Sex determination

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Reproductive development process
Genetic / Somatic / Male / Female / Primary / Mating Type Gender Determination
Gene Ontology
Schematic representation of genital development in mammals. Important factors: SRY , a gene on the Y chromosome ; MIS (Müllerian inhibiting substance), the anti- Müllerian hormone ; Testosterone .

Under Geschlechtsdetermination those operations which, in the development of somatic sex determine in an organism. In mammals, these processes take place during embryogenesis ; in other organisms, the sex changes in the course of their lives.

Two different mechanisms are known for determining gender.

  • The genetic or chromosomal sex determination, which is based on the genetic differences between the sexes.
  • The modifying sex determination, which depends on external factors (e.g. temperature).

The former leads u. a. in mammals , the latter u. a. in many reptiles to a somatic sex determination.

Known biological genders

Most of the organisms that reproduce their offspring through sexual reproduction are of two biological sexes: their individuals are divided into male and female . In other species there are hermaphrodites that combine male and female characteristics in one individual. Some fish, lizard, and insect species are all female and reproduce through parthenogenesis . In some arthropods , the female sex is forced by infection with bacteria of the genus Wolbachia . In the case of ant populations resulting from hybrids of certain species of ants ( Pogonomyrmex barbatus and P. rugosus ), the fathers of workers and queens are genetically different from one another. Fungi do not have any sexes, but there are different types of mating in the case of the sac and mushroom mushrooms , which can only initiate reproduction if they are compatible with each other. The common split-leaved leaf has over 23,000 mating types, the single-cell Tetrahymena has seven different mating types.

Genetic Determination Systems

Genetic or chromosomal sex is used when the sex determination is based on the type or number of chromosomes present .

Sex determination by sex chromosomes

If females and males have the same number of chromosomes, but at least one of the chromosomes in females and males is different, one speaks of sex chromosomes (gonosomes). For example, in mammals, females have two X chromosomes , so they are homozygous for the sex chromosomes . Males have an X and a Y chromosome and thus different sex chromosomes, they are hemizygous (XX / XY system). The Y chromosome contains the SRY gene , which is important for the development of the male genital tract. If there is no SRY gene, its effect can be fully or partially compensated for by SOX9 and other genes involved. In the absence of both genes, according to the results of a study, the FOXL2 gene becomes active and ensures the development of a female genital tract. A ZW / ZZ system occurs in birds, for example. Here the females are hemizygous (one W and one Z chromosome ) and the males homozygous (ZZ). Sex chromosomes are also found in some groups of reptiles. Most fish and amphibians lack sex chromosomes.

There are also distinguishable sex chromosomes in separate-sex plant species ( dioeciousness , see diocesan ). The evolutionary very young dioecious plant species provide more exceptions are. This includes, from among the angiosperms (angiosperms) z. B. the white light carnation ( Silene latifolia ), the great nettle ( Urtica dioica , with reference to the diocyte in the species name), hemp ( Cannabis sativa ), the papaya ( Carica papaya ) or the ginkgo ( Ginkgo biloba ), as a representative the naked man (gymnosperms).


Haplodiploidy is a form of genetic sex determination in which one sex has only one set of chromosomes ( haploid ) and the other sex has a double set of chromosomes ( diploid ). Usually the male sex is haploid. The most famous examples are bees and ants .

The mite species Brevipalpus phoenicis , a pest of tropical crops, consists only of haploid females that reproduce parthenogenetically . According to one study, these are actually genetic males that are changed into females when infected with bacteria . Feminization through bacterial infection is also known in other arthropods , mostly through Wolbachia .

Modifying Determination Systems

In crocodiles , the sex depends on the temperature of the eggs: up to around 30 ° C, females are formed, from 34 ° C only males are formed. At temperatures in between, crocodiles of both sexes hatch. It is the other way around with turtles . Higher temperatures lead to female offspring, lower temperatures to males.

In addition to temperature, other elements of the habitat can also determine the sex of an organism. The crab Gammarus duebeni, for example, produces males at the beginning of the mating season and later females. The transition is determined by the length of daylight. The great water flea produces male offspring in autumn with a combination of food shortages, increased population density and reduced day length. In the green hedgehog worm , the sex of the larvae depends on whether they meet an adult female or end up on an unoccupied sea floor.

