{"id":278,"date":"2024-03-23T09:38:26","date_gmt":"2024-03-23T09:38:26","guid":{"rendered":"https:\/\/pressbooks.justwrite.in\/interactive-biology-secondary\/?post_type=chapter&p=278"},"modified":"2024-10-28T18:24:22","modified_gmt":"2024-10-28T18:24:22","slug":"5-4-sex-determination","status":"publish","type":"chapter","link":"https:\/\/pressbooks.justwrite.in\/interactive-biology-secondary\/chapter\/5-4-sex-determination\/","title":{"raw":"5.4 Sex Determination","rendered":"5.4 Sex Determination"},"content":{"raw":"

Sex Chromosomes<\/h1>\r\nIn humans and in many other animal species the sex of the organism is determined by specific chromosomes.\r\n\u2022 The chromosomes that carry those genes related to sexual characteristics are called the sex chromosomes.\r\n\u2022 Experiments conducted by German biologist Hermann Henking in 1891 showed that sex chromosomes were unique from all other\r\nchromosomes.\r\n\r\nHenking was studying sperm formation in wasps. Henking saw that some wasp sperm cells had 12 chromosomes, while others had only 11.\r\n\u2022 While observing the stages of meiosis that formed these sperm cells, Henking noticed that the twelfth chromosome looked different from all the others.\r\n\u2022 He thus named this chromosome the \"X element,\" to represent its unknown nature.\r\n\u2022 Henking hypothesized that this extra chromosome must play some role in sex determination in insects.\r\n\r\nHumans and most other mammals have two sex chromosomes, the X and the Y.\r\n\u2022 Females have two X chromosomes in their cells, while males have both X and a Y chromosomes in their cells.\r\n\u2022 Egg cells all contain an X chromosome, while sperm cells contain an X or Y chromosome\r\n\r\nAnimals with two different sex chromosomes are of heterogametic sex. These are able to produce two types of gametes.\r\n\u2022 While members of the homogametic sex can only produce one type of gamete.\r\n\u2022 In humans (as well as many other animals), males are heterogametic, while females are homogametic .\r\n\u2022 This system is reversed in butterflies and moths (order Lepidoptera): females are the heterogametic sex, while males are\r\nhomogametic\r\n

Sex Determination in Mammals<\/h1>\r\n\u2022 In placental mammals, the presence of a Y chromosome determines sex.\r\n\u2022 Normally, cells from females contain two X chromosomes, and cells from males contain an X and a Y chromosome.\r\n\u2022 Occasionally, individuals are born with sex chromosome aneuploidies, and the sex of\r\nthese individuals is always determined by the absence or presence of a Y chromosome\r\n

The Y Chromosome<\/h1>\r\n\u2022 The Y chromosome of the males is the smallest chromosome .\r\n\u2022 It\u2019s a kind of hemizygous- single chromosome.\r\n\u2022 It is inherited from fathers to their male child.\r\n\u2022 It has very few genes present on its single-arm, in comparison with other chromosomes.\r\n\u2022 The Y chromosome evolved before 300,000 years ago from autosomes.\r\n\u2022 The Y chromosome is the smallest acrocentric chromosome\r\n\u2022 The size of it is 57,227,415bp.=57.2 Mb\r\n\u2022 Genes on Y chromosome: around 200 in which the structure and function of only 50 to 60 genes are known to us.\r\n\u2022 The centromere is located at the position of 10.4Mb.\r\n\u2022 Only a single Y chromosome is present in the male individuals of the mammalian population\r\n\r\nNote:\r\n\u2022 kilo base pairs = 1,000bp\r\n\u2022 1 Mb (Mbp) = mega base pairs = 1,000,000bp\r\n\r\n\"undefined\"\r\n\r\n\"Human male Karyotype after G-banding. Y chromosome highlighted\"<\/a>\u00a0by\u00a0<\/span>National Human Genome Research Institute, via Wikimedia Commons<\/a><\/a><\/a>\u00a0is in the\u00a0<\/span>Public Domain, CC0<\/a>\r\n\r\n \r\n\r\nStructure:\r\n\u2022 Structurally, the Y chromosome is a smaller- acrocentric chromosome having a short p arm and a long q arm.\r\n\u2022 The p arm mostly contains genes and DNA sequences that are necessary for maleness, for example, the TDF and SRY.\r\n\u2022 The short Yp regions are more euchromatin in nature, which is transcriptionally active.\r\n\u2022 Around 95% portion of the Y chromosomes is dedicatedly present for maleness while the rest of the 5% region is known as PAR\r\npseudoautosomal regions. Usually, the PAR sequences are transcriptionally inactive.\r\n\u2022 The centromere between the p and q arm helps in correct chromosomal segregation during the cell division. The centromere is\r\nalso heterochromatin, a transcriptionally inactive region.\r\n\u2022 The approximate size of the Y chromosome is 59 million base pairs with 100 to 200 known genes. However, only 40 to 75 genes\u2019 functions are known to us.\r\n\u2022 Usually, the Y chromosome is almost the same in all other mammals as well.\r\n\u2022 For instance, the Y chromosome of our closest relative chimpanzee is only 30% different in terms of sequences, not size.\r\n

