Sex Determination in Human beings

Sex Determination in Human:

In human Male sex  chromosome is 'XY', where as in female sex chromosome is XX. During gamete formation in Male half of the sperm contain 'X' chromosome while other half contain 'Y' Chromosome. In female all gametes contain only one type of chromosome that is 'X' .Thus when a Male hamete i.e.sperm carrying 'X'Chromosome fertilize an over, the zygote develop into female. When a sperm carrying 'Y' chromosome fertilizes an egg, zygote develops into Male.



Sex Determination:


Establishment of male and female  individuals or male and female organs  of an individual  are called sex Determination. It is of three types :

A): ● Environmental 

B): ● Genic

C):● Chromosomes 

The initial clue about the genetic/ chromosomal mechanism of sex determination can be traced  back to some of the experiments  carried out in insects. Honking ( 1891) could trace a specific structure all through spermatogenesis in a few insects, and it was also observed by him that 50 per cent of the sperm received this structure  after spermatogenesis, whereas the other 50 percent sperm did not receive it. Honking gave  a name to this structure  as the X body but  he could not explain its significance. Further investigations  by other scientists  led to the conclusion that the X body of Henking was in fact a chromosome and that it why it was given the name X _ chromosome.

A): Environmental or Nongenetic Determination of Sex:

In some lower animals, sex determination is nongenetic  and depends on factors in the external environment. Males and females  have similar   genotypes, but stimuli from environmental sources initiate development  towards one sex or the other.


1): Marine mollusk Crepidula becomes female  if reared alone. In company of a female, it develops into male.


2): Marine worm Bonellia develops into 3cm long female if its larva settles down in an isolated  place. It grows into much smaller male if it comes closer to an already established female. The male enters the body of the female and lives as a parasite in the reproductive  tract of the female. Extracts made from the female proboscis influence young worms towards maleness.



3): Ophryortocha is male in the young state female later on.


4): In Crocodiles  and some lizards ( e.g., Agama agama) high temperature induces maleness and low temperature femaleness. In turtle, Chrysema picta males are predominant  below 28°C, female above 33°C and equal number  of the two sexes are formed between 28_ 33°C.


B): Nonallosomic Genic Determination of Sex:

In  bacteria, fertility factor which is present in a plasmid determines  the sex. The alga Chlamydomonas possesses sex determining genes. Maize plant possesses  separate genes for development of  tassel ( male inflorescence) and cob ( female inflorescence). 


C): Chromosomal Determination of Sex:

Wilson and Stevens gave the chromosome theory of sex and named the X and Y_ bodies ( observed by other workers) as sex chromosomes, X and Y. Chromosomal or allosomic determination of sex is based on heterogamesis or occurrence of two types of gametes  in one of the two sexes. It is of the following  types: 

1): XX_ XY Type ( Lygeaus Type):

In most insects  including fruitfly Drosophila and mammals  including  human beings the females possess  two homomorphic ( = isomorphic) sex chromosomes,  named XX.The males contain two heteromorphic sex chromosomes, i.e., XY. The Y__ chromosome is usually  shorter and hererochromatic.Inspite of differences  in structure,  the XY chromosomes  are homogeneous  and synapse during zygotene.They actually  possess  two parts, homogeneous  and differential. Homologous regions of the two chromosomes help in pairing. These regions carry same genes and are termed pseudoautosomal as  they are inherited  like autosomal  genes. These homologous regions are present on the tips of short arms of Xp and Yp. The  genes which are present on both X and Y chromosomes are XY linked genes  .They are inherited like autosomal   genes, e.g., genes  causing xeroderma  pigmentosum, epidermolysis bullosa.

The differential region of Y __ chromosome carries only Y__ linked or holandric genes, e.g., Sry ( sex determining region) which codes for a product called TDF ( Testis determining factor). It is perhaps the smallest  gene containing  only 14 base pairs. TDF is required for development   of male sex  and its  absence  leads to development  of female sex. This concept which explains the formation  of male sex in the presence  of TDF and female sex in the absence  of TDF is known as Master  Gene Concept .Other holandric genes are hypertrichosis ( excessive hairiness) on pinna,porcupine skin, keratoderma  disipatum ( thickened skin of hands and feet) and webbed toes. Holandric genes are directly inherited by  a son  from his father. Some non_ functioning genes are also present  on the Y chromosome. These non_ functioning  genes are remnants of  a time in  the evolutionary past when these were probably  active.


