Genetics of Human Sexual Development
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Levels of Sexual Development
- There are 3 levels to sexual development:
- chromosomal sex: presence or absence of the Y chromosome
- gonadal sex (primary sex determination): whether the gonads develop as testes or ovaries depends on the presence or absence of the SRY gene, usually found on the Y chromsome
- phenotypic sex (secondary sex determination): all of the internal and external structures develop along male or female lines depending on which hormones are secreted by the gonads.
- Phenotypic sex also has a couple of distinct systems: the internal ducts, and the external genitalia
- Two important times: pre-natal development and puberty
Chromosomal Sex Determination
- We have 46 chromosomes: 23 pairs, one set from each parent.
- One pair of chromosomes is the sex chromosomes, X and Y.
- the other chromosomes just have numbers: 1-22.
- A person with 2 X chromosomes (46,XX) is female, and a person with an X and a Y (46,XY) is male.
Human karyotype: chromsomes
stained to show bands, from a male
- Reproduction involves putting one copy of each chromosome into each sperm cell or egg cell: the process of meiosis. So, meiosis starts with 46 chromosomes, chooses one from each pair, and puts 23 chromosomes in each sperm or egg.
- For males, 1/2 the sperm get an X chromosome, and the other 1/2 get a Y chromosome.
- Sometimes meiosis goes wrong (non-disjunction), and a sperm or egg might get 2 sex chromosomes, or 0 sex chromosomes, leading people with 47 or 45 chromosomes. More on this later...
- Fertilization means the sperm joins the egg, creating the zygote, which is the first cell of the new individual person.
- 23 chromosomes from sperm plus 23 from the egg restores the total of 46 chromosomes.
- And, the new individual is now either 46,XX (female) or 46,XY (male)
The SRY Gene
- How the Y chromosome determines sex.
- The SRY gene, located on the Y chromosome, is the primary determinant of sexual development.
- That is, if a developing embryo has a functional SRY gene in its cells, it will develop as a male. And, if there is no functional SRY, the embryo develops as female.
- Although the SRY gene is usually on the Y chromosome, it occasionally gets transferred to the X.
- this leads to 46,XX males
- Also, sometimes the SRY gene is inactivated by mutation.
- Leading to 46,XY females (Swyer syndrome)
- it is also possible to have a partially inactive SRY gene, leading to ambiguous genitalia
Early Gonad Development
- Before 6-7 weeks of development, the gonad is indifferent: neither male nor female.
- It develops from the same tissue as the kidneys and adrenal glands.
- Also developing by this time: 2 sets of ducts that will eventually lead to the outside world.
- Wolffian ducts = male
- Mullerian ducts = female
- If SRY is present in the indifferent gonad at 6 weeks, it gets activated. This in turn activates other genes, and the indifferent gonad is converted to a testes.
- In the absence of SRY, a different set of genes is activated, and the indifferent gonad becomes an ovary.
- The germ cells, which actually become sperm or eggs, migrate into the gonad about this time.
Development of Phenotypic Sex
- The cells of the newly formed testes start secreting the hormone testosterone.
- Testosterone secretion peaks about week 16, with levels similar to those found in adult males. After this, the testosterone level drops to about the same level as female fetuses.
- The testes also secrete another hormone: Mullerian inhibiting substance (MIS) (aka anti-Mullerian hormone, AMH).
- Another important process in the developing male: during the last trimester of pre-natal life, the testes migrate (“descend”) from the kidney region into the scrotum.
- Under the control of a third testes hormone: “insulin-like hormone 3”
- The developing ovary secretes estrogen, which is important after birth, but estrogen from the mother completely swamps it out before birth.
- In the early embryo, two duct systems form. After the gonad differentiates into a testis or ovary, one set of ducts develops further while the other set degenerates.
- Testosterone causes the Wolffian ducts to develop into male structures: epididymus, vas deferens, seminal vesicles.
- In the absence of testosterone, the Wolffian ducts disappear (except a bit becomes the adrenal glands in both sexes)
- Mullerian inhibiting substance causes the Mulerian ducts to disappear.
- In the absence of MIS, the Mullerian ducts develop into the Fallopian tubes, uterus, and upper vagina.
Another Duct Picture
Development of the External Genitalia
- This process is controlled by the presence or absence of dihydrotestosterone (DHT).
