In field crickets , for example, insects with a single X chromosome develop as male, while those with two develop as female. These animals transport the pollen as they move to other flowers, which also contain female reproductive organs, resulting in pollination. The advantageous alleles A and B, arising independently, can be rapidly combined by sexual reproduction into the most advantageous combination AB. Thus, XX mammals are female and XY are male. Sometimes an organism's development is intermediate between male and female, a condition called intersex. The plants attract these insects or larger animals such as humming birds and bats with nectar-containing flowers. Sex-determination system Sex helps the spread of advantageous traits through recombination.
In birds, which have a ZW sex-determination system , the opposite is true: The majority of butterflies and moths also have a ZW sex-determination system. This is called X0 sex-determination —the 0 indicates the absence of the sex chromosome. The height of the mushroom aids the dispersal of these sexually produced offspring. 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. Flowers are usually hermaphroditic, producing both male and female gametes. As with flowers, seeds form within the female cone after pollination. Within the mushroom diploid cells are formed, later dividing into haploid spores. Other plants have heavier, sticky pollen that is specialized for transportation by insects. 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. In humans, biological sex is determined by five factors present at birth: The biological cause for an organism developing into one sex or the other is called sex determination. When a pollen grain lands upon the stigma on top of a carpel's style, it germinates to produce a pollen tube that grows down through the tissues of the style into the carpel, where it delivers male gamete nuclei to fertilize an ovule that eventually develops into a seed. In field crickets , for example, insects with a single X chromosome develop as male, while those with two develop as female. Asexual reproduction takes longer to achieve this combination, because it can only produce AB if A arises in an individual which already has B, or vice versa. The "default sex," in the absence of a Y chromosome, is female-like. Exceptions are common—for example, the roundworm C. Genetic sex-determination, because it is determined by chromosome assortment, usually results in a 1: In the majority of species with sex specialization, organisms are either male producing only male gametes or female producing only female gametes. In genetic sex-determination systems, an organism's sex is determined by the genome it inherits. Because plants are immobile, they depend upon passive methods for transporting pollen grains to other plants. One or more of these reproductive units may be merged to form a single compound pistil. 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. Thus, XX mammals are female and XY are male. The more familiar female cones are typically more durable, containing ovules within them.
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