Hermaphrodites and sequential hermaphrodites

Sequential hermaphrodites : male (in the foreground) and female anemonefish

Some species are hermaphrodites and have both male and female sex organs, while others are sequential hermaphrodites. Thus, anemone fish and some worms ( annelids ) than young animals male and female adult animals. Here the gender depends on the size of the individual.

The great majority of plant species are hermaphroditic. The hermaphroditic plants are further subdivided into hermaphrodite, namely those with hermaphrodite flowers, and monoecious ( monoecious ), those with separate male and female flowers on one plant. In contrast, dioecious ( dioecious ) plant species have female and male individuals. There are distinguishable sex chromosomes (see above). It is assumed that separated sex is evolutionarily younger than hermaphroditic plants.

Gonadal sex determination

As sexual differentiation , the assignment is to the female or male sex on the basis of sex characteristic " gonad " (gonad, ovary or testis ), respectively. Because of the hormone production by the gonads, the gonadal sex is therefore also referred to as hormonal or endocrine sex .

The starting point for sex differentiation in mammals is the bipotent gonadal system. In mammals the Y chromosome induces the development of the testes and thus the male sexual development. The gonadal sex determination by the XY system was first independently of one another in 1905 by Dr. Nettie Stevens and Edmund Beecher Wilson described.

Impressive evidence for this thesis comes from experiments with rabbits. Alfred Jost castrated embryonic rabbits in utero at a stage of development in which the differentiation of the internal and external genital organs has not yet started. The removal of the gonads at a specific point in time during embryonic development resulted in the formation of female reproductive organs in both male and female rabbits. These experiments showed that the presence of the testes suppresses the formation of the female reproductive organs while promoting the development of the male phenotype .

Sex differentiation in humans

In humans, gonadal sex is determined by chromosomal or genetic sex . The gonadal sex leads in the undisturbed further embryonic development with the production of the corresponding sex hormones to the development of a male or female phenotype - the gonoductal sex , for the determination of which the internal sex organs are used, and the genital sex, which is defined on the basis of the external sex organs. The development of the internal sexual organs is characterized in the male embryo by the development of the Wolff's ducts and in the female embryo by the Müllerian ducts . If, for example, the testis-determining factor (HDF) is present, the testes develop from the initially indifferent gonadal structures , otherwise the ovaries .

Male development

In male embryos , after the sixth week, the so-called SRY gene is read on the Y chromosome and a protein is formed that is known as the testicular-determining factor (HDF). As a transcription factor, this protein regulates the expression of the DMRT1 gene and numerous other genes in the genome and initiates gender differentiation . Under the influence of HDF, the male embryo undergoes a remodeling of the internal sexual organs (especially the paired testicles , epididymis , vas deferens and the prostate gland ). In the male embryo, the development of the female reproductive organs is also suppressed by the anti- Müllerian hormone (AMH), which is produced in the Sertoli cells of the embryonic testicle.

Furthermore, the HDF stimulates certain somatic cells to develop into testosterone-producing Leydig cells . After the start of testosterone production in these cells around the seventh week, this androgen promotes the further differentiation of Wolff's ducts and the development of the epididymis , the vas deferens and the seminal vesicle is intensified. In addition, with the help of the enzyme steroid 5α-reductase (SRD5), the testosterone in the target cells is converted to the most biologically active form dihydrotestosterone (DHT). Under the influence of this DHT, the protophallus of the genital cusp ( urogenital sinus / genital tubercle ) elongates to a penis ( phallus ), the urogenital groove closes to the pars spongiosa of the urethra and forms the urethral erectile tissue . From the central part of the phallus, the penile cavernous body and possibly the penis bones arise .


The sexual dimorphism of the male brain, for example. in SDN-POA is triggered by estrogen . Since estrogen is largely deactivated prenatally in the bloodstream by proteins in order to prevent a general virilization , its intracellular occurrence can only take place through enzymatic conversion of testosterone by aromatase .

