GN 301 Module 5: Sex Determination

Methods of sex determination

  • Isogametes such as the organism Chlamydomonas were single-celled organisms that determined sex

  • Environmental factors of sex determination

    • Eg: Temperature: controlling sex determination in turtles and alligators
    • Proteins such as aromatase function differently in sex determination pathways depending on temperature
    • Some PCBs are pollutants that act like estrogens, which can reverse the sex of turtles that should be maleThe herbicide atrazine induces aromatase and can feminize tadpoles
    • Eg: Location: controlling sex determination in slipper limpet
    • Their location in a stack dictates sex
    • The limpets on the outside are males, ones with limpets on top of them are female
    • This is sequential hermaphroditism, since sex changes as the position in the stack changes

  • Haplodiploidy in bees and wasps

    • Males develop from unfertilized eggs and females from fertilized eggs
    • Females must be heterozygous for a particular sex determination locus

  • Nuclear and Cytoplasmic genes in corn

    • Examples of nuclear genes
    • Cytoplasmic genes (on the chloroplast or mitochondria) can also affect sex development
    • Some plants have cytoplasmic male sterility genes in the mitochondria, making them female

  • Sex chromosomes

    • Protenor in grasshoppers and orthoptera
    • Details: XX/X, where one sex has 2 copies of the same chromosome and the other has only one copy of the chromosome

  • Lygaeus in fruit flies, mealworms, melandrium

    • Details: XX/XY, where one sex has 2 X chromosomes and the other has one copy of each chromosome

  • XX/XY  vs ZZ/ZW

    • Chickens, birds and some fish have a ZZ/ZW system in which females are ZW and males are ZZ
    • Platypuses have 10 X chromosomes, males have 5 X and 5 Y chromosomes. The sex chromosomes form into a chain during meiosis

Normal Sex Development

  • Ovotestes:

In early embryological development, there is a sex neutral structure called the ovotestes. Around 7-8 weeks after fertilization, the ovotestes begins developing along the male or female pathway

  • SRY gene:

The ovotestes is activated by the SRY gene on the Y chromosome, directing to male testis development. Lack of the SRY gene leads to female development. Hormones direct development of secondary sexual characteristics after puberty

Abnormalities in sex development

  • Possible Causes
    • SRY gene can be moved, developing an XX male or XY female. These individually are anatomically similar to their gender but are sterile and have secondary sex characteristics that don’t fully develop**.**
    • There could be an extra sex chromosome (XXY)
    • There could be an unusual hormone influence in the uterus.
    • There could be specific mutations that alter sex development.
  • Comparable Tissue Types
    • Testis and ovaries come from the same developmental tissues (penis and clitoris, vas deferens and fallopian tubes)
Sex Development Disorders
  • Androgen Insensitivity (testicular feminization)
    • XY genotype. They look like females and identify as female, but they’re sterile.
    • The X chromosome codes for androgen receptors. In this disorder, androgens receptors aren’t functional, so the effects of testosterone aren’t perceived
    • The SRY pathway is functional, but after androgen receptors don’t work, sex development shifts to female development. Testis have already formed, so they can’t form ovaries. A clitoris and vagina are formed, and no penis. No uterus is formed either
  • Guevodoces
    • XY, but appear more female. Affected individuals are born with a larger than normal clitoris, but not large enough to be a penis, so they have an ambiguous sex
    • They have a defective 5-alpha-reductase enzyme, which converts testosterone to dihydrotestosterone, which leads to male development. This enzyme functions less than it should, so during puberty when there’s finally a lot of testosterone, the clitoris develops into a penis at age 12. They typically switch to living life as a male at that point.
    • They can reproduce!
    • This only occurs in a small village in Salinas.
  • Congenital Adrenal Hyperplasia
    • XX individual that is missing an enzyme, which causes overproduction of male hormones produced by the adrenal glands
    • Can occur prior to birth, causing XX individuals to have both a normal uterus and a penis
    • At puberty they develop normal female secondary characteristics
  • Turner Syndrome
    • Only one X chromosome and no other (XO)
    • Arises through nondisjunction of the X chromosome
    • People have female sex characteristics, and sometimes have a thickening of the skin at their neck, short stature and don’t develop secondary sex characteristics
  • Klinefelter Syndrome
    • Individuals have 2 X chromosomes and a Y (XXY) from a nondisjunction
    • They have feminized characteristics and are tall, and have a penis/testicles
    • Don’t produce many sperm, often have learning disabilities.

Barr Body: inactivated X chromosome

  • Use

 Table: Column Headings Disorder, Sex Chromosome Composition and Number of Barr Bodies. Row Headings Normal Female, Normal Male, Turner Syndrome, Guevedoces, CAH, Klinefelter Syndrome, Androgen Insensitivity

  • Normal female: XX.
    • Number of Barr bodies: 1
  • Normal Male: XY.
    • Number of Barr bodies: 0
  • Turner Syndrome: X0.
    • Number of Barr bodies: 0
  • Guevedoces: XY.
    • Number of Barr bodies: 0
  • CAH: XX.
    • Number of Barr bodies: 1
  • Klinefelter: XXY.
    • Number of Barr bodies: 1
  • Androgen Insensitivity: XY.
    • Number of Barr bodies: 0

Unnamed disorders

  • XXX.
    • Number of Barr bodies: 2
  • XYY.
    • Number of Barr bodies: 0
  • XXXX.
    • Number of Barr bodies: 3

 \n