DETERMINATION Sex refers to the contrasting features of male and individuals of the same species. two types male and female. Sex determination is a process of sex differentiation which utilizes various genetical concepts to decide whether a particular individual will develop into male or female. Plants also have sex as there are male and female parts in flowers. The may reside in different individuals or within the same individual. An animal possessing both male and female reproductive organs is usually referred to as . In plants where staminate and pistillate flowers occur in the same plant, the term of preference is monoecious Eg. maize, castor, coconut etc. However, most of the flowering plants have both male and female parts within the same flower (perfect flower). Relatively few angiosperms are dioecious i.e. having male and female elements in different individuals Eg: cucumber, papaya, asparagus, date palm, hemp and spinach. The sex cells and reproductive organs form the primary sexual characters of male and female sexes. Besides these primary sexual characters, the male and female sexes differ from each other in many somatic characters known as secondary sexual characters. The importance of sex itself is that it is a mechanism, which provides for the great amount of genetic variability characterizing most natural populations. The mechanisms of sex determination

1. Chromosomal sex determination 2. Genic balance mechanism 3. Male haploidy or mechanism 4. Single gene effects (or) monofactorial mechanism of sex determination 5. Metabolically controlled mechanism 6. Hormonally controlled mechanism 7. Sex determination due to environmental factors I. Chromosomal sex determination:

The chromosomes, which have no relation with sex and contain genes, which determine the somatic characters of an individual are known as autosomes. These chromosomes do not differ in morphology and number in male and female sex.

Those chromosomes, which differ in morphology and number in male and female sex and contain genes responsible for the determination of sex are known as allosomes or sex chromosomes. Differences between Autosomes and Allosomes or Sex chromosomes

. Refer to other than sex . These are sex chromosomes. chromosomes . Morphology is similar in . Morphology is different in male and female sex. male and female sex. . The number is same in . The number is sometimes both the sexes. different in male and female . Generally control traits sex. other than sex . Usually determine sex of an . Number of autosomes individual. differs from species to . Each diploid organism species. usually has two allosomes. . Do not exhibit . . Exhibit sex linkage. The chromosomal influence on sex, in certain insects, has been shown for the first time by McClung in 1902 to be associated with a special sex determining ‘X’ chromosome

McClung proposed that a male had one ‘X’ chromosome per cell (XO) and a female has two ‘X’ chromosomes (XX).

Later Stevens and Wilson(1905) found same number of chromosomes in both sexes of milk weed bug. In all chromosomes were paired and the homologues were equal in size (homomorphic). In the male, all the chromosomes were paired, but the chromosome identified as homologous to the “ X” Chromosome was distinctly smaller and was called the “Y” Chromosome (Heteromorphic). Thus, allosomes are generally of X and Y types, but in birds they are of Z and W types. Sex with similar type of sex chromosomes (XX or ZZ) is known as homogametic sex and with dissimilar type of sex chromosomes (XY or ZW) as heterogametic sex. These are two types: a) Heterogametic male and b) Heterogametic female a) Heterogametic male: In this mechanism, the female sex has two ‘X’ chromosomes, while the male sex has only a single ‘X’ chromosome. As the male lacks a ‘X’ chromosome during , 50% of lthe carry ‘X’chromosome, while the rest do not have the ‘X’ chromosome. Such a mechanism,which produces two different types of gametes in terms of is called heterogametic sex. The female sex here is called homogametic sex because it produces similar type of gametes. The heterogametic male may be i) XX – XO ii) XX – XY b) Heterogametic female: In this mechanism the male possess two homomorphic sex chromosomes and are thus homogametic, while the female possesses either a single ‘X’ chromosome or one ‘X’ and one ‘Y’ chromosome and are hence heterogametic.

