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Sex chromosome
X and Y chromosomes
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Male
- XY
- Y chromosome carries male-determining genes
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Female
- XX
- female is the "default sex"
- "she" develops in the absence of the male-determining gene
- male development must be triggered (they cause more work)
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X chromosomes
- carry a number of genes
- most of them are not involved with sex determination, involved with everyday life of the cell - "housekeeping genes"
- there are more housekeeping genes than sex determination
- larger in size - due to heterochromatin ("rubbish", non-coding genes)
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Y chromosomes
- contain few genes that are not involved in sex determination
- contains more sex deterimination genes than X chromosome
- smaller in size - very little heterochromatin
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Gene Dosage Compensation Theory
- aka the Lyon Hypothesis of X inactvation
- the X chromosome is larger, carries more genes than the Y...
- the XY complement is deficient in the "dose" of genes inherited from female, but ...
- in the female, one chromosome becomes highly condensed, inactivated, but...
- is facultative heterochromatin
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Facultative heterochromatin
has the facility to return to active chromatin when/if needed
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Fertilization, early embryonic development
- both X chromosomes are active, functional
- one X chromosome ceases to function...
- ...random as to which one is picked for each cell in existence at that point
- cell nucleus multiplies and cell divides
- - same X chromosome remains inactive in both daughter cells
- "housekeeping" genes on X chromosome (also found on X chromosome) are not inactivated
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Pseudoautosomal region
- part of sex chromosomes that pair up durng meiosis
- prevents crossing over of sex-determining genes during meiosis
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Sex differentiation in the male
- starts early in the embryo
- in mammals, Y chromosome carries the primary male-determining genes
- main gene - testis-determining gene (Tdy) aka sex-determining region (SRY)
- controls expression of other genes (both on Y chromosome and some autosomes)
- effect of expression of Tdy gene is that undifferentiated gonad develops as a testis
- - XY embryo
- -- undifferentiated gonad support cells (not gonad cells) direct goads to differentiate into a testis
- - meiotic inhibition substance is secreted
- - germ cells do not undergo meiotic division in the embryo once testis have differentiated
- - Sertoli cells produce a Mullerian Inhibition Substance (MIS)
- -- blocks the development of the Mullerian duct system (female reproductive system)
- Leydig cells secrete testosterone
- - stimulates the Wolffian duct system (male reproductive duct system)
- at puberty, a spermatogenesis gene on the X chromosome of spermatogonia starts spermatogenesis by stimulating meiotic division
- - sperm development starts at puberty
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Sex differentiation in the female
- development happens by default - because male development is not directed, female development occurs
- - XX embryo
- -- no suppression of meiosis
- -- germ cells in the undifferentiated gonads begin meiosis
- -- cells orient themselves into the morphology of an ovary with a medulla and a cortex
- -- oogonia do not complete the full meiotic process
- --- remain resting at diplotene until just before ovulation in the post pubertal animal
- -- female has all the eggs (not yet fully developed) she will ever have at birth
- -- in the absence of suppression, the Mullerian ducts develop and the female duct system is established
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Embryo
- gonadal sex is undifferentiated in the early embryo
- although it is determined at fertilization, sex is not distinguishable until developed enough
- - 30 days of gestation in the dog
- - 40 days in the cat
- because of embryonic similarities during development, homologies of genital organs in the male and female mammal exist
- - see slide 11 or book for chart
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Tortoiseshell cats
- coat color - mix of orange (aka ginger) and black or tabby
- may have patches of white fur (piebalding)
- white gene is on a separate autosome
- color arises because:
- - it is sex-linked
- - it is epistatic to the autosomal black or tabby genes
- female cats (tortis) are heterozygous for the orange gene
- - one X chromosome has the orange gene and one does not
- - only one X chromosome is functional in each cell
- - functional X chromosome w/ orange gene
- -- suppresses the black or tabby gene
- -- only orange hair will be produced
- - functional X chromosome without orange gene
- -- black or tabby gene, whichever is present, will be expressed
- - inactivation of X chromosome doesn't take place until ~ day 12 of embryogenesis in cats
- -- result is patches of orange or patches of tabby or black
- -- due to same active X chromosome in patches of cells
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Calico cats
Sheila has never seen one w/o white (w/o piebalding)
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Male tortoiseshell cats
- 1 in 3000 tortis are male
- only 1 in 10,000 are fertile
- chromosomally abnormal
- - sex chromosome complement
- -- XXY - Kleinfelter Syndrome
- -- 2 distinct cell lines
- --- eg XX/XY, XX/XXY, XY/XXY
- -- must have at least one cell line with a Y chromosome
- -- most are infertile (XXY - Kleinfelter)
- -- those with a normal male cell line (XY) may be fertile
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XX/XY
two cell lines - chimera
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XXY
- single cell line
- every cell has an extra X chromosome
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XX/XYY
- two cell lines
- one a normal female line, one trisomic
- every other cell had an extra Y chromosome
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XY/ XYY
- two cell lines
- one was a normal male line and the other was trisomic
- three copies of each chromosome in each trisomic cell
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XY/XXY/XXYY
- three cell lines
- one normal male line, one line where each cell has an extra X chromosome and one line where each cell has two extra chromosomes
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Abnormalities of sex differentiation
- freemartin
- mutation of the Tdy gene (or any others involved in sex differentiation)
- mutation of an X-linked gene
- abnormal number of chromosomes - excess of X chromosomes or too few
- abnormal number of X chromosomes together with a Y chromosome (XXY)
- true hemaphrodites
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Freemartin
- sterile female
- fusion of blood vessels in multiple conceptions
- secretions from male embryo can affect the development of a female twin embryo
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Mutation of Tdy or othersex-differentiation gene
animals develop as intersexes
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Mutation of an X-linked gene
- target organs cannot respond to the secretion of testosterone
- will see feminized males - not normal development of testes
- mutated gene - testicular feminization gene (Tfm)
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Kleinfelter syndrome
- XXY
- affected individuals are:
- - phenotypic males (Y chromosome)
- - infertile (presence of 2 X chromosomes blocks spermatogenesis)
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True Hermaphrodites
animal has both a complete ovary and a complete testis
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Intersexes
pseudohermaphrodites
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Intersexuality
animal has morphological aspects of both a male and a female
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Intersexuality in the dog and cat
- more common in the dog than the cat
- more common in pigs
- dog presents as a female that does not come into season
- 1. gonads do not develop properly and remain undifferentiated or are ovo-testes (properties of both)
- - to determine this, send out for histopathology
- 2. tubular genetalia do not develop normally (urinary problems)
- - urine from bladder pools in a urogenital sinus instead of being expelled through a vagina and vestibule
- - causes bacterial urine infection
- 3. Urethra is narrower than normal
- - bladder stones
- - urethral blockage (urinary stasis)
- 4. enlarged clitoris
- - rubbing
- - licking
- -soreness
- 5. genetic females (XX)
- 6. genetic males (XY)
- - female phenotype with some male characteristics
- male tortis are not intersexes because they do not have any development of the female reproductive system
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