XY sex-determination system - AbsoluteAstronomy.com

The XY sex-determination system is the sex-determination system

Sex-determination system

A sex-determination system is a biological system that determines the development of sexual characteristics in an organism. Most sexual organisms have two sexes. In many cases, sex determination is genetic: males and females have different alleles or even different genes that specify their sexual...

found in human

Human

Humans are the only living species in the Homo genus...

s, most other mammal

Mammal

Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

s, some insects (Drosophila
Drosophila
Drosophila is a genus of small flies, belonging to the family Drosophilidae, whose members are often called "fruit flies" or more appropriately pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species to linger around overripe or rotting fruit...

) and some plants (Ginkgo
Ginkgo
Ginkgo , also spelled gingko and known as the Maidenhair Tree, is a unique species of tree with no close living relatives...

). In this system, females have two of the same kind of sex

Sex

In biology, sex is a process of combining and mixing genetic traits, often resulting in the specialization of organisms into a male or female variety . Sexual reproduction involves combining specialized cells to form offspring that inherit traits from both parents...

chromosome

Chromosome

A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...

(XX), and are called the homogametic sex. Males have two distinct sex chromosomes (XY), and are called the heterogametic sex

Heterogametic sex

Heterogametic sex refers to the sex of a species in which the sex chromosomes are not the same. For example, in humans, males, with an X and a Y sex chromosome, would be referred to as the heterogametic sex, and females having two X sex chromosomes would be referred to as the homogametic...

. However, an opposite scheme

ZW sex-determination system

The ZW sex-determination system is a system that determines the sex of offspring in birds, some fish and crustaceans such as the giant river prawn, some insects , and some reptiles, including Komodo dragons...

is found in birds.

The XY sex determination system was first described independently by Nettie Stevens

Nettie Stevens

Nettie Maria Stevens was an early American geneticist. She and Edmund Beecher Wilson were the first researchers to describe the chromosomal basis of sex....

and Edmund Beecher Wilson

Edmund Beecher Wilson

Edmund Beecher Wilson was a pioneering American zoologist and geneticist. He wrote one of the most famous textbooks in the history of modern biology, The Cell.- Career :...

in 1905.

Ancient ideas on sex determination

Since ancient times, people have believed that the sex of an infant is determined by how much heat a man's sperm had during insemination. Aristotle wrote that:

Aristotle claimed that the male principle was the driver behind sex determination, such that if the male principle was insufficiently expressed during reproduction, the fetus

Fetus

A fetus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth.In humans, the fetal stage of prenatal development starts at the beginning of the 11th week in gestational age, which is the 9th week after fertilization.-Etymology and spelling variations:The...

would develop as a female. In contrast, modern genetics has developed a view on sex determination in which no one single factor is responsible for determining sex; a number of pro-male, anti-male and pro-female genes being responsible, though the largest factor is whether the male's gamete carries an x or y chromosome.

Beginnings of genetics of sex

Edmund Beecher Wilson

Edmund Beecher Wilson was a pioneering American zoologist and geneticist. He wrote one of the most famous textbooks in the history of modern biology, The Cell.- Career :...

and Nettie Stevens

Nettie Stevens

Nettie Maria Stevens was an early American geneticist. She and Edmund Beecher Wilson were the first researchers to describe the chromosomal basis of sex....

are credited with discovering, in 1905, the chromosomal XY sex-determination system; the fact that males have XY sex chromosomes and females have XX sex chromosomes.

The first clues to the existence of a factor that determines the development of testis in mammalians came from experiments carried out by Alfred Jost

Alfred Jost

Alfred Jost was a French endocrinologist, famous for his discovery of the Mullerian inhibitor, now called anti-Mullerian hormone or Mullerian inhibiting substance...

, who castrated embryonic rabbits in utero and noticed that they all developed as female. In the wake of Jost's experiments, C. E. Ford and his team discovered that the Y gene was needed for a fetus to develop as male when they examined patients with Turner's syndrome

Turner syndrome

Turner syndrome or Ullrich-Turner syndrome encompasses several conditions in human females, of which monosomy X is most common. It is a chromosomal abnormality in which all or part of one of the sex chromosomes is absent...

, who grew up as phenotypic females, and found them to be X0 (hemizygous for X and no Y). All these observations lead to a consensus that a dominant gene that determines testis development (TDF) must exist on the mammalian chromosome Y.

The search for a testis-determining factor (TDF) lead a team of scientists in 1990 to discover a region of the Y chromosome that is necessary for the male sex determination, which was named SRY

SRY

SRY is a sex-determining gene on the Y chromosome in the therians .This intronless gene encodes a transcription factor that is a member of the SOX gene family of DNA-binding proteins...

(Sex-determining Region of the Y chromosome).

