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14 SEX CHROMOSOMES AND SEX DETERMINATION4.1 Sex chromosomes and Sex DeterminationSex- chromosomes. If present, sex chromosomes may not have the same size, shape, orgenetic potential. In humans, females have 2 so-called X chromosomes and males have oneX chromosome and one Y chromosome. The human X and Y not strictly homologous. Y ismuch smaller and lacks most of loci contained on the X. They do behave as homologs duringmeiosis, howeverSoon after the rediscovery of Mendel's work, experiments with Drosophila produced results inwhich the phenotypic proportions differed between males and females. These results wereexplained by postulating that the genes were located on the X chromosome of Drosophila,which is present in two copies in females, but only one copy in males.Mammals, birds, some insects, and a few plants have this kind of sex chromosome system.One sex has a pair homologous chromosomes, whereas the other sex has one chromosomethat resembles the homologous pair, and one different chromosome.Mammals and Drosophila females have two X chromosomes: XXmales have one X chromosome and one Y: XYThese are the "sex chromosomes", all other chromosomes are called "autosomes".The sex with two different chromosomes is the heterogametic sex. The other is thehomogametic sex.For the X chromosome then, female mammals and Drosophila have two copies of each geneon the X, but males have only one. Females can be homozygous or heterozygous, but malesare hemizygous.Birds and Lepidoptera (Moths and Butterflies) have the opposite pattern: Males arehomogametic (ZZ) and females are heterogametic (ZW).4.2 The Mammalian X ChromosomeThe X chromosome carries hundreds of genes but few, if any, of these have anything to dodirectly with sex. However, the inheritance of these genes follows special rules. These arisebecause: * males have only a single X chromosome * almost all the genes on the X have no counterpart on the Y; thus * any gene on the X, even if recessive in females, will be expressed in males.Genes inherited in this fashion are described as sex-linked or, more precisely, X-linked.X-Linkage: An ExampleHemophilia A is a blood clotting disorder caused by a mutant gene encoding the clotting factorVIII. This gene is located on the X chromosome. With only a single X chromosome, maleswho inherit the defective gene (always from their mother) will be unable to produce factor VIII2and suffer from difficult-to-control episodes of bleeding. In heterozygous females, the un-mutated copy of the gene will provide all the factor VIII they need. Heterozygous females arecalled "carriers" because although they show no symptoms, they pass the gene on toapproximately half their sons, who develop the disease, and half their daughters, who alsobecome carriers. X YX XX XYXhXhX XhYWomen rarely suffer from hemophilia A because to do so they would have to inherit a defectivegene from their father as well as their mother. Until recently, few hemophiliacs ever becamefathers.4.3 The Mammalian Y ChromosomeIn making sperm by meiosis, the X and Y chromosomes must separate in Meiosis I just ashomologous autosomes do (if you don’t remember what happens in Meiosis I vs. Meiosis II,this would be a good time to review. You will need to know this in order to understand much fothe remainder of this course!) This occurs without a problem because, like homologousautosomes, the X and Y chromosome synapse during prophase of meiosis I. There is a smallregion of homology shared by the X and Y chromosome and synapsis occurs at that region.Crossing over between the X and the Y occurs in two regionsof pairing, called the pseudoautosomal regions. These arelocated at opposite ends of the chromosome.The Pseudoautosomal Regions:The pseudoautosomal regions get their name because anygenes located within them (so far only 9 have been found)are inherited just like any autosomal genes. Males have twocopies of these genes: one in the pseudoautosomal region oftheir Y, the other in the corresponding portion of their Xchromosome. So males can inherit an allele originallypresent on the X chromosome of their father and femalescan inherit an allele originally present on the Y chromosomeof their father.This diagram shows the structure of the human Ychromosome.Genes outside the pseudoautosomal regions:Although 95% of the Y chromosome lies between the pseudoautosomal regions, fewer than 80genes have been found here. Some of these encode proteins used by all cells (and bothsexes). The others encode proteins that appear to function only in the testes. A key player inthis latter group is SRY.34.4 SRY and Mammalian Sex DeterminationIt is often stated that sex determination in humans is based on the presence or absence of theY chromosome. However, the situation is more complex. Sex determination in humans andother mammals is actually due to a single gene that is normally located on the Ychromosome. Near one of the pseudoautosomal regions, but not in it, is the SRY (“sex-determining region”). The SRY produces a gene product, TDF (testis determining factor),that triggers undifferentiated gonadal tissue in embryos to form testes. SRY has been found inall mammals investigated.The gene functions early in the developmental program that ultimately causes tissue that isdeveloping into ovaries to 'switch' their developmental, so that they develop as testes.Individuals without this gene develop as females. Sometimes, the SRY gene becomesassociated with a chromosome other than the Y. In these cases, an individual can have thechromosome complement XY, and be a perfectly normal female (or be XX and be a normalmale). Beyond the XY pairing region and the TDF, the Y contains very few genes.SRY is a gene located on the short (p) arm just outside the pseudoautosomal region. It is themaster switch that triggers the events that converts the embryo from it’s ‘defaul’ developmentalpattern as a female into a developmental sequence that results in a male. If this gene is absentor inactivated, a mammal develops into a female, even if the Y chromosome itself is present.Therefore it is not the Y chromosome that determines maleness, or the presence of two Xchromosomes that determines femaleness, it is the presences or absence of the protein codedfor by the SRY gene that determines gender in mammals.What is the evidence?On very rare occasions aneuploid humans are born with such karyotypes as XXY, XXXY, andeven XXXXY. Despite their extra X chromosomes,


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UIUC IB 201 - SEX CHROMOSOMES AND SEX DETERMINATION

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