To avoid confusion with earlier types, instead of X and Y, the alphabets Z and W are used. This mechanism of sex determination is also known as “Abraxas mechanism of sex determination” (Kuspira and Walker, 1973) The heterogametic females may be ZO – ZZ ZW – ZZ ZO – ZZ: This mechanism is found in certain moths and butterflies. In this case,female possess one single ‘Z’ chromosome and hence is heterogametic. Male possesses two Z chromosomes and thus homogametic. ZW – ZZ: This system is found in certain insects (gypsy moth) and vertebrates such as fishes, reptiles and birds. In this system, the female is heterogametic and produces two types of gametes, one with ’Z’ chromosome and the other with ‘W’ chromosome. male is homogametic and produces all sperms of same type carrying one ‘Z’ chromosome. The sex of the offspring depends on the kind of egg being fertilized. The ‘Z’ chromosome bearing eggs produce males, but the ‘W’ chromosome bearing eggs produce females. II) Genic balance mechanism:

By studying the sex chromosomal mechanism of sex determination, it may appear at first glance that some genes carried by sex chromosomes i.e. X and Y are entirely responsible for determining sex. Extensive experiments on different organisms by different workers have revealed the fact that most organisms generally have inherent potentialities for both sexes and each individual is found to be more or less intermediate between male and female. Hence may be referred to as inter sex. There seems to exist a delicate balance of masculine and feminine tendency in the hereditary compliment of an individual. Such a genic balance mechanism of determination of sex was first observed and studied by C.B. Bridges in 1921 while working with Drosophila for the inheritance of vermillion eye colour. According to this mechanism, the sex of an individual in is determined by a balance between the genes for femaleness located in the X-chromosome and those for maleness located in autosomes. Hence, the sex of an individual is determined by the ratio of number of its X chromosomes and that of its autosomal sets, the ‘Y’ chromosome being unimportant. The ratio is termed as sex index and is expressed as follows.

Sex index = X/ A Different doses of X – Chromosomes and autosome sets and their effect on sex determination Individuals with sex index of 0.5 develop into normal males 1 into normal females. If the sex index is between 0.5 and 1, the resulting individuals will be neither a female nor a male, but have an intermediate sex expression and is called inter sex. Such individuals are sterile. Some flies have sex index of >1, such flies have more pronounced female characteristics than normal females and are called super females or metafemales. These are generally weak, sterile and non- viable. Super male flies have a sex index value of <0.5 and are also weak, sterile and non-viable. Bridges drew the observation by crossing triploid females (3A + XXX) with normal diploid males (2A + XY). From such a cross he obtained normal diploid females, males, triploid females, , super males and super females. The occurrence of triploid intersexes from such a cross clearly established that autosomes also carry genes for sex determination. Triploid individuals, which had two ‘X’ Chromosomes as in the case of normal female, here were inter sexes as they had an extra set of autosomes indicating that the autosomes play a definite role in the determination of sex.

III. Single gene effect or monofactorial mechanism of sex determination:

In Neurospora, Asparagas, Drosophila , maize and Asparagus, sex determination is influenced by the differential action of a single autosomal gene, which over rules the effect of sex chromosomes present in the individual. Autosomal recessive transformer (tra) gene of Drosophila; when it is present in the homozygous recessive state, it transforms the female (XX) zygote to develop into males which are sterile. The gene tra is recessive and hence does not have any effect in heterozygous condition (Tra/tra ) on either sex i.e male or female. SEX DETERMINATION IN HUMANS The discovery that human females are XX and that human males are XY suggested that sex might be determined by the number of X chromosomes or by the presence or absence of a . As we now know, the second hypothesis is correct. In humans and other placental mammals, maleness is due to a dominant effect of the Y chromosome . The evidence for this fact comes from the study of individuals with an abnormal number of sex chromosomes. XO animals develop as females, and XXY animals develop as males. The dominant effect of the Y chromosome is manifested early in development, when it directs the primordial gonads to develop into testes. Once the testes have formed, they secrete testosterone, a hormone that stimulates the development of male secondary sexual characteristics. The process of sex determinationin humans. Male sexual development depends on the production of the testis determining factor (TDF) by a gene on the Y chromosome. In the absence of this factor, the embryo develops as a female Researchers have shown that the testis-determining factor (TDF) is the product of a gene called SRY (for sex-determining region Y), which is located in the short arm of the Y chromosome. The discovery of SRY was made possible by the identification of unusual individuals whose sex was inconsistent with their chromosome constitution—XX males and XY females( Figure ). Some of the XX males were found to carry a small piece of the Y chromosome inserted into one of the X chromosomes. This piece evidently carried a gene responsible for maleness. Some of the XY females were found to carry an incomplete Y chromosome. The part of the Y chromosome that was missing corresponded to the piece that was present in the XX males; its absence in the XY females apparently prevented them from developing testes. These complementary lines of evidence showed that a particular segment of the Y chromosome was needed for male development. Molecular analyses subsequently identified the SRY gene in this male-determining segment. Additional research has shown that an SRY gene is present on the Y chromosome of the mouse, and that— like the human SRY gene—it triggers male development.