Mechanisms

Some species (including most mammals) have a gene

Gene

A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...

or genes on the Y-chromosome

Y chromosome

The Y chromosome is one of the two sex-determining chromosomes in most mammals, including humans. In mammals, it contains the gene SRY, which triggers testis development if present. The human Y chromosome is composed of about 60 million base pairs...

that determine maleness. In the case of humans, a single gene (SRY
SRY
SRY is a sex-determining gene on the Y chromosome in the therians .This intronless gene encodes a transcription factor that is a member of the SOX gene family of DNA-binding proteins...

) on the Y-chromosome acts as a signal to set the developmental pathway towards maleness. Other mammals use several genes on the Y-chromosome for that same purpose. Not all male-specific genes are located on the Y-chromosome.

Other species (including most Drosophila
Drosophila
Drosophila is a genus of small flies, belonging to the family Drosophilidae, whose members are often called "fruit flies" or more appropriately pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species to linger around overripe or rotting fruit...

species) use the presence of two X chromosome

X chromosome

The X chromosome is one of the two sex-determining chromosomes in many animal species, including mammals and is common in both males and females. It is a part of the XY sex-determination system and X0 sex-determination system...

s to determine femaleness. One X chromosome gives putative maleness. The presence of Y-chromosome genes is required for normal male development.

Humans, as well as some other organisms, can have a chromosomal arrangement that is contrary to their phenotypic sex, that is, XX males or XY females. See, for example, XX male syndrome

XX male syndrome

XX male syndrome is a rare sex chromosomal disorder. Usually it is caused by unequal crossing over between X and Y chromosomes during meiosis in the father, which results in the X chromosome containing the normally-male SRY gene...

and androgen insensitivity syndrome

Androgen insensitivity syndrome

Androgen insensitivity syndrome is a condition that results in the partial or complete inability of the cell to respond to androgens. The unresponsiveness of the cell to the presence of androgenic hormones can impair or prevent the masculinization of male genitalia in the developing fetus, as...

.

Birds have a similar system of sex determination (ZW sex-determination system

ZW sex-determination system

), in which it is the females that are heterogametic (ZW), while males are homogametic (ZZ).

Recent studies on the genetic factors that influence gender traits

For a long time, biologists believed that the female form was the default template for the mammalian fetuses of both sexes. After the discovery of the testis-determining gene SRY, many scientists shifted to the theory that the genetic mechanism that determines a fetus to develop into a male form was initiated by the SRY gene, which was thought to be responsible for the production of testosterone

Testosterone

Testosterone is a steroid hormone from the androgen group and is found in mammals, reptiles, birds, and other vertebrates. In mammals, testosterone is primarily secreted in the testes of males and the ovaries of females, although small amounts are also secreted by the adrenal glands...

and its overall effects on body and brain development. This perspective still shared the classical way of thinking; that in order to produce two sexes, nature has developed a default female pathway and an active pathway by which male genes would initiate the process of determining a male sex, as something that is developed in addition to and based on the default female form. This view is no longer considered accurate by most scientists who study the genetics of sex. In an interview for the Rediscovering Biology website, researcher Eric Vilain described how the paradigm changed since the discovery of the SRY gene:
In mammals, including humans, the SRY gene is responsible with triggering the development of non-differentiated gonads into testes, rather than ovaries. However, there are cases in which testes can develop in the absence of an SRY gene (see sex reversal). In these cases, the SOX9

SOX9

Transcription factor SOX-9 is a protein that in humans is encoded by the SOX9 gene.- Function :SOX-9 recognizes the sequence CCTTGAG along with other members of the HMG-box class DNA-binding proteins...

gene, involved in the development of testes, can induce their development without the aid of SRY. In the absence of SRY and SOX9, no testes can develop and the path is clear for the development of ovaries. Even so, the absence of the SRY gene or the silencing of the SOX9 gene are not enough to trigger sexual differentiation of a fetus in the female direction. A recent finding indicates that ovary development and maintenance is an active process, regulated by the expression of a "pro-female" gene, FOXL2. In an interview for the TimesOnline edition, study co-author Robin Lovell-Badge explained the significance of the discovery:

Implications for human health and social policy

Looking into the genetic determinants of human sex can have wide-ranging consequences. Scientists have been studying different sex determination systems in fruit flies and animal models to attempt an understanding of how the genetics of sexual differentiation can influence biological processes like reproduction, ageing and disease. Since many of the same genetic mechanisms involved in determining sexually dimorphic traits have been preserved during evolution to this day in fruitflies, mice, and humans, understanding how these genetic mechanisms work can lead to improved healthcare that takes into account sex differences. The research could also lead to changes in how people understand and perceive sex differences.

External links

Sex Determination and Differentiation
SRY: Sex determination from the National Center for Biotechnology Information
National Center for Biotechnology Information
The National Center for Biotechnology Information is part of the United States National Library of Medicine , a branch of the National Institutes of Health. The NCBI is located in Bethesda, Maryland and was founded in 1988 through legislation sponsored by Senator Claude Pepper...
Sex Chromosomes: What Are They For?