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in genetics, regrouping of the maternal and paternal genes during the formation of gametes (sex cells). Recombination occurs randomly in nature as a normal event of meiosis, the process by which gametes are produced. Recombination is enhanced by the phenomenon of crossing over, in which gene sequences called linkage groups are disrupted, resulting in an exchange of segments between paired...
...cycle. In the first stage, each cell has a single set of chromosomes and is called haploid, whereas in the second stage each cell has two sets of chromosomes and is called diploid. When one haploid gamete fuses with another haploid gamete during fertilization, the resulting combination, with two sets of chromosomes, is called a zygote. Either immediately or at some later time, a diploid cell...
In considering the development of reproductive organs, distinctions must be made between: (1) the origin of sex cells (gametes), (2) the origin and differentiation of the sex glands, or gonads (ovaries and testes), and (3) the origin and development of the supporting parts of the reproductive system (e.g., genital ducts, copulatory organs).
After a pollen grain has reached the stigma, it germinates, and a pollen tube protrudes from it. This tube, containing two male gametes (sperms), extends into the ovary and reaches the ovule, discharging its gametes so that they fertilize the egg cell, which becomes an embryo. (Normally many pollen grains fall on a stigma; they all may germinate, but only one pollen tube enters any one ovule.)...
Fungi employ a variety of methods to bring together two compatible haploid nuclei (plasmogamy). Some produce specialized sex cells (gametes) that are released from differentiated sex organs called gametangia. In other fungi two gametangia come in contact, and nuclei pass from the male gametangium into the female, thus assuming the function of gametes. In still other fungi the gametangia...
...from zygote formation via syngamy (fertilization) to spore formation via meiosis. Syngamy and meiosis are successive events in a sexual life history. Syngamy involves the union of two 1n gametes to form a 2n zygote, which eventually develops into a 2n sporophyte. Meiosis involves the division of a 2n sporocyte (meiocyte, spore mother cell, pollen mother cell) to...
...systems in metazoans (multicelled animals) from sponges to mammals, exclusive of humans. It focuses on the gonads (sex organs), associated ducts and glands, and adaptations that aid in the union of gametes—i.e., reproductive cells, male or female, that are capable of producing a new individual by union with a gamete of the opposite sex. Brief mention is made of how the organism...
in reproductive system, animal: Role of gonads in hormone cycles )Gonadal hormones participate in the maturation of gametes still in the gonads by augmenting the metabolic effects of other hormones.
...The two members of a pair are called homologous chromosomes. Each cell of an organism and all individuals of the same species have, as a rule, the same number of chromosomes. The reproductive cells (gametes) are an exception; they have only half as many chromosomes as the body (somatic) cells. But the number, size, and organization of chromosomes varies between species. The parasitic nematode...
Gametes differentiate in parts of the body referred to as gonads, despite the fact that cnidarians cannot be said to have true ovaries and testes because they lack organs. In anthozoans, cubozoans, and scyphozoans, gametes develop in the endoderm, whereas in hydrozoans they ripen in the ectoderm, although they do not necessarily originate there. Sexes are commonly separate, but hermaphroditism...
the fusion of male and female gametes (sex cells) from different individuals of the same species. Cross-fertilization must occur in dioecious plants (those having male and female organs on separate individuals) and in all animal species in which there are separate male and female individuals. Even among hermaphrodites—i.e., those organisms in which the same individual produces both...
...nucleus, of maternal origin, to form the primary nucleus of an embryo. In all organisms the essence of fertilization is, in fact, the fusion of the hereditary material of two different sex cells, or gametes, each of which carries half the number of chromosomes typical of the species. The most primitive form of fertilization, found in micro-organisms and protozoans, consists of an exchange of...
in pregnancy: Initiation of pregnancy )...sex chromosomes. Before fertilization a type of cell division called a reduction division brings the number of chromosomes in the female pronucleus down to 23, including one X-chromosome. The male gamete, or sex cell, also has 44 autosomes and two (X, Y) sex chromosomes. As a result of a reducing division occurring before fertilization, it, too, has 23 chromosomes, including either an X or a Y...
...(body) cells cannot be inherited, but they can cause congenital malformations and cancers (see below Abnormal growth of cells); however, mutations that occur in germ cells—i.e., the gametes, ova and sperm—are transmitted to offspring and are responsible for inherited diseases. Each gamete contributes one set of chromosomes and therefore one copy (allele) of each gene to...
...phenomena: the union of sex cells and their nuclei, with concomitant association of their chromosomes, which contain the genes, and the nuclear division called meiosis. The sex cells are called gametes, and the product of their union is a zygote. All gametes are normally haploid (having a single set of chromosomes) and all zygotes, diploid (having a double set of chromosomes, one set from...
in plant development )a multiphasic process in which two distinct forms succeed each other in alternating generations. One form, created by the union of sexual cells (gametes), contains two sets of similar chromosomes (diploid). At sexual maturity, this form, called the sporophyte, produces an offspring (gametophyte) with cells containing only one set of genetic instructions (haploid). At their sexual maturity,...
...organisms, the number of chromosomes in the body (somatic) cells is diploid (2n; a pair of each chromosome), twice the haploid (1n) number found in the sex cells, or gametes. The haploid number is produced during meiosis (q.v.). During fertilization, two gametes combine to produce a zygote, a single cell with a diploid set of chromosomes. See also...
in reproduction: Binary fission )...may be an unequal cytoplasmic division with an equal division of the chromosomes. This occurs, in fact, in a large number of higher organisms during meiosis—the process by which sex cells (gametes) are formed: originally each chromosome of the cell is in a pair (diploid); during meiosis these diploid pairs of chromosomes are separated so that each sex cell has only one of each pair of...
in reproduction: The evolution of variation control )Because inherited variation is largely handled by genes in the chromosomes, organisms that reproduce sexually require a single-cell stage in their life cycle, during which the haploid gamete of each parent can combine to form the diploid zygote. This is also often true in organisms that reproduce asexually, but in this case the asexual reproductive bodies (e.g., spores) are small and...
...with a long and a short arm. The Y chromosome has one long arm and a very short second arm. This path to maleness or femaleness originates at the moment of meiosis, when a cell divides to produce gametes, or sex cells having half the normal number of chromosomes. During meiosis the male XY sex-chromosome pair separates and passes on an X or a Y to separate gametes; the result is that one-half...
in sex: Sex cells )The term sex is variously employed. In the broad sense it includes everything from the sex cells to sexual behaviour. Primary sex, which is generally all that distinguishes one kind of individual from another in the case of many lower animals, denotes the capacity of the reproductive gland, or gonad, to produce either sperm cells or eggs or both. If only sperm cells are produced, the...
in sex: Hormones )This susceptibility of the reproductive glands, and sexuality in general, to the influence of sex hormones is particularly acute in mammals, where the egg and embryo, unprotected by any shell, develop in the uterus exposed to various chemicals filtering through from the maternal blood stream. A developing embryo eventually produces its own sex hormones, but they are not manufactured in any...
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