After the testes have formed, testosterone secretion initiates the development of male sexual characteristics. Testosterone is a hormone that binds to receptors in many kinds of cells. Once bound, the hormone–receptor complex transmits a signal to the nucleus, instructing the cell in how to differentiate. The concerted differentiation of many types of cells leads to the development of distinctly male characteristics such as heavy musculature, beard, and deep voice. If the testosterone signaling system fails, these characteristics do not appear and the individual develops as a female. One reason for failure is an inability to make the testosterone receptor complex. - Testicular , XY individuals with this biochemical deficiency initially develop as males—testes are formed and testosterone is produced. However, the testosterone has no effect because it cannot transmit the developmental signal inside its target cells. Individuals lacking the testosterone receptor therefore acquire female sexual characteristics. They do not, however, develop ovaries and are therefore sterile. This syndrome, called testicular feminization, results from a mutation in an X-linked gene, Tfm, which encodes the testosterone receptor. The tfm mutation is transmitted from mothers to their hemizygous XY offspring (who are phenotypically female) in a typical X-linked pattern.

V. Metabolically controlled mechanism:

Riddle, found that metabolism has some definite role in the determination of sex in pigeon and dove because increased rate of metabolism lead to the development of maleness while decreased rate of metabolism caused femaleness In pigeons.

Increased metabolism - Males

Decreased metabolism - Females VI. Hormonally controlled mechanism VIII. Sex determination due to environmental Factors:

In many reptiles, sex is determined by environmental factors like temperature. In most turtle species only females are produced at high temperature (30 – 35ºC) while only males are produced at low temperatures (23 – 28ºC). Sex ratio changes suddenly from all males to all females with just change in temperature of 2ºC during the incubation. In Crocodiles and Lizards, the males are produced at high temperature while females are produced at low temperature. In some plants, sex determination depends on day length, temperature and hormones. For example, in cucumber (Cucumis sativa) and muskmelon, treatment with ethylene enhances production of female flowers. STRUCTURE OF X-CHROMOSOME

• The X & Y chromosome exhibit structural differences.

Cytological study show that

1. X-chromosome of most organism are straight rod like & comparatively longer than Y- chromosome. 2. X have large euchromatin and small amount of heterochromatin. Euchromatin have large amount of DNA & hence much genetic information. Y- chromosome contain small euchromatin & large heterochromatin hence it has little genetic information, sometimes referred as genetically inert or inactive.

Functions of the Y chromosome

. In mammals, the Y chromosome contains the SRY gene which is key to the development of the testes in males. Without this gene, the testes would not develop and the fetus would become a female.

. The Y chromosome spans approximately 58 million base pairs, contains 86 genes, and represents around 2% of the total DNA in a human male.

. Traits that are passed from father to son on the Y chromosome are referred to as holandric traits, meaning they only occur in males.