- Testosterone gets converted into DHT by the enzyme 5-alpha reductase, which is found in the testes and the skin.
- Both sexes start out with the same structures, which develop along different lines under the influence of testosterone and DHT.
- The default condition in female: in the absence of DHT, the external genitalstructures develop along female lines.
- DHT also causes hair loss: male pattern baldness. Testosterone is converted to DHT locally. Rogaine works by blocking 5-alpha reductase
- In the absence of DHT, the genital swellings form the labia majora; the genital folds remain unfused and form the labia minora; the genital tubercle forms the clitoris and the urogenital sinus forms the lower part of the vagina.
- With DHT present, the genital swellings migrate and become the scrotum; the urogenital folds enlarge and enclose the penile urethra and become the shaft of the penis; the genital tubercle becomes the glans penis; and the urogenital sinus forms the prostate gland
Childhood and Puberty
- During childhood, sex hormone levels are very low in both sexes.
- There is a surge of sex hormones in both boys and girls for a few weeks just after birth. Significance is unknown.
- Puberty begins when the brain and hypothalamus start producing the neurohormone GnRH (gonadotropin releasing hormone). This hormone then stimulates production of LH and FSH by the pituitary gland.
- LH and FSH stimulate the testes and ovaries to start producing large amounts of testosterone and estradiol (a form of estrogen).
- In boys, some of the testosterone is converted to estradiol, which causes a growth spurt, and sometimes leads to temporary breast development.
- The adrenal glands also secrete male sex hormones, in both boys and girls, starting in late childhood.
- After puberty starts, the ovaries also produce androgens.
Review of Normal Sexual Development
- The default condition is female. Male development occurs only when the SRY gene (on the Y chromosome) is present.
- The primary sex organ, the gonad, becomes an ovary in the absence of SRY, and a testis in the presence of SRY.
- The testes secrete two hormones: testosterone and Mullerian-inhibiting substance. These control the development of the internal reproductive ducts.
- Two duct systems are present in the embryo: Wolffian and Mullerian
- In males, testosterone causes the Wolffian ducts to develop into the vas deferens, seminal vesicles, and epididymus. Also in males, MIS causes the Mullerian ducts to degenerate.
- In females, the absence of MIS causes the Mullerian ducts to develop into the fallopian tubes, uterus, and upper vagina. The absence of testosterone causes the Wolffian ducts to degenerate.
- The external genitalia develop in the male pattern if dihydrotestosterone (DHT) is present. Female genitalia develop in the absence of DHT.
- Very little change occurs in childhood, but puberty brings a big surge in sex hormones, which modify the structures formed before birth.
- The large majority of people develop as either completely male or completely female. However, 1% or more of the population has some variant condition.
- Chromosomal variations
- Gene mutations
- External conditions
- A few important terms:
- Gynecomastia: development of breasts in a male
- Hypospadias: the urethra exits the male body at the base of the penis instead of at the tip, due to failure of the urethra to become enclosed by the urogenital folds.
- Hermaphrodite: a person exhibiting both male and female characteristics
- a true hermaphrodite has both testicular and ovarian tissue, sometimes as separate organs but more frequently as an ovotestis: a single organ with different parts. Very rare.
- Male pseudo-hermaphrodite has testes and no ovaries; a female pseudo-hermaphrodite has ovaries and no testes.
- Worth noting: the Intersex Society of North America finds the term “hermaphrodite” offensive, and prefers “intersex”.
Meiosis, the form of cell division that generates the sperm and eggs, carefully puts exactly 1 copy of each chromosome pair into each cell.
Sometimes meiosis goes wrong and puts 0 or 2 copies of some chromosome into a sperm or egg cell.
the best example of this: Down syndrome, which starts with a sperm or egg with 2 copies of chromosome 21.
Maternal age effect: more frequent in older mothers
The sex chromosomes are quite tolerant of variants.
Most common types involve 45 or 47 chromosomes
There are many other, rarer types, with 48 or even 49 chromosomes, such as 49,XXXXY. Such conditions almost always lead to serious mental deficiencies.
The general rule: if the Y is present, the person is internally and externally male.
Klinefelter Syndrome: 47,XXY
- Occurs about 1 per 500 male births. It is the most common type of sex chromosome variant.
- The presence of the Y chromosome causes a 47,XXY person to be male, both externally and internally, because the testes are formed.
- Root symptom: small testes, leading to low testosterone levels. Most, but not all, are sterile.
