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Mammalian Reproduction > Sex Determination > Flashcards

Sex Determination Flashcards

1
Q

Give an example of size-dependent sex determination.

A

Reef fish - male when little & female when big

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2
Q

What is temperature-dependent sex determination (TSD)?

A

Where temperature experienced by developing embryo during incubation determines the sex

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3
Q

Give an example of an animal that’s sex is determined by the environment?

A

Turtles

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4
Q

What is genetic sex determination?

A

Found in humans & most mammals
In humans:
XX = female
XY = male

In birds (& some insects/fish):
ZW

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5
Q

Give an example of a mammal that lacks a Y chromosome.

A

Shrews

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6
Q

What species lack a second sex chromosome?

A

Transcaucasian mole voles

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7
Q

What are the 2 proposed explanations for Transcaucasian mole voles not having a second sex chromosome?

A

1 - It may be that a new gene not linked to the Y
chromosome took over the role of the male-
determining gene.
2- The male-determining gene was translocated
from the Y chromosome to the X
chromosome.

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8
Q

Are females homogametic or heterogametic?

A

Homogametic

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9
Q

Are males homogametic or heterogametic?

A

Heterogametic

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10
Q

What are sensitive periods?

A

Early periods, when hormones have long lasting effects

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11
Q

What hormone does sexual differentiation mostly depend on during a sensitive period?

A

Testosterone

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12
Q

In humans, what is the sensitive period for genital formation?

A

6 to 12 weeks

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13
Q

During the early stages of fetal development, what do embryos have?

A
  1. Bi-potential gonads
  2. Wolffian ducts
  3. Müllerian ducts
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14
Q

What are Wolffian ducts?

A

The precursors to other male reproductive organs. They will develop into the epididymis, vas differens, and seminal vesicles.

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15
Q

What are Mullerian ducts?

A

Precursors to the females oviducts (fallopian tubes, uterus, cervix and upper vagina).

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16
Q

Describe sex determination in males.

A
  • The Y chromosome carries a testis-determining factor on it (TDF), now known to be the SRY gene.
  • This allows expression of autosomal genes necessary for testis differentiation.
  • Presence of the TDF on the SRY gene of the Y chromosome causes the undifferentiated gonad to develop into testes (otherwise they develop into ovaries).
  • Expression of the TDF causes the development of primary sex cords in the central region of the undifferentiated gonad.
  • The sex cords later develop into seminiferous tubules, turning it into a testis.
  • In males, the undifferentiated gonads develop into testes from about week 7 of gestation.
  • Leydig cells, in the undifferentiated gonad, develop & secrete testosterone.
  • Testosterone binds to androgen receptors, which leads to development of the Wolffian duct into male reproductive organs.
  • I.e. the action of sex determining gene on the Y
    chromosome and the secretion of testosterone
    stimulates the development of the male organs
    – prostate, penis, scrotum, and associated
    ducts.
17
Q

Describe sex determination in females.

A

In chromosomally female embryos (XX):
- There is no SRY gene.
- No Leydig cells form in the gonads.
- No testosterone is produced and androgen
receptors are not activated.
- Lack of anti-Müllerian hormone preserves the
Müllerian duct.
- Wolffian ducts regress in the absence of
testosterone.
- Ovaries form after 9th week and the Müllerian
duct differentiates into oviduct, uterus, cervix
and upper vagina.

18
Q

What is antigen insensitivity syndrome?

A
  • Genetically male - resistant/insensitive to testosterone
  • No masculinisation of male genitals in foetus
  • No development of male secondary characteristics at puberty
  • Person has some (or all) of physical characteristics of a woman, but genetic makeup of a man
  • Caused by mutation in antigen receptor gene
19
Q

What is complete AIS?

A

Testosterone has no effect on sexual development, so the external genitals are entirely female, but they do not have a uterus or upper vagina so therefore do not menstruate & cannot conceive.

20
Q

What are the sexual characteristics of an individual with complete AIS?

