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B Sc-II ClassNotes # 23, , Suresh DVNS, , 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. Occasionally there are hermaphrodites in place of one or both, sexes. There are also some species that are only one sex due to parthenogenesis, the act of a female reproducing, without fertilization. In many cases, sex determination is genetic: males and females have different alleles or even, different genes that specify their sexual morphology. In animals, this is often accompanied by chromosomal, differences. Determination genetically is generally through chromosome combinations of XY, ZW, XO, or, haplodiploid. Sexual differentiation is generally started by a main gene, a sex locus, then a multitude of other genes, follow in a domino effect. In other cases, sex is determined by environmental variables (such as temperature) or, social variables (the size of an organism relative to other members of its population). Environmental sex, determination occurred before genetic; it is thought that a temperature-dependent reptile was the common, ancestor to sex chromosomes. Some species do not have a set sex, and instead change it based on certain cues. The, details of some sex-determination systems are not yet fully understood., Origin of sex chromosomes, The ends of the XY chromosomes, highlighted here in green, are all that's left of the original, autosomes that can still cross-over with each other., The accepted hypothesis of XY and ZW sex chromosome evolution is that they evolved at the, same time, in two different branches. However, there is some evidence to suggest that there, could have been transitions between ZW and XY, such as in Xiphophorus maculatus, which, have both ZW and XY systems in the same population, despite the fact that ZW and XY have different gene locations., The platypus' genes also back up the possible evolutionary link between XY and ZW, because they have the DMRT1, gene possessed by birds on their X chromosomes. Regardless, XY and ZW follow a similar route. All sex chromosomes, started out as an original autosome of an original reptile that relied upon temperature to determine the sex of, offspring. After the mammals separated, the branch further split into Lepidosauria and Archosauromorpha. These, two groups both evolved the ZW system separately, as evidenced by the existence of different sex chromosomal, locations. In mammals, one of the autosome pair, now Y, mutated its SOX3 gene into the SRY gene, causing that, chromosome to designate sex. After this mutation, the SRY-containing chromosome inverted and was no longer, completely homologous with its partner. The regions of the X and Y chromosomes that are still homologous to one, another are known as the pseudoautosomal region. Once it inverted, Y chromosome continued to receive both, positive and negative mutations throughout the years, and without a partner to combine with, it slowly decayed., However, because both the chromosomes were necessary for reproduction, the Y chromosome remained, despite, its inability to pair with the X. This is becoming a problem, due to the fact that the Y chromosome is steadily, shrinking and losing its genes. Without the ability to swap genes with its pair, it is racking up with mutations and may, stop functioning in 10 million years., There are some species, such as the medaka fish that evolved sex chromosomes separately, whose Y chromosome, never inverted and can still swap genes with the X. These species are still in an early phase of evolution of their sex, chromosomes. Because the Y does not have male-specific genes and can interact with the X, XY and YY females can, be formed as well as XX males., Chromosomal determination, XX/XY sex chromosomes, Drosophila sex-chromosomes