Genes of Y Chromosome<\/h1>\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n
SRY<\/td>\r\nSex determining region on Y<\/td>\r\n<\/tr>\r\n
ZFY<\/td>\r\nZinc finger protein<\/td>\r\n<\/tr>\r\n
PRKY<\/td>\r\n \r\n\r\nProtein kinase, Y linked\r\n\r\n <\/td>\r\n<\/tr>\r\n
TDF<\/td>\r\nTesting determining region<\/td>\r\n<\/tr>\r\n
AMGL<\/td>\r\nAmelogenin<\/td>\r\n<\/tr>\r\n
AZF1<\/td>\r\nAzoospermia factor 1<\/td>\r\n<\/tr>\r\n
AZF2<\/td>\r\n \r\n\r\nAzoospermia factor 2\r\n\r\n <\/td>\r\n<\/tr>\r\n
DAZ<\/td>\r\nDeleted in azoospermia<\/td>\r\n<\/tr>\r\n
UTY<\/td>\r\nUbiquitously transcribed TPR gene on the Y chromosome<\/td>\r\n<\/tr>\r\n
RBM<\/td>\r\nThe RNA-binding motif on Y<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n

\"File:Y<\/h1>\r\n

\"Y Chromosomes\"<\/a>\u00a0by\u00a0<\/span>Christinelmiller<\/a><\/a><\/a>\u00a0is licensed under\u00a0<\/span>CC BY-SA 4.0<\/a><\/p>\r\n\r\n

MSY- Male-specific region of Y:<\/h1>\r\n\u2022 This is the major portion of the Y chromosome\r\n\u2022 It is made up of the euchromatin and heterochromatin regions,\r\n\u2022 These are all collectively known as male-specific regions of Y.\r\n\u2022 This region possesses genes for the traits such as \u2013 male gonadal development, spermatogenesis, and testis development.\r\n\u2022 The euchromatin region has SRY, TDF and AZF like genes\r\n\u2022 While the heterochromatin region contains around 5000 copies of tandemly repetitive sequences called SINES throughout the MSY region.\r\n

SRY gene:<\/h1>\r\n\u2022 The SRY gene is located in the MSY ,on the p arm\r\n\u2022 It is very important for the development of maleness.\r\n\u2022 The SRY gene is the Sex-determining Region on Y,\r\n\u2022 It has a single function exon which encodes a protein of approximately 204 amino acids.\r\n\u2022 Structurally, the SRY gene is located within the specific region on the Y chromosome known as Testis determining region\r\n\u2022 Scientists believe that the SRY gene might have evolved from the SOX gene of the X chromosome.\r\n\u2022 The SRY gene is one of the most important genes in the pathway of male sex determination and differentiation, thus it is known as the \u2018candidate gene\u2019 in the gonadal development.\r\n\r\nThe early embryo has two systems of ducts,\r\n\u2022 Namely the Wolffian and M\u00fcllerian ducts,\r\n\u2022 These are capable of developing into the male and female reproductive tracts, respectively.\r\n\u2022 The SRY gene product stimulates the indifferent gonad to develop into a testis.\r\n\u2022 The testis then begins producing two hormones, testosterone and anti-M\u00fcllerian hormone, or AMH.\r\n\u2022 Testosterone and one of its derivatives, dihydrotestosterone, induce formation of other organs in the male reproductive\r\nsystem.\r\n\u2022 While AMH causes the degeneration of the M\u00fcllerian duct.\r\n\u2022 In females, who do not contain the SRY protein, the ovary forming pathway is activated by a different set of proteins.\r\n\u2022 The fully developed ovary then produces estrogen\r\n\u2022 Estrogen triggers development of the uterus, oviducts, and cervix from the M\u00fcllerian duct.\r\n

PAR:<\/h1>\r\n\u2022 The PAR is known as a pseudoautosomal region\r\n\u2022 It is located on extreme ends of Y chromosomes.\r\n\u2022 The PAR on p arm is around 2.5Mb in size while the PAR on q arm is around 1Mb in size.\r\n\u2022 The PAR has genes homologous to the X chromosomes that take part in the recombination during meiosis cell division.\r\n\u2022 During the meiosis in males the PAR regions of X and Y combine and exchange genetic material.\r\n\u2022 However, the rest of the regions of Y remain conserved\r\n

The AZF gene<\/h1>\r\n\u2022 This gene is known as AZF- azoospermia factor.\r\n\u2022 It is concerned with sperm formation.\r\n\u2022 The AZF locus is comparatively larger in size than the SRY gene\r\n\u2022 It is divided into three regions namely\u2013 AZFa, AZFb and AZFc.\r\n\u2022 Mutation in the regions of AZF leads to failure in spermatogenesis.\r\n\u2022 This condition is commonly known as azoospermia\r\n

ZFY gene<\/h1>\r\n\u2022 This is another candidate gene,which is located on the Y chromosome,\r\n\u2022 It encodes a zinc finger protein.\r\n\u2022 It lies close to the pseudoautosomal boundary on the short arm of the human Y chromosome.\r\n\u2022 In the mouse, Zfy was found to consist of two duplicated genes, Zfy-1 and Zfy-2, which are both present on the normal human Y chromosome.\r\n

Functions of Y chromosome<\/h1>\r\n\u2022 Development of maleness\r\n\u2022 Male gonadal differentiation and development\r\n\u2022 Male sex determination\r\n\u2022 Sperm cell development\r\n\u2022 Testis development\r\n\u2022 Development of other secondary male reproductive phenotypes\r\n

Inheritance pattern of Y chromosome:<\/h1>\r\n