41 public questions tagged with this topic.
The Y-chromosome carries the SRY gene, which triggers male development in humans. This follows from NCERT principle where relation explains outcome clearly for students.
Ref: NCERT Biology Textbook for Class XI and XII (Botany section), Chapter: Biology - Botany portion covering relevant concept, Topic: Plant structure, physiology and applications.
Honeybees follow the haplodiploidy system, where females are diploid (fertilized eggs) and males are haploid (unfertilized eggs). This follows from NCERT principle where relation explains outcome clearly for students.
Ref: NCERT Biology Textbook for Class XI and XII (Botany section), Chapter: Biology - Botany portion covering relevant concept, Topic: Plant structure, physiology and applications.
Acronym SRY coined after positional cloning of testis-determining factor on Y chromosome through deletion mapping in XY females and translocation in XX males. It stands for Sex-determining Region of Y chromosome, located on short arm Yp encompassing single exon encoding highly conserved HMG box DNA-binding domain essential for bending DNA and triggering Sox9 expression. Terminology distinguishes master switch from larger Y functions including azoospermia factor regions. Understanding acronym clarifies localization and function as genetic determinant present in minimal Y fragment sufficient to induce male development when translocated.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 6: SRY definition - Sex-determining Region Y.
XX male syndrome occurs frequently due to aberrant recombination during paternal meiosis where SRY-containing segment of short arm Y translocates onto distal Xp or autosome. Offspring inheriting recombined chromosome are karyotypically XX but carry functional SRY capable of initiating Sox9 activation and testis determination despite absence of other Y genes such as AZF needed for spermatogenesis. Individuals develop phenotypic males with testes, male internal ducts and external genitalia though typically azoospermic and sterile. Incidence around 1 in 20,000 males reveals sufficiency of SRY for gonadal male development.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 6: XX males due to SRY translocation.
Sex-lethal protein contains two conserved RNA recognition motifs binding uridine-rich sequences near regulated splice sites modulating spliceosome assembly. Rather than binding DNA as transcription factor, Sxl interacts with U1 snRNP and U2AF complex to block inclusion of male-specific exon with premature stop codons in its own transcript and transformer. This post-transcriptional RNA-splicing factor activity shifts pattern toward female isoforms. Through precise alternative splicing control, Sxl integrates dosage compensation, germline sexual identity and somatic differentiation, orchestrating entire female program via RNA processing rather than direct transcriptional activation of target genes.
Ref: Alberts, Molecular Biology of the Cell, 6th ed., Chapter 8: Sxl is RNA-splicing regulator.
X:A ratio 1.0 in Drosophila meaning two X chromosomes and two autosome sets provides strong numerator signal activating Sex-lethal establishment promoter. Double dose of transcription factors sis-a, sis-b, scute and runt cooperatively binds SxlPe enhancer driving robust early Sxl transcription around blastoderm. Resulting Sxl protein autoregulates its maintenance splicing avoiding stop-codon exon and triggers female transformer and doublesex isoforms DsxF. Phenotype therefore female with functional ovaries, female pigmentation, dosage compensation off. Ratio 0.5 yields male, intermediate 0.67 intersex, 1.5 metafemale inviable.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: X:A ratio 1.0 indicates female Drosophila.
Drosophila sex hierarchy consists of master regulator Sex-lethal, intermediate regulators transformer and transformer-2, and terminal effectors doublesex and fruitless producing sex-specific isoforms via alternative splicing. Numerator X-linked genes activate Sxl in XX; Sxl directs female tra splicing; Tra with constitutive Tra2 directs female dsx and fru splicing. Genes Sox9, Sry, Fgf9 are mammalian male determinants operating via transcriptional activation and β-catenin antagonism; Wnt4/Rspo1 are mammalian ovarian regulators; Wt1, Lhx9, Gata4 are mammalian bipotential gonad factors. Thus canonical fly genes are Sxl, tra, dsx.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: Drosophila sex genes Sxl-tra-dsx.
Hymenoptera including honeybees, ants and wasps lack heteromorphic sex chromosomes and determine sex via haplodiploid fertilization status conceptually distinct from X:A or ZZ/ZW. Diploid fertilized eggs heterozygous at complementary sex determiner locus csd produce females, either workers or queens, while haploid unfertilized eggs hemizygous become males drones. Molecular mechanism csd complementation regulates feminizer gene transformer ortholog splicing into female form producing DsxF, whereas hemizygous genotype yields male splicing and DsxM. Queen controls fertilization voluntarily releasing stored sperm, thereby controlling colony sex ratio.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 6: Fertilization-dependent sex in Hymenoptera.
Testis determination initiates when Sry protein briefly expressed in XY bipotential gonads binds testis-specific enhancer of Sox9 together with Sf1, triggering dramatic Sox9 upregulation in pre-Sertoli precursors. Elevated Sox9 establishes feed-forward loop involving Fgf9 and prostaglandin D2 maintaining high Sox9 and repressing β-catenin/Wnt4 ovarian signals. Sox9 subsequently activates Dhh for Leydig specification, Amh for Müllerian regression and collagen genes organizing testis cords and supporting germ cell quiescence. Without Sry, Sox9 remains low permitting ovarian differentiation via Wnt4, β-catenin and Foxl2 pathways.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 6: Sry initiates testis by activating Sox9.
In Drosophila alternative splicing hierarchy defaults to male mode when Sex-lethal inactive. Absence of Sxl allows transcripts of Sxl itself to retain exon 3 with premature termination producing no protein, transformer pre-mRNA splices to truncated nonfunctional male form, and doublesex retains male-specific exon producing DsxM driving male differentiation and courtship behaviors. This default splicing requires no activator, occurring constitutively unless Sxl redirects spliceosome by binding intronic repressor elements. Hence XY males display male splicing pathway as ground state, while females actively divert splicing via Sxl and Tra to female isoforms.
Ref: Alberts, Molecular Biology of the Cell, Chapter 8: Default male splicing in Drosophila sex determination.
Active Sex-lethal gene in Drosophila initiates cascade culminating in female development. Sxl protein functions only in XX embryos where early burst of transcription overcomes splicing block, establishing positive autoregulatory loop maintaining female-specific Sxl mRNA lacking male exon with stop codon. Sxl then directs splicing of transformer into functional Tra protein which together with Tra2 drives female-specific splicing of doublesex and fruitless producing DsxF and FruF. DsxF governs yolk protein synthesis, female genitalia, receptivity and repression of male structures, establishing morphological and behavioral female identity.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: Active Sxl results in female Drosophila.
Fruit fly sexual fate depends upon counting X chromosomes relative to autosome ploidy, not Y. In XX embryos double dose of X-encoded numerator transcription factors SisA, SisB, Scute and Runt activate early Sex-lethal promoter SxlPe around cycle 12. Resulting Sxl protein maintains female splicing program and inhibits dosage compensation complex. In XY embryos single dose fails to activate Sxl, causing male-specific splicing of downstream effectors and activation of MSL complex for X hypertranscription. Y chromosome provides fertility factors such as kl-5 for spermatogenesis but does not influence male versus female identity.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: Drosophila sex determined by X chromosome number.