 Genes  present on the differential region of X__ chromosome also find  expression in males whether they are dominant  or recessive, e.g.,red green colour  blindness, haemophilia. It is because the males are hemizygous for these gen. Such type of phenomenon where a recessive allele expresses itself in  a  phenotype, when only one copy of the allele  is present, is called pseudodominance.

  Human  beings have 22 pairs of autosomes and one pair of  sex chromosomes. All the ova formed by female are similar in their chromosome type ( 22 +X). They are   thus homogametic. The sperms produced by human males are of two  types (22+X) and ( 22+Y).They are thus heterogametic ( male digamety).


Sex of offspring:

Sex of the  offspring  is determined at the time of fertilization. It is not dependent on the genetic constitution of the female parent because  the latter is homogametic and produces only one types of eggs. The male gametes  are of two types, androsperms ( 22+ Y) and gynosperms ( 22+ X).Fertilization  of the egg with  a gynosperm with produce a female  child ( 44+ XX) while fertilization with an androsperm gives rise to male child ( 44+XY). As the two types of sperms are produced in equal proportions, there are equal chances  of getting  a male or female  child in a particular  mating.As Y_ chromosome determines the male  sex of the individual , it is also called androsome.

Chromatin:

Chromatic is hereditary  DNA protein fibrillar complex which contains DNA, RNA and  proteins. It occurs  in the form of fine overlapping  and coiled  fibres, which produce  network called chromatin  reticulum. Chromatin fibres are distributed  throughout  the nucleoplasm  and are differentiated into two  regions : 

☆ Euchromatin which is a  narrow lightly, stained  and diffused fibrous part forming the bulk  of chromatin 

☆ Heterochromatin which is wider, darkly stained  and condensed  granular  part attached  here and there on the euchromatin.

Generally , portion  of  euchromatin  with acid proteins  takes part in transcription  or formation of RNAs. The chromatin  fibers during prophase of nuclear condense to form a definite number of thread__ like structures called chromosomes.
 

 The staining differences in the two types probably  result from the coiling of chromosome material  during the cell cycle; heterochromatin usually  remains compactly  coiled, whereas  euchromatin undergoes a cycle of condensation and decondensation.It has also been established that genes  in heterochromatin region are inactive, but the earlier belief that no genes  are found in heterochromatic  region is not correct,  because  in several  cases like Drosophila  and Tomato,the genes  in heterochromatic region perhaps become  active for a short period. Y__ chromosome is another example  of heterochromatin having inactive genes. The switching  off of genes  in heterochromatin provides  a mechanism that might  explain the regulation  of genes in cell differentiation.  Heterochromatin shows clumping  during  interphase, called heteropyknosis. Clump, thus formed is called chromocentre or karyosome or false nucleolus.

   Some heterochromatic regions remain  heterochromatic, i.e., condensed throughout  the entire  cell cycle  and are known as constitutive  heterochromatin. Constitutive  heterochromatin mainly surrounds the centromere,  telomeres and satellites  and is rich in satellite or repetitive DNA. Other heterochromatic regions change their staining  behaviour either in different  cell stages or in different  cells. These regions  are known as facultative  heterochromatin. The most important  example  of facultative  heterochromatin is the Barr body or sex chromatin effect which leads to the unique    staining  behaviour of only one of the X__ chromosomes in the cells of mammalian females. 


■ Barry body is found in female somatic cells.

■ Fertilization is done in test tube in laboratory but further development takes place inside mother womb in test tube baby.

■ Sometime sex Determination is regulated by environmental factor .In some reptiles temperature determine the sex at which the fertilized egg is incubated. 

Sex chromatin and Dosage Compensation in Organisms  with Heterochromatic  Males:

The homogametic sex has two X_ chromosomes  and heterogametic sex has only one X and a Y chromosome. In most species  heterochromatic sex ( XY) is male. However,  in certain cases ( as discussed in next section) male is homogametic ( ZZ) and female is heterogametic ( ZW). It is obvious that in the homogametic sex, there will be two X or two Z_ chromosomes  carrying  two sets of identical  genes. On the contrary,  in the heterogametic sex, there will be only one set of these genes. If both the sets of these genes are expressed in the heterogametic sex, there will be twice as much X_ coded gene products in the homogametic sex, as in the heterogametic  sex. This would create an abnormal  situation. Therefore, a mechanism  must exist to bring about equality  in the quantity of X_ coded gene products in the two sexes. This phenomenon of bringing  about equality in products  synthesized under the control  of genes carried on X__ chromosomes,  was termed dosage compensation. This  compensation  in dosage of genes in mammals is achieved by hypoproduction due to inactivation of one X__ chromosome in homogametic sex. In the homogametic  XX female  individual , one X__ chromosome gets condensed and inactivated. Such chromatin  material is described as facultative  heterochromatin since it becomes inactive in certain  part of the life cycle and resumes activity before entering the germ line. The inactivated, heterochromatized X_ chromosome was called Barr body. The above concept was confirmed by the observation that whenever  the number of X_ chromosomes  was two or more than two the number of Barr bodies  was one less than the number  of X_ chromosomes, i.e., one for normal XX; two for XXX and so on. It was also established that in a normal  female  only one active X_ chromosome is found. The number of Barr bodies  in normal  male cells has been found   to be zero. This concept is also referred  to Lyon's hypothesis. 

Which of the two X_ chromosomes become inactive in females is a matter of chance and is determined  the early stages of development. ( However,  some authors say in the embryo, placental  cells show inactivation  of paternal X_ chromosome and this inactivation is random  in the rest of the body). Once an X_ chromosome has become inactivated, all the cells arising  from that cell will keep the same inactive X_ chromosome. In human embryos,  sex chromatin bodies ( Barr bodies) have been observed  by sixteenth day of foetal life. Later ( after the sixteenth  day), only one X__ chromosome is functional in a given body cell. That means some human traits could, therefore,  be influenced  by both X__ chromosomes during the first 16 days.It may result in  mosaic pattern ( i.e., alternate alleles  expressed  in different  body parts) of development  in females  e.g., tortoise shell, female cats with black and  brown patches over white  background. However,  the condensed Barr body, characteristic of somatic cells, is absent from the female   pre meiotic cells. The process of inactivation, therefore , seems to be reversed in the germ cells, so that all female  gametes will carry  an active X_ chromosome.The fate of the X_ chromosome carried in an egg would depend  upon whether  it goes to a male individual  or to a female  individual.In female  individual , they may again have a chance  of inactivation while in male they will have no chance of becoming  inactive.

   The phenomenon of dosage compensation in Drosophila has been shown to be due to hyperactivity  of one X__ chromosome  in male Drosophila ,rather  than due to the inactivation  of X_ chromosome in female.

 Barr body  is found attached to nuclear envelope  in oral mucosa, any where in the nucleus in nerve cells  and as drumstick or small rod at one side of nucleus in neutrophil or polymorphonuclear leucocyte. However,  the occurrence  is  not cent  percent  _ 20_ 50% in cells of oral mucosa, 10% in neutrophil leucocytes,  85% in nervous tissue  and 96% in amniotic and chorionic  epithelium. 

In males  the cells stained with quinacrine mustard  shows fluorescent Y_ chromatin ( Y_ spot) because long arm of the Y_ chromosome gets differentially  stained. The number of Y__ chromatins of Y_ spots is equal to numbe of Y__ chromosomes. It is one in normal  male; zero in normal  female and 2 in super male (XYY).

Drosophila   Karyotypes: Drosophila melanogaster has four pairs of chromosomes. Three pairs of them are autosomes. The fourth pair is of sex chromosomes. The two sex chromosomes of female Drosophila are homomorphic. They are called XX. The female  are thus homogametic. The two sex chromosomes of male fly are heteromorphic .One of them is called X__ chromosome and the other is known as Y__ chromosome. The gametes formed by the male are of two types depending  upon the sex chromosome they possess  ( 3+X or  3+Y). Therefore , the males are heterogametic. The Y_ chromosome has two parts, homologous and differential.The homologous region helps in synapsis and carries   some genes  present  on the X_ chromosome as well. Such genes  which occur  on both X and Y chromosomes are called XY_ linked genes,  e.g., bobbed  bristles.  The genes  present  on the differential region of Y _ chromosome  are called  holandric  genes. They are  passed  directly  from male parent to male offspring .All the  genes  present on the differential region of X__ chromosome find expression  in males irrespective  of their being dominant  or recessive. 