Female development

In the absence of Y-chromosome and thus of the SRY gene as well as the testicular determinative factor (HDF) and the Anti-Mullerian Hormone (AMH) is differentiated in the female embryo under the influence of special female genes such as the FOXL2 gene the initially indifferent, bipotent gonadal anlage to ovaries . The uterus , fallopian tubes and the upper two-thirds of the vagina develop from the Müllerian ducts .

Gender determination and intersexuality in humans

The SRY gene, which is mainly responsible for sex determination in humans, normally remains on the Y chromosome during the maturation of the father's germ cells. In rare cases (frequency approx. 1: 10,000 men) the gene is transferred to the X chromosome through a crossing-over . This creates individuals with a female genotype (XX) and a male phenotype. They have male internal and external genitals, but mostly small testicles and are infertile due to azoospermia . XX men have already been described in whom SRY was not detectable. In these cases, X-linked or autosomal genes take over the function of SRY.

In addition, there are also XY women . This syndrome, first described by the endocrinologist G. Swyer in the 1950s and named after him, is characterized by a lack of testicular development and genital development despite the male genotype. It occurs very rarely (frequency about 1 in 100,000 women). The primary genital organs ( uterus , clitoris , vagina ) are “feminine”, however, since the XY chromosomal, no ovaries are formed, but so-called cord gonads, which can be more or less hormone-active. Up to puberty the development is “female”. During puberty a more or less pronounced virilization occurs , a "feminine" development of the secondary sexual characteristics (breast development, menstruation ) does not occur. In 30% of cases, the genetic cause is a defective or missing SRY gene. Furthermore, various mutations of the 5α-reductase-2 gene cause a disruption of sexual development due to the resulting 5α-reductase-2 deficiency .

The focus of the SRY gene is now controversial. Instead, network models are currently favored in biology in which numerous genetic and environmental factors interact. In gene expression analyzes, around 1000 genes were expressed in the relevant cell groups during the period in question of the sex determination, around 80 of these genes are described in somewhat more detail (with quite contradicting results). An example is the case of a human who has the genotype XY but the phenotype of a woman. A mutation in the CBX2 gene was found here. Another gene that can be involved in so-called autosomal sex reversal is SOX9 ( Kampomele Dysplasia ).

Historical ideas about the genesis of sex

The earliest natural-philosophical explanation of gender origins can be found in Alkmaion , who lived in the 5th century BC. Chr. Assumed that, according to the so-called two-seed doctrine, the quantitative predominance of the “seed” of one sexual partner over that of the other is the cause of the development of a female or male seedling. The author of the between the 2nd and 6th centuries BC The Hippocratic writing About the Seed , which originated in BC , was based on the fact that both women and men have male and female seeds and the sex determination depends on the ratio of the two parts of the seed when the seedling is formed. The idea goes back to Parmenides that the genesis of sex is determined by the placement of the (sex-indifferent) semen in the uterus ( bicornuate uterus ) (according to animal anatomy, thought to be two-chambered) . This would create boys in the right part of the womb and girls in the left part. Empedocles had argued that male (in the humoral-pathological sense, constitutionally warmer) individuals would arise in a warmer uterus and females in a colder one. Corresponding to this theory of the evolution of gender, this idea can also be found in Galenos , which he theoretically underpins with the different blood supply (with blood at different temperatures) to the halves of the uterus.

Linguistic sex determination

If the term determination is viewed as an assignment (see also determination (logic) ), separate terms for "male" or "male animal" and "female" or "female animal" and gender-neutral for the offspring are used for the animal kingdom. For example

  • Bull - cow - calf among cloven-hoofed animals and whales
  • Cock - hen - chicks in birds (such as black cock and black hen)
  • Boar - sow - piglets in pigs
  • Male - heifer - puppy among dogs and wolves

See also


  • Julianne Imperato-Mcginley, Vivian Sobel, Yuan-Shan Zhu: Fetal hormones and sexual differentiation. In: Obstetrics and Gynecology Clinics of North America. Volume 31, No. 4, January 2005, pp. 837-856, DOI: 10.1016 / j.ogc.2004.08.005 ( full text ).

Individual evidence

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