CHAPTER 5
Sexual Differentiation
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FIGURE 5-1 HUMAN CHROMOSOMES (TOP), WHICH HAVE BEEN ARRANGED IN HOMOLOGOUS PAIRS (BOTTOM).
THESE ARE FROM A CELL OF ONE OF THE AUTHORS’ SON (EVAN PRYOR JONES), WHICH WAS OBTAINED WHILE HE WAS
STILL A FETUS BY AMNIOCENTESIS (SEE CHAPTER 10). NOTE THE X AND Y CHROMOSOMES AT LOWER RIGHT.
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FIGURE 5-2 POSSIBLE VARIATIONS
IN NUMBER OF SEX CHROMOSOMES IN MALES AND FEMALES AND THE
RESULTANT NUMBER OF SEX CHROMATINS, OR
THAN THE NUMBER OF
BARR BODIES. NOTE THAT THE NUMBER OF BARRBODIES IS ONE LESS
X CHROMOSOMES
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FIGURE 5-3 LOCATION OF GENES INVOLVED IN GONADAL SEX DIFFERENTIATION. THE SEX-DETERMINING REGION
OF THE Y (SRY) GENE CODES FOR THE PRODUCTION OF TESTIS-DETERMINING FACTOR (TDF), WHICH IN TURN
CAUSES TESTIS DIFFERENTIATION. ABSENCE OF THIS GENE IN AN INDIVIDUAL LACKING THE Y CHROMOSOME
RESULTS IN THE FORMATION OF OVARIES. THE DAX-1 GENE ON THE X CHROMOSOME SUPPRESSES SRY GENE
EXPRESSION, BUT THE NORMAL INTERACTION OF DAX-1 AND SRY HAS NOT BEEN FULLY DISCOVERED.
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BOX 1: WHY THE Y CHROMOSOME?
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FIGURE 5-4 INHERITANCE OF A SEX-LINKED TRAIT (RED–GREEN COLOR BLINDNESS). A NORMAL VISION
FATHER (XCY) AND A NORMAL VISION, CARRIER MOTHER (XCXC) WILL HAVE NORMAL VISION DAUGHTERS,
HALF OF WHOM WILL BE CARRIERS. HALF OF THEIR SONS WILL BE COLOR BLIND. XC REPRESENTS THE COLOR
BLIND ALLELE CARRIED ON THE X CHROMOSOME; XC REPRESENTS THE NORMAL ALLELE.
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FIGURE 5-5 DIAGRAM ILLUSTRATING DIFFERENTIATION OF INDIFFERENT GONADS INTO TESTES OR OVARIES: (A)
INDIFFERENT GONADS FROM A 6-WEEK-OLD EMBRYO; (B) AT 7 WEEKS, SHOWING TESTES DEVELOPING UNDER THE
INFLUENCE OF TDF; (C) AT 12 WEEKS, SHOWING OVARIES DEVELOPING IN THE ABSENCE OF TDF; (D) TESTIS AT 20
WEEKS, SHOWING THE RETE TESTIS AND SEMINIFEROUS TUBULES DERIVED FROM MEDULLARY CORDS; (E) OVARY AT
20 WEEKS, SHOWING PRIMORDIAL FOLLICLES; (F) SECTION OF A SEMINIFEROUS TUBULE FROM A 20-WEEK FETUS; AND
(G) SECTION FROM THE OVARIAN CORTEX OF A 20-WEEK FETUS, SHOWING TWO PRIMORDIAL FOLLICLES
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FIGURE 5-6 DIFFERENTIATION
OF SEX ACCESSORY STRUCTURES IN MALE AND FEMALE HUMAN EMBRYOS.
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FIGURE 5-7 DIFFERENTIATION
OF THE INDIFFERENT STAGE INTO MALE AND FEMALE EXTERNAL GENITALIA.
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FIGURE 5-8 SUMMARY OF SEX DETERMINATION
AND DEVELOPMENT.
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FIGURE 5-9 A PERSON
WITH TESTICULAR FEMINIZATION SYNDROME, A FORM OF MALE
PSEUDOHERMAPHRODITISM.
(PHOTOGRAPH COURTESY OF DR. HOWARD W. JONES.)
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BOX 2: GUEVEDOCES
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FIGURE 5-10 EXTERNAL
GENITALIA OF FEMALE PSEUDOHERMAPHRODITES WITH ADRENOGENITAL SYNDROME: (A)
A
NEWBORN FEMALE EXHIBITING ENLARGEMENT OF THE CLITORIS AND FUSION OF THE LABIA MAJORA; (B) A FEMALE
INFANT SHOWING CONSIDERABLE ENLARGEMENT OF THE CLITORIS; AND (C AND D) A
ENLARGED CLITORIS AND FUSION OF THE LABIA MAJORA TO FORM A SCROTUM. (D)
6-YEAR-OLD GIRL SHOWING AN
NOTE THE CLITORAL GLANS AND
THE OPENING OF THE UROGENITALSINUS (ARROW).
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FIGURE 5-11 NONDISJUNCTION IS THE FAILURE OF A HOMOLOGOUS PAIR OF CHROMOSOMES TO SEPARATE DURING
MEIOSIS. (A) NONDISJUNCTION OF SEX CHROMOSOMES DURING SPERM FORMATION. THE FERTILIZED OVA WILL HAVE
TWO X CHROMOSOMES AND ONE Y CHROMOSOME (KLINEFELTER’S SYNDROME) OR ONLY AN X CHROMOSOME
(TURNER’S SYNDROME). (B) NONDISJUNCTION OF SEX CHROMOSOMES DURING MEIOSIS IN THE OVARY RESULTS IN
OVA WITH EITHER TWO X CHROMOSOMES OR NONE. THE FERTILIZED OVA THEREFORE CAN EXHIBIT ONE OF FOUR
CHROMOSOMAL ABERRATIONS,THREE OF WHICH IS VIABLE
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FIGURE 5-12 ILLUSTRATIONS OF VARIOUS TYPES OF CONGENITAL UTERINE ABNORMALITIES: (A) DOUBLE
UTERUS AND DOUBLE VAGINA, (B) DOUBLE UTERUS WITH SINGLE VAGINA, (C) BRANCHED (BICORNUATE)
UTERUS, (D) BICORNUATE UTERUS WITH A SMALL LEFT BRANCH, (E) SEPTATE UTERUS (DIVIDED IN THE MIDDLE), AND
(F) UTERUS FORMED FROM MÜLLERIAN DUCT ON ONE SIDE ONLY
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