Sex is the biological 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 present in many mammals, including humans, some insects (Drosophila), some snakes, some fish (guppies), and some plants.
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.
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 chromosome because the presence of the Y chromosome causes offspring produced in sexual reproduction to be of male sex. 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.
Sexual differentiation is the process of development of the sex differences between males and females from an undifferentiated zygote. Sex determination is often distinct from sex differentiation; sex determination is the designation for the development stage towards either male or female, while sex differentiation is the pathway towards the development of the phenotype.
Sex linked describes the sex-specific reading patterns of inheritance and presentation when a gene mutation (allele) is present on a sex chromosome (allosome) rather than a non-sex chromosome (autosome). In humans, these are termed X-linked recessive, X-linked dominant and Y-linked. The inheritance and presentation of all three differ depending on the sex of both the parent and the child. This makes them characteristically different from autosomal dominance and recessiveness.
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.
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.
The XO sex-determination system is a system that some species of insects, arachnids, and mammals use to determine the sex of offspring. In this system, there is only one sex chromosome, referred to as X. Males only have one X chromosome (XO), while females have two (XX). The letter O signifies the lack of a Y chromosome. Maternal gametes always contain an X chromosome, so the sex of the animals' offspring depends on whether a sex chromosome is present in the male gamete. Its sperm normally contains either one X chromosome or no sex chromosomes at all.
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.
In embryology, the genital ridge is the developmental precursor to the gonads. The genital ridge initially consists mainly of mesenchyme and cells of underlying mesonephric origin. Once oogonia enter this area they attempt to associate with these somatic cells. Development proceeds and the oogonia become fully surrounded by a layer of cells.
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.
Parthenogenesis is a natural form of asexual reproduction in which the embryo develops directly from an egg without need for fertilization. In animals, parthenogenesis means development of an embryo from an unfertilized egg cell. In plants, parthenogenesis is a component process of apomixis. In algae, parthenogenesis can mean the development of an embryo from either an individual sperm or an individual egg.
The western clawed frog is a species of frog in the family Pipidae, also known as tropical clawed frog. It is the only species in the genus Xenopus to have a diploid genome. Its genome has been sequenced, making it a significant model organism for genetics that complements the related species Xenopus laevis, a widely used vertebrate model for developmental biology. X. tropicalis also has a number of advantages over X. laevis in research, such as a much shorter generation time, smaller size, and a larger number of eggs per spawn.
An organism's sex is female if it produces the ovum, the type of gamete that fuses with the male gamete during sexual reproduction.
Sex reversal is a biological process whereby the pathway directed towards the already determined-sex fate is flipped towards the opposite sex, creating a discordance between the primary sex fate and the sex phenotype expressed. The process of sex reversal occurs during embryonic development or before gonad differentiation. In GSD species, sex reversal means that the sexual phenotype is discordant with the genetic/chromosomal sex. In TSD species, sex reversal means that the temperature/conditions that usually trigger the differentiation towards one sexual phenotype are producing the opposite sexual phenotype.
Silene is a flowering plant genus that has evolved a dioecious reproductive system. This is made possible through heteromorphic sex chromosomes expressed as XY. Silene recently evolved sex chromosomes 5-10 million years ago and are widely used by geneticists and biologists to study the mechanisms of sex determination since they are one of only 39 species across 14 families of angiosperm that possess sex-determining genes. Silene are studied because of their ability to produce offspring with a plethora of reproductive systems. The common inference drawn from such studies is that the sex of the offspring is determined by the Y chromosome.
The evolution of sex-determining mechanisms, characterized by the evolutionary transition to genetic sex determination or temperature-dependent sex determination from the opposite mechanism, has frequently and readily occurred among multiple taxa across a transitionary continuum.
