Sex Determination and Sex Chromosomes

wide range of reproductive modes and life cycles in biological world

• some organisms are entirely asexual

• others alternate b/w short periods of sexual reproduction and prolonged periods of asexual reproduction

sexual differentiation

• successful fertilization depends on sexual differentiation in reproductive organisms

• differentiation of sexes is more evident as phenotypic dimorphism of males and females

heteromorphic chromosomes (XY in mammals)

characterize one sex or the other in wide range of species, labeled as sex chromosomes

genes serve basis for

sex determination

males

• produce unlike gametes (heterogamic sex)

• determine sex of progeny

females

produce like gametes (homogametic sex), uniform gametes

human karyotype

one pair of chromosomes differ in males and females

• metaphase show 46 was humN DIPLOID NUMBER

of 23 pair chromosomes, one pair varied in configuration in males and females

• females have 2 X chromosomes

• males have 1 X and 1 Y chromosome

  • Y determines male

Klinefelter Syndrome

• individuals have more than 1 chromosome

  • XXY complement in addition to 44 autosomes

Klinefelter Syndrome

• example

  • tall with long arms and legs-large hands and feet

  • testes are rudimentary, fail to produce sperm

  • slight breast enlargement and hips often

Turner Syndrome

• individuals often 45 chromosomes with single X chromosome

• Female external genitalia and internal ducts but ovaries rudimentary

• short stature, skin flaps on back of neck, underdeveloped breast, broad shield-like chest

• normal intelligence

Klinefelter and Turner Syndromes

• nondisjunction during meiosis (failure of sex chromosomes to segregate during meiosis

Y Chromosome in Klinefelter and Turner Syndrome

Klinefelter: Y chromosome is sufficient to determine maleness

Turner: Absence of Y chromosome, no masculinization occurs

Karyotype 47, XXX (Triplo-X)

• Three X chromosomes along with normal set of autosomes

• Results in female differentiation

• Often 47, XXX women perfectly normal- unaware of condition

• underdeveloped secondary sex characteristics, sterility, intellectual disability

• Tetra-X (48, XXXX) and penta-X (49,XXXXX) karyotypes have been reported

Karyotype 47, XYY Condition

• Presence of additional Y chromosome is only deviation from diploidy

• consistently shared characteristic

  • males over 6 feet tall

  • may subnormal intelligence

  • may have personality disorders

Sexual Differentiation in Humans

• Y chromosome houses genetic information for maleness-gene provides “signal”

  • in early embryonic development human embryo hermaphrodic-gonadal phenotype sexually indifferent

Sexual Differentiation in Humans

• Presence of Y chromosome- medulla develops into testes

• Absence of Y chromosome

  • cortex of ridge forms ovarian tissue

  • mullerian duct forms oviducts, uterus, cervix, and portiions of vagina

Y Chromosome and Male Development

Y chromosome: 75 genes

X chromosome: 900-1400 genes

Pseudoautosomal regions (PARs)

• present on both ends of Y chromosome: share homology with regions on X chromosome

• Synapse and recombine with X chromosome during meiosis

Male-Specific Region of Y (MSY)

• 95% of Y chromosome which does not recombine with X chromosome

SRY: Sex determining region Y

• critical gene controlling sexual development

• Gene becomes active in XY embryos at 6-8 weeks of development

Testis-Determining Factor (TDF)

• SRY gene encodes protein TDF

  • present in all mammals-tiggers testes formation

  • causes undifferentiated gonadal tissue of embryo to form testes

Deviations from normal sex determination

• males with two X chromosomes and no Y

• females with one X chromosome and one Y chromosome are missing SRY gene

Transgenic Mice Research

• fertilized eggs injected with foreign DNA

• Mice have Sry gene comparable to human SRY

• Mouse DNA with SRy injected into XX mice eggs: most offsprings being males

• TDF believe to be transcription factor:

  • behaves as master switch controlling genes involved in sexual differentiation

Human Sex Ratio (sex ratio)

• Sex ratio

  • proportion of male to female offspring assessed in two ways

    • primary and secondary sex ratio

Human Sex Ratio (Primary)

• reflects proportion of males to females conceived in population

Human Sex Ratio (Secondary)

• reflects proportion of each sex born

  • does not account for fetal mortality

Sex Ratio-male to female ratio

• more males born than females

  • segregation, males produce equal numbers of X and Y-bearing sperm

  • each type of sperm has equivalent viability and motility in female reproductive tract

  • egg surface is receptive to both X and Y bering sperm