. Aside from very small regions present at the telomeres, the Y chromosome is unable to recombine with an . This majority portion of the Y chromosome is referred to as the non-combining region of theY chromosome. Genes on the Y chromosome Genes present on the Y chromosome that correspond to a similar gene on the X chromosome include: AMELY/AMELX (amelogenin) RPS4Y1/RPS4Y2/RPS4X (ribosomal protein S4)

Genes that are exclusive to the Y chromosome include: AZF1 (azoospermia factor 1) BPY2 (basic protein on the Y chromosome) DAZ1 (deleted in azoospermia 1) DAZ2 (deleted in azoospermia 2) PRKY (Y-linked protein kinase) RBMY1A1 (RNA Binding Motif Protein, Y-linked, family 1, member A1) SRY (sex-determining region Y) TSPY (testis-specific protein, Y-linked) USP9Y (ubiquitin specific peptidase 9, Y-linked) UTY (ubiquitously transcribed tetratricopeptide repeat containing, TPR gene on Y-linked) ZFY (zinc finger protein, Y-linked)

Y-linked disease or disorders Several conditions that are specifically linked to the Y chromosome and only transmitted from father to son include: Defective or deformed Y chromosome that leads to features of feminization and infertility Dosage compensation

. A mechanism by which species with sex chromosomes ensure that the homogametic sex does not have too much or the heterogametic sex too little activity of loci on the homogametic sex chromosome.

. Dosage compensation is the process, where the total gene expression from single x in one sex is made equal to total gene expression from the two x chromosomes in the other sex. This can be achieved either by up regulating the single x chromosomes in one sex or down regulating the two x chromosomes in the other or by a combination of these mechanisms. Failure to achieve this compensation often lethal. . For many years, geneticists have observed that in some case, female homozygous for the genes in the X-chromosomes do not express a trait more markedly than do hemizygous males. So, it must be a mechanism of “dosage compensation”, through which the effective dosage of genes of the two sexes is made equal or nearly so. . This mechanism of compensating the differential doses of functional sex chromosomes in male and female human is effected by the inactivation of one X-chromosome in the normal female. The genetically inactive X- chromosome or condensed X-chromosome is called hetero-pychnotic X- chromosome or heterochromatin or sex-chromatin body or Barr body (according to the name of the geneticist M. L. Barr who first observed it) or Drum-stick (according to the shape of the inactive X-chromosome). . In humans, inactive form of X-chromosome as a Barr-body have been observed by the sixteenth day of gestation. X-chromosome inactivation occurs in human when two or more X-chromosomes are present. Diagrammatic representation of the results that led H.J. Muller to formulate the hypothesis of dosage compensation.

The mutant allele of the X-linked white gene (wa) is a hypomorph and allows partial eye-pigment synthesis; its presence on the X chromosomes is indicated. The level of pigmentation is directly proportional to the dosage of the wa allele within each sex; yet, males with one dose and females with two doses have comparable amounts of pigment because of dosage compensation.

Barr Body

. The Barr body, also sometimes called the sex chromatin, is the inactive X chromosome in female somatic cells.

. Human females have two X chromosomes, while males have one X and oneY.

. In all of the female somatic cells, which don’t take part in , one of the X chromosomes is active, and the other is inactivated in a process called lyonization, becoming the Barr body.

. The reason for shutting off one X chromosome is so that only the necessary amount of genetic information is expressed, rather than double or even more. This is why X-inactivation doesn’t only occur in humans, but in all organisms whose gender is determined by the presence or absence of a Y or W chromosome in the cell. In short, the amount of X chromosome genes expressed has to be equal in both males and females. . The Barr body is packaged in heterochromatin, while the active X chromosome is packaged in euchromatin. This means that although both X chromosomes have the same gene content, the inactive X chromosome is condensed and not easily accessible to molecules involved in transcription, while the active X chromosome has a larger volume, and is more dispersed, or open, allowing for transcription to take place. Left: DAPI stained female human fibroblast with Barr body (arrow). Right: histone macroH2A1 staining. Arrow points to sex chromatin in DAPI-stained cell nucleus, and to the corresponding sex chromatin site in the histone macroH2A1-staining.