromosomes which carry genetic features that influence development ; sex may be determined either by the presence of a sex chromosome or by how many the organism has. In the majority of species with sex specialization, organisms are either male producing only male gametes or female producing only female gametes. Typically, prior to an asexual division, a cell duplicates its genetic information content, and then divides. This sex-determination system results in highly biased sex ratios , as the sex of offspring is determined by fertilization rather than the assortment of chromosomes during meiosis. Male cones are smaller and produce pollen which is transported by wind to land in female cones. Many fish change sex over the course of their lifespan, a phenomenon called sequential hermaphroditism. One or more of these reproductive units may be merged to form a single compound pistil. In pines and other conifers the sex organs are conifer cones and have male and female forms. Conversely, there are also species in which there is no sexual specialization, and the same individuals both contain masculine and feminine reproductive organs, and they are called hermaphrodites. The female gametes of seed plants are contained within ovules ; once fertilized by pollen these form seeds which, like eggs, contain the nutrients necessary for the development of the embryonic plant. One proposed explanation for this is the handicap principle. Environmental sex determination For many species, sex is not determined by inherited traits, but instead by environmental factors experienced during development or later in life. This process of cell division is called mitosis. In female mammals the vagina connects with the uterus , an organ which directly supports the development of a fertilized embryo within a process called gestation. Different forms of isogamy: The most basic sexual system is one in which all organisms are hermaphrodites , producing both male and female gametes—[ citation needed ] this is true of some animals e. Genetic Like humans and other mammals, the common fruit fly has an XY sex-determination system. Within the mushroom diploid cells are formed, later dividing into haploid spores. Female left and male right cones are the sex organs of pines and other conifers. Yeast with the same mating type will not fuse with each other to form diploid cells, only with yeast carrying the other mating type. In addition to animals, plants, and fungi, other eukaryotes e. In clownfish , smaller fish are male, and the dominant and largest fish in a group becomes female. Sexual dimorphisms in animals are often associated with sexual selection — the competition between individuals of one sex to mate with the opposite sex. In some ferns the default sex is hermaphrodite, but ferns which grow in soil that has previously supported hermaphrodites are influenced by residual hormones to instead develop as male. The biological cause for an organism developing into one sex or the other is called sex determination.
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