  Sex of Drosophila  is not wholly  dependent  on sex chromosomes. It is determined  on the basis of Genic Balance  Theory  of Sex Determination, which  was put forward   by Bridges.He suggested that every individual  whether male or female,possesses in its genotype, genes for  both male and female characteristics. Which sex will actually  develop, is decided by the preponderance of that type of genes. In Drosophila, the X__ chromosome carry female tendency  genes, while autosomes carry male tendency genes.Therefore, the deciding factor is the ratio between  the number of X_ chromosomes to number of sets of autosomes ( a set of autosomes is represented  by A and  contains 3 chromosomes). According  to this concept, when X/ A ratio is 1.p, the individual  will be female and when it  is 0.5, it would  be male.When this ratio falls between  1.0to 0.5,it would be intersex; when below 0.5, it would be an infertile metamale or supermale and when above 1.0, it would be an infertile and weak metafemale  or superfemale.



 In Drosophila, occasionally  files  are obtained  whic have  female characters in one part of the body and male characters in the remaining  parts.Such individual  are called gynandromorphs and are believed  to result due to loss of an X_ chromosome in a cell during first  mitotic division  of zygote. One of the two cells of two_ celled proembryo will have 24A + XX with X/ A = 1.0 and the other cell will have 2A+ X with X/A= 0.5. The fly derived from such a situation  will have half of its  body as female and the other half as male.


   In Drosophila, the  X_ chromosomes have been shown  to carry a sex_ switch gene,  also known as Sxl ( sex lenthal) .This gene seem to have two states of activity. When it is on, it directs female development; when it is  off,male development  ensues. Other genes located on the X_ chromosome and  the autosomes regulate this sex__ switch gene. Genes one the X_ chromosome that act to regulate Sxl into the on condition  ( female development) are called numerate elements as they act on the numerator of X/A equation. Genes  on the autosomes  that act to regulate Sxl into the off condition  ( male development) are called denominators elements. 


     Comparison  of sex determination in Man and Drosophila: In case of Man, individuals  having Turner's syndrome ( XO) are sterile females  and individuals  having Klinefelters syndrome(XXY) are male despite the presence  of two X_ chromosomes. In contrast to this, a Drosophila fly with 2A+ XXY,  which  is  exactly  like Klinefelter's syndrome of human beings  in chromosome constitution, is a normal female individual  and XO Drosophila fly is male. These findings  also corroborate the evidence  that Y__ chromosome controls sex in human beings  while it plays no such role in Drosophila.
 

2): XX_ XO Type (Protenor Type): 

In roundworm and some insects  ( true bugs, squash bug ( Anasa) grasshoppers, roaches), the females  have two sex chromosomes, XX and the males have only one sex chromosome, X. Therefore, the male are designated as XO. The females are homogametic.The  males are heterogametic with half the male gametes ( gynosperms) carrying X_ chromosome ( A+ X) while the other half ( androsperms) being devoid of it ( A+0). The sex ratio produced in the progeny is 1:1.


3): ZW_ ZZ Type ( = Wz_ WW  type): 

In birds, butterflies and some reptiles both the sexes possess  two sex chromosomes. The females  contain  heteromorphic sex chromosomes ( ZW) while the males have homomorphic  sex chromosomes ( ZZ). Because  of having heteromorphic sex chromosomes, the females are heterochromatic and produce two types of  eggs, ( A+Z) and ( A+W) .The male gametes are of one type ( A+ Z) only. ( The symbols ZZ and ZW in  place of  XX and XY respectively  are used to avoid while XY indicates  maleness).

4): ZO_ ZZ Type:

This type is seen in moths and   some butterflies. It is opposite  to the condition found  in Cockroaches.Here the females  have one sex chromosome ( AA+ Z) while the males have two homomorphic  sex chromosomes ( AA+ZZ). The females  are thus heterogametic.They produce two types of gametes, male forming with one sex chromosome ( A+Z) and female forming without the sex chromosome( A+0). The males are heterogametic,  forming similar  types of sperms ( A+Z).

5): Haplodiploidy ( Hymenopteran Type):

In this type the male is haploid while the female is diploid. It occurs in some insects like bees, ants and wasps. Male insects or drones are haploid as they develop parthenogenetically from unfertilized eggs. It is usually  said that the males of this group do not have a father but they do have a grandfather . Meiosis  does not occur during the formation  of sperms.Females grow from fertilized  eggs and are diploid. Female bees are of two types:

 (i) Workers, which are sexually non_ functional  and are unable to produce ova.