A

Female body except:
- Testes in abdomen
- Shortened vagina or no upper vagina, no
cervix, uterus, fallopian tubes
- No ovaries (testes instead)
- Minimum androgenic hair at puberty

21
Q

What happens to the testosterone in people who have CAIS?

A
  • Testosterone is converted into oestrogen
  • Oestrogen feminises body + accounts for normal female phenotype observed in CAIS
22
Q

How is CAIS managed?

A
  • A gonadectomy to remove gonads from the
    abdomen;
  • Hormone replacement therapy;
  • Genetic and psychological counselling.
23
Q

What is partial AIS?

A

Where testosterone still has some effect on
sexual development, so genitals are often
between male & female.

24
Q

What is the typical PAIS phenotype?

A
  • Abnormal male genitals (e.g. the urethra on the
    underside of the penis (hypospadias))
  • Micropenis
  • Decreased body hair
  • Small scrotum (may not contain gonads)
  • Undescended testes (cryptorchidism)
25
Q

How is PAIS managed?

A
  • Requires surgery in 2-3yrs to bring undescended
    testes into scrotum
  • Androgen supplementation at puberty
  • Surgery for gynacomastia (male breast
    reduction) in adolescence
26
Q

What is mild AIS?

A

The external genitalia are that of a
normal male. May be infertility problems but
sperm count can be restored through androgen
treatment

27
Q

What is Swyers’s syndrome?

A
  • When developing sperm undergoes meiosis, the SRY gene is transferred to the X chromosome.
  • The Y chromosome then lacks the SRY gene and cannot initiate testes development.
  • Children that inherit this Y chromosome without the SRY have an XY genotype but a female phenotype, although with underdeveloped and non-functioning ovaries, mainly composed of fibrous tissue.
28
Q

Why are the ovaries of people with Swyer’s syndrome surgically removed?

A
  • ## High risk of tumour development
29
Q

What are the issues with not having functional ovaries in people with Swyer’s syndrome?

A
  • No oestrogens are produced so secondary sexual
    characteristics fail to develop at puberty unless
    treated with hormone replacement therapy
  • Unable to conceive naturally
  • Can develop testes
  • Can result in both testicular & ovarian tissue being present
30
Q

What is intersex?

A
  • People who have a variety of combinations of what are considered both male and female characteristics that do not fit the traditional (binary) definitions of female or male
  • I.e. they may have both testicular and ovarian
    tissue. Used to be called hermaphrodite but
    now the preferred term is intersex
31
Q

What are 3 possible causes intersex?

A
  • Complete Androgen Insensitivity Syndrome
  • 2 ova, fertilized by 2 sperm, which may
    occasionally fuse to form a tetragamete. If 1 male zygote and 1 female zygote fuse,
    an intersex individual may result
  • Crossing over of the SRY gene from the Y to
    the X chromosome during meiosis (Swyer’s
    syndrome). The SRY is then activated in only certain areas, causing development of testes in these areas,
    but in areas with no activation, there is growth
    of ovarian tissues
32
Q

What is congenital adrenal hyperplasia (CAH)?

A
  • A group of congenital inherited conditions in which the adrenal gland is over- or under-active
33
Q

How is congenital adrenal hyperplasia (CAH) occur?

A
  • Adrenal gland starts to produce cortisol
    after ~6-8 weeks of foetal development
  • Lack of specific enzymes, involved in cortisol synthesis, leads to insufficient cortisol production which results in increased levels of adrenocorticotropic hormone (ACTH)
  • Stimulates over-development (hyperplasia)
    and over-activity of adrenal gland
  • Leads to overproduction of androgens
    (testosterone) & oestrogens (oestradiol)
  • Excessive production of these
    hormones results in CAH
34
Q

Name 3 symptoms of congenital adrenal hyperplasia

A
  • Short stature in males
  • Early/delayed/failed puberty
  • Menstrual irregularities

Mammalian Reproduction (16 decks)

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