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B Sc-II ClassNotes # 23, , Suresh DVNS, , The XX/XY sex-determination system is the most familiar, as it is found in humans. In the system, females have two, of the same kind of sex chromosome (XX), while males have two distinct sex chromosomes (XY). The XY sex, chromosomes are different in shape and size from each other, unlike the autosomes, and are termed allosomes., Some species (including humans) have a gene SRY on the Y chromosome that determines maleness; others (such as, the fruit fly) use the presence of two X chromosomes to determine femaleness. Because the fruit fly, as well as other, species, use the number of Xs to determine sex, they are nonviable with an extra X. SRY-reliant species can have, conditions such as XXY and still live. Human sex is determined by containing SRY or not. Once SRY is activated, cells, create testosterone and anti-müllerian hormone to turn the genderless sex organs into male. With females, their, cells excrete estrogen, driving the body down the female pathway. Not all organisms remain gender indifferent for a, time after they're created; for example, fruit flies differentiate into specific sexes as soon as the egg is fertilized. In Ycentered sex determination, the SRY gene is not the only gene to have an influence on sex. Despite the fact that SRY, seems to be the main gene in determining male characteristics, it requires the action of multiple genes to develop, testes. In XY mice, lack of the gene DAX1 on the X chromosome results in sterility, but in humans it causes adrenal, hypoplasia congenita. However, when an extra DAX1 gene is placed on the X, the result is a female, despite the, existence of SRY. Also, even when there are normal sex chromosomes in XX females, duplication or expression of, SOX9 causes testes to develop.Gradual sex reversal in developed mice can also occur when the gene FOXL2 is, removed from females. Even though the gene DMRT1 is used by birds as their sex locus, species who have XY, chromosomes also rely upon DMRT1, contained on chromosome 9, for sexual differentiation at some point in their, formation., The XX/XY system is also found in most other mammals, as well as some insects. Some fish also have variants of this,, as well as the regular system. For example, while it has an XY format, the Xiphophorus variatus also has a second Y, chromosome, known as Y', that creates XY' females and YY' males. At least one monotreme, the platypus, presents a, particular sex determination scheme that in some ways resembles that of the ZW sex chromosomes of birds, and, also lacks the SRY gene, whereas some rodents, such as several Arvicolinae (voles and lemmings), are also noted for, their unusual sex determination systems. The platypus has ten sex chromosomes; males have an XYXYXYXYXY, pattern while females have ten X chromosomes. Although it is an XY system, the platypus' sex chromosomes share, no homologues with eutherian sex chromosomes. Instead, homologues with eutherian sex chromosomes lie on the, platypus chromosome 6, which means that the eutherian sex chromosomes were autosomes at the time that the, monotremes diverged from the therian mammals (marsupials and eutherian mammals). However, homologues to, the avian DMRT1 gene on platypus sex chromosomes X3 and X5 suggest that it is possible the sex-determining gene, for the platypus is the same one that is involved in bird sex-determination. More research must be conducted in, order to determine the exact sex determining gene of the platypus., , Heredity of sex chromosomes in XO sex determination, XX/X0 sex determination, In this variant of the XY system, females have two copies of the sex chromosome (XX), but males have only one (X0). The 0 denotes the absence of a second sex chromosome. Generally in this method, the, sex is determined by amount of genes expressed across the two chromosomes. This system is observed in a number, of insects, including the grasshoppers and crickets of order Orthoptera and in cockroaches (order Blattodea). A small, number of mammals also lack a Y chromosome. These include the Amami spiny rat (Tokudaia osimensis) and the, Tokunoshima spiny rat (Tokudaia tokunoshimensis). Voles also have a form of XO determination in which both, genders lack a second sex chromosome. The mechanism of sex determination is not yet understood., The nematode C. elegans is male with one sex chromosome (X0); with a pair of chromosomes (XX) it is a, hermaphrodite. Its main sex gene is xol, which controls the expression of the genes tra-2 and her-1. These genes, reduce male gene activation and increase it, respectively.