( ii) Queen bees, which  are fully developed functional  females. Queen Bee picks up sperms from the drone during nuptial flight and stores them in her reproductive tract. It does not fertilized the eggs, which are destined to develop into males.Workers and future Queen Bee develop from fertilized eggs. The difference  between  them is because  of the difference in nourishment they receive  ( royal jelly for queen and bee bread for workers).

  There are certain forms where males are altogether absent and females are produced by parthenogenesis. These females are always diploid. 

■ In Human reach cell contains 46 chromosomes. Any addition or removal in the number of sex chromosome or autosomal cause genetic disorder.



1.Klinefelter Syndrome:

When a male have an extra Xor Y chromosome in sex chromosome then the condition will be XXY ro XYY instead of XY. The male individual with this Syndrome have masculine development but feminine development is not completely suppressed and the individual become sterile. 

In female when extra X  chromosome is present instead of XX they show normal development but limited fertility. Mental retardness is also seen in this type of syndrome. Number of chromosome because 47 instead of 46.







2. Turner's Syndrome:

When female has single sex chromosome ( X0) their ovaries are rudimentary, lack of secondary sexual orientation character.



3. Down's Syndrome:

When an extra chromosome is added to 21st autosomal chromosomes lead to develop Down's syndrome. In this Syndrome person became Mangolism. The person is mentally retarded,eyes protruded an irregular physical structure is present. 



4. Patau's Syndrome:

This type of Syndrome is develop by an addition of autosomal chromosome in 13th  chromosome. There is a cut mark in the lip and person is mentally retarded. 



5. Sickle Cell Anaemia:

In this disorder erythrocytes destroyed more rapidly than normal leading to Anaemia. These occur due to change in 11 the autosomal chromosome.




6. Phenylketonuria:

It is an inborn error of metabolism which result in mental retardation cause due to change in  12 the autosomal chromosomes.



7. Haemophilia:

Gene responsible for this disorder is linked with sex chromosomes. This disease lead to failure of blood clothing.  Stems cell and cloning  



8. Color blindness: 


This disorder lead to failure to distinguish red & green coloured.The gene responsible for this disease is situated on sex chromosomes.  genetic cell 




 Hormones and Sex Differentiation:

The sex determination  theories  of chromosomes and genic balance  successful apply to the lower animals but in higher vertebrates,  the embryo may develop some features of the opposite sex together with the features of its  own sex__  chromosome. That means, sex may change under specific  circumstances. This  is due to the hormones secreted by the gonads of that animal.The artificial removal of gonads of either sex before puberty in mammals, results in development of secondary sex characters of the opposite sex. The female  characters  can be  induced in a castrated male by injecting female sex hormones and vice_ versa. Free Martinism is an important  and interesting example  of early effect of hormones on sex differentiation, which is explained  as follows:

  Free martin is a sterile female  with many male characters born to the cattle. This female calf is born as a twin along with a male calf. The male calf is normal . The reason for the formation  of free martins is the effect of hormones of the male foetus on the female foetus. In cattle the foetal membranes of the twins are fused in such a manner that they have a common  circulation of blood.The female sex hormones are produced  at a  slightly  later stage as compared  to the male sex hormones, which are produced  slightly  in advance  to the female  hormones. Because of common circulation  between  the twins, the male hormones enter the body of  the female  foetus. This  event takes place  even before  the female hormones are able to switch  on the development of female characters in the female twin. The male hormones suppress the development  of female  characters in the female twin and make it sterile.


Sex Determination in plants:

Most flowering  plants are monoecious  and therefore, do not have sex cchromosomes.A   number  of dioecious plant species  have been examined  for the presence of sex chromosomes. A detailed  study has been undertaken in Coccinia ( family Cucurbitaceas) and Malendrium ( family Caryophyllaceae). In both these cases Y_ chromosome determines  a  tendency to maleness  just as it does in humans. Thus among these plants, males are heterogametic, while in some species like strawberry, females are heterogametic.In  all these plants having sex chromosomes, two sex  determinating mechanisms have been found. One is  similar  to the system  in mammals in which the Y chromosome has a genes present  that actively determine male flowering  plants. The other system is similar  to that found  in fruit flies, in which the X: A ratio determines sex. In certain  cases like Chlamydomonas,  Neurospora, Yeast, Asparagus and Maize, individual  genes  are found to be responsible for the determination of sex.


As discussed under " Growth and Development  in Flowering  plants" hormones  also affect expression  of sex.







































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