Sex is the trait that determines whether a sexually reproducing organism produces male or female gametes. During sexual reproduction, a male and a female gamete fuse to form a zygote, which develops into an offspring that inherits traits from each parent. By convention, organisms that produce smaller, more mobile gametes are called male, while organisms that produce larger, non-mobile gametes are called female. An organism that produces both types of gamete is hermaphrodite.
The XY sex-determination system is a sex-determination system used to classify many mammals, including humans, some insects (Drosophila), some snakes, some fish (guppies), and some plants. In this system, the sex of an individual is determined by a pair of sex chromosomes. Females have two of the same kind of sex chromosome (XX), and are called the homogametic sex. Males have two different kinds of sex chromosomes (XY), and are called the heterogametic sex.
A sex-determination system is a biological system that determines the development of sexual characteristics in an organism. Most organisms that create their offspring using sexual reproduction have two common sexes and a few less common intersex variations.
Silene is a genus of flowering plants in the family Caryophyllaceae. Containing nearly 900 species, it is the largest genus in the family. Common names include campion and catchfly. Many Silene species are widely distributed, particularly in the northern hemisphere.
The Y chromosome is one of two sex chromosomes in therian mammals and other organisms. Along with the X chromosome, it is part of the XY sex-determination system, in which the Y is the sex-determining because it is the presence or absence of Y chromosome that determines the male or female sex of offspring produced in sexual reproduction. In mammals, the Y chromosome contains the SRY gene, which triggers development of male gonads. The Y chromosome is passed only from male parents to male offspring.
Haldane's rule is an observation about the early stage of speciation, formulated in 1922 by the British evolutionary biologist J. B. S. Haldane, that states that if — in a species hybrid — only one sex is inviable or sterile, that sex is more likely to be the heterogametic sex. The heterogametic sex is the one with two different sex chromosomes; in therian mammals, for example, this is the male.
Dosage compensation is the process by which organisms equalize the expression of genes between members of different biological sexes. Across species, different sexes are often characterized by different types and numbers of sex chromosomes. In order to neutralize the large difference in gene dosage produced by differing numbers of sex chromosomes among the sexes, various evolutionary branches have acquired various methods to equalize gene expression among the sexes. Because sex chromosomes contain different numbers of genes, different species of organisms have developed different mechanisms to cope with this inequality. Replicating the actual gene is impossible; thus organisms instead equalize the expression from each gene. For example, in humans, female (XX) cells randomly silence the transcription of one X chromosome, and transcribe all information from the other, expressed X chromosome. Thus, human females have the same number of expressed X-linked genes per cell as do human males (XY), both sexes having essentially one X chromosome per cell, from which to transcribe and express genes.
Dioecy is a characteristic of certain species that have distinct unisexual individuals, each producing either male or female gametes, either directly or indirectly. Dioecious reproduction is biparental reproduction. Dioecy has costs, since only the female part of the population directly produces offspring. It is one method for excluding self-fertilization and promoting allogamy (outcrossing), and thus tends to reduce the expression of recessive deleterious mutations present in a population. Plants have several other methods of preventing self-fertilization including, for example, dichogamy, herkogamy, and self-incompatibility.
Male is the sex of an organism that produces the gamete known as sperm, which fuses with the larger female gamete, or ovum, in the process of fertilisation. A male organism cannot reproduce sexually without access to at least one ovum from a female, but some organisms can reproduce both sexually and asexually. Most male mammals, including male humans, have a Y chromosome, which codes for the production of larger amounts of testosterone to develop male reproductive organs.
Silene latifolia, commonly known as white campion, is a dioecious flowering plant in the family Caryophyllaceae, native to most of Europe, Western Asia and northern Africa. It is a herbaceous annual, occasionally biennial or a short-lived perennial plant, growing to between 40–80 centimetres tall. It is also known in the US as bladder campion but should not be confused with Silene vulgaris, which is more generally called bladder campion.
The ZW sex-determination system is a chromosomal system that determines the sex of offspring in birds, some fish and crustaceans such as the giant river prawn, some insects, the schistosome family of flatworms, and some reptiles, e.g. majority of snakes, lacertid lizards and monitors, including Komodo dragons. It is also present in some plants, where it has probably evolved independently on several occasions. The letters Z and W are used to distinguish this system from the XY sex-determination system. In the ZW system, females have a pair of dissimilar ZW chromosomes, and males have two similar ZZ chromosomes.
The mechanisms of reproductive isolation are a collection of evolutionary mechanisms, behaviors and physiological processes critical for speciation. They prevent members of different species from producing offspring, or ensure that any offspring are sterile. These barriers maintain the integrity of a species by reducing gene flow between related species.
Cytoplasmic male sterility is total or partial male sterility in hermaphrodite organisms, as the result of specific nuclear and mitochondrial interactions. Male sterility is the failure to produce functional anthers, pollen, or male gametes. Such male sterility in hermaphrodite populations leads to gynodioecious populations.
Sex chromosomes are chromosomes that carry the genes that determine the sex of an individual. The human sex chromosomes are a typical pair of mammal allosomes. They differ from autosomes in form, size, and behavior. Whereas autosomes occur in homologous pairs whose members have the same form in a diploid cell, members of an allosome pair may differ from one another.
Sexual differentiation in humans is the process of development of sex differences in humans. It is defined as the development of phenotypic structures consequent to the action of hormones produced following gonadal determination. Sexual differentiation includes development of different genitalia and the internal genital tracts and body hair plays a role in sex identification.
Androdioecy is a reproductive system characterized by the coexistence of males and hermaphrodites. Androdioecy is rare in comparison with the other major reproductive systems: dioecy, gynodioecy and hermaphroditism. In animals, androdioecy has been considered a stepping stone in the transition from dioecy to hermaphroditism, and vice versa.
A hermaphrodite is a sexually reproducing organism that produces both male and female gametes. Animal species in which individuals are either male or female are gonochoric, which is the opposite of hermaphroditic.
Gynodioecy is a rare breeding system that is found in certain flowering plant species in which female and hermaphroditic plants coexist within a population. Gynodioecy is the evolutionary intermediate between hermaphroditism and dioecy.
The ZO sex-determination system is a system that determines the sex of offspring in several moths. In those species, there is one sex chromosome, Z. Males have two Z chromosomes, whereas females have one Z. Males are ZZ, while females are ZO.
A sexual system is a distribution of male and female function across organisms in a species. The terms reproductive system and mating system have also been used as synonyms.