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B Sc-II ClassNotes # 23, , Suresh DVNS, , ZW sex chromosomes, The ZW sex-determination system is found in birds, reptiles, some insects and other organisms. The ZW sexdetermination system is reversed compared to the XY system: females have two different kinds of chromosomes, (ZW), and males have two of the same kind of chromosomes (ZZ). In the chicken, this was found to be dependent on, the expression of DMRT1. In birds, the genes FET1 and ASW are found on the W chromosome for females, similar to, how the Y chromosome contains SRY. However, not all species depend upon the W for their gender. For example,, there are moths and butterflies that are ZW, but some have been found female with ZO, as well as female with ZZW., Also, while mammals inactivate one of their extra X chromosomes when female, it appears that in the case of, Lepidoptera, the males produce double the normal amount of enzymes, due to having two Z's. Because the use of, ZW sex determination is varied, it is still unknown how exactly most species determine their sex. Despite the, similarities between ZW and XY, the sex chromosomes do not line up correctly and evolved separately. In the case of, the chicken, their Z chromosome is more similar to humans' autosome 9. The chicken's Z chromosome also seems to, be related to the X chromosomes of the platypus. When a ZW species, such as the Komodo Dragon, reproduce, parthenogenetically, males are usually only produced. This is due to the fact that the haploid eggs double their, chromosomes, resulting in ZZ or WW. The ZZ become males, but the WW are not viable and are not brought to term., Haplodiploid sex chromosomes, Haplodiploidy, Haplodiploidy is found in insects belonging to Hymenoptera, such as ants and bees., Unfertilized eggs develop into haploid individuals, which are the males. Diploid, individuals are generally female but may be sterile males. Males cannot have sons or, fathers. If a queen bee mates with one drone, her daughters share ¾ of their genes, with each other, not ½ as in the XY and ZW systems. This is believed to be significant, for the development of eusociality, as it increases the significance of kin selection, but, it is debated. Most females in the Hymenoptera order can decide the sex of their offspring by holding received, sperm in their spermatheca and either releasing it into their oviduct or not. This allows them to create more, workers, depending on the status of the colony., Sex determination and differentiation (human), , The Human Y Chromosome showing the SRY gene which codes for a protein regulating, sexual differentiation., Human sex refers to the processes by which an individual becomes either a male or, female during development., The Jost Paradigm, Under typical circumstances, the sex of an individual will be determined and expressed through the following, mechanisms:, , , , , Chromosomal Sex (genetic): Presence or absence of Y chromosome, Gonadal Sex (Primary Sex Determination): Controlled by presence or absence of testis determining factor, (TDF), Phenotypic Sex (Secondary Sex Differentiation): Determined by the hormonal products produced by the, gonads.
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B Sc-II ClassNotes # 23, , Suresh DVNS, , Sex differentiation, Sex differentiation refers to the expression of phenotypic attributes specific to the sex of an individual. While gonad, development is a result of the presence or absence of the sex determination gene SRY on the Y chromosome, sex, differentiation is determined by the hormonal products produced by the gonads., Testosterone, In the 1930s, Alfred Jost determined that the presence of testosterone was required for Wolffian duct development, in the male rabbit., Müllerian inhibiting substance, Jost also observed that while testosterone was required for Wolffian duct development, the regression of the, Müllerian duct was due to another substance. This was later determined to be Müllerian inhibiting substance (MIS),, a 140 kD dimeric glycoprotein that is produced by sertoli cells. MIS blocks the development of Müllerian ducts,, promoting their regression., 5-alpha dihydrotestosterone (DHT), Testosterone is converted to the more potent DHT by 5-alpha reductase. DHT is necessary to exert androgenic, effects farther from the site of testosterone production, where the concentrations of testosterone are too low to, have any potency. A 5-alpha reductase deficiency results in an androgen disorder characterized by female phenotype, or severely undervirilized male phenotype with development of the epididymis, vas deferens, seminal vesicle, and, ejaculatory duct, but also a pseudovagina., Pathologies, The following disorders are caused by a malfunction in the sex determination and differentiation process:, Congenital adrenal hyperplasia - Inability of adrenal to produce sufficient cortisol, leading to increased production of, testosterone resulting in severe masculinization of 46 XX females., , , , , , , Persistent müllerian duct syndrome - A rare type of pseudohermaphroditism that occurs in 46 XY males,, caused by either a mutation in the Müllerian inhibiting substance (MIS) gene, on 19p13, or its type II, receptor, 12q13. Results in a retention of Müllerian ducts (persistence of rudimentary uterus and fallopian, tubes in otherwise normally virilized males), unilateral or bilateral undescended testes and sometimes, causes infertility., Male pseudohermaphroditism - Failure of androgen production or inadequate androgen response, which, can cause incomplete masculinization in XY males. Varies from mild failure of masculinization with, undescended testes to complete sex reversal and female phenotype (Androgen insensitivity syndrome), Swyer syndrome. A form of complete gonadal dysgenesis, mostly due to mutations in the first step of sex, determination; the SRY genes.
