Sex Determination Flashcards
Give an example of size-dependent sex determination.
Reef fish - male when little & female when big
What is temperature-dependent sex determination (TSD)?
Where temperature experienced by developing embryo during incubation determines the sex
Give an example of an animal that’s sex is determined by the environment?
Turtles
What is genetic sex determination?
Found in humans & most mammals
In humans:
XX = female
XY = male
In birds (& some insects/fish):
ZW
Give an example of a mammal that lacks a Y chromosome.
Shrews
What species lack a second sex chromosome?
Transcaucasian mole voles
What are the 2 proposed explanations for Transcaucasian mole voles not having a second sex chromosome?
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.
Are females homogametic or heterogametic?
Homogametic
Are males homogametic or heterogametic?
Heterogametic
What are sensitive periods?
Early periods, when hormones have long lasting effects
What hormone does sexual differentiation mostly depend on during a sensitive period?
Testosterone
In humans, what is the sensitive period for genital formation?
6 to 12 weeks
During the early stages of fetal development, what do embryos have?
- Bi-potential gonads
- Wolffian ducts
- Müllerian ducts
What are Wolffian ducts?
The precursors to other male reproductive organs. They will develop into the epididymis, vas differens, and seminal vesicles.
What are Mullerian ducts?
Precursors to the females oviducts (fallopian tubes, uterus, cervix and upper vagina).
Describe sex determination in males.
- 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.
Describe sex determination in females.
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.
What is antigen insensitivity syndrome?
- 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
What is complete AIS?
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.
What are the sexual characteristics of an individual with complete AIS?
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
What happens to the testosterone in people who have CAIS?
- Testosterone is converted into oestrogen
- Oestrogen feminises body + accounts for normal female phenotype observed in CAIS
How is CAIS managed?
- A gonadectomy to remove gonads from the
abdomen; - Hormone replacement therapy;
- Genetic and psychological counselling.
What is partial AIS?
Where testosterone still has some effect on
sexual development, so genitals are often
between male & female.
What is the typical PAIS phenotype?
- 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)
How is PAIS managed?
- Requires surgery in 2-3yrs to bring undescended
testes into scrotum - Androgen supplementation at puberty
- Surgery for gynacomastia (male breast
reduction) in adolescence
What is mild AIS?
The external genitalia are that of a
normal male. May be infertility problems but
sperm count can be restored through androgen
treatment
What is Swyers’s syndrome?
- 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.
Why are the ovaries of people with Swyer’s syndrome surgically removed?
- ## High risk of tumour development
What are the issues with not having functional ovaries in people with Swyer’s syndrome?
- 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
What is intersex?
- 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
What are 3 possible causes intersex?
- 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
What is congenital adrenal hyperplasia (CAH)?
- A group of congenital inherited conditions in which the adrenal gland is over- or under-active
How is congenital adrenal hyperplasia (CAH) occur?
- 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
Name 3 symptoms of congenital adrenal hyperplasia
- Short stature in males
- Early/delayed/failed puberty
- Menstrual irregularities