- At puberty, reduced facial and body hair, broader hips, breast development.
- 47,XXY children tend to be taller, less physically strong and coordinated, and more quiet and shyer than their peers. Some language and learning problems are common: often slow to learn to speak and read.
- Testosterone replacement therapy helps with some of the physical symptoms. Speech therapy and educational services also help.
46,XX males, with the SRY
gene on the X, have the
Turner Syndrome: 45,X
- Only one X chromosome, sometimes called XO. Since there is no Y chromosome, the primary gonad is the ovary, and 45,X people are female.
- About 1 in 2500 live female births.
- 10% of all spontaneous abortions (miscarriages) are due to Turner syndrome; about 98% of all Turner’s embryos die before birth
- Ovaries completely non-functional, so 45,X women are sterile, with no production of sex hormones and development of secondary sexual characteristics at puberty.
- Some characteristic physical abnormalities: short stature, low hairline, webbed skin at neck. Kidney and circulatory system problems
- Often have problems with spatial reasoning and mathematics. Also social difficulties: inability to understand others’ emotions.
- Can be treated with growth hormone and estrogen.
- You need 2 X chromosomes for
- proper ovarian development.
- 46,XY females (non-functional
- SRYgene) resemble Turner’s
- About 1 in 1000 live male births. Most XYY’s are never detected: a very mild condition.
- since 1960, newly discovered chromosome variants aren’t given the discoverer’s name
- It was once thought to create hyper-aggressive males with a tendency towards criminal behavior.
- Richard Speck, the killer of eight student nurses in 1966, pretended (falsely) to be an XYY to obtain leniency.
- A 1968 letter to the Lancet claimed that XYY men were in prison at a rate "25-60 times as high as the prevalence in the general population”, based on finding 2 XYY’s.
- the plot of Aliens 3 involves a prison planet for XYY’s.
- XYY’s are generally normal in appearance, but with average height about 7 cm above expected and normal build. Perhaps acne is more common than average, but this is disputed.
- They are often more physically active, somewhat delayed in emotional maturity, and have a slight increase in learning and speech problems.
- Fertile, normal sex drive, very rarely pass 2 Y’s to sons.
1970’s British TV series:
He had an extra Y, which
made him a macho criminal!
- About 1 in 1000 live female births. So mild as to be only rarely detected. Also called triplo-X.
- Originally called “superfemale” (early 1960’s). <rolls eyes>
- Widely varyingsymptoms, including none at all.
- Slightly more passive and quiet as babies, less assertive, delayed motor and linguistic skills. Delayed emotional maturity and social skills. Some have slightly decreased intelligence and learning difficulties.
- Lower back problems are common. Fertility normal, don’t generally pass 2 X’s to children.
Mosaics and Chimeras
- Both terms refer to people who have 2 different chromosome sets in different cells. For example, a 46,XX/47,XXY person has some cells with 46 chromosomes and other cells with 47.
- A mosaic starts out with a single fertilized egg. During an early cell division in the embryo, one cell gained or lost a chromosome. (This is non-disjunction, the same event that happens in meiosis to generate Klinefelter’s, etc.)
- A chimera starts out with two separate fertilized eggs, fraternal twins. The two embryos fuse together to form a single individual.
- It is not uncommon to have fraternal twins sharing some blood cells, a “blood chimera”
- fused embryo chimeras are very rare: there are about 30-40 known XX/XY chimeras (and undoubtedly an equal number same sex chimeras). “tetragametic chimera”
- Chimerism is probably the way most true hermaphrodites, who have both ovarian and testicular tissue, are formed. However, actual XX/XY chimeras have been everything from normal male, through various degrees of ambiguous genitalia, to normal female.
- Sexual development can be quite variable in such people, because the characteristics depend on which cells have which chromosome complement.
- The variants up to now all involve whole chromosomes, which have lots of genes on them. The effects of changing the dosage of many genes tend to be widespread but mild. (or completely lethal, as with most non-sex chromosomes).
- Now we are going to look at several gene mutations. In these cases, only one gene is affected, but it is completely knocked out. This can lead to large effects, but limited to a few subsystems in the body.
- Rates are different: for chromosome changes, about 1 in 1000 births is a typical frequency. For gene mutations, each parent needs to contribute a mutated copy of the gene, so rates are usually 1 in 10,000 births or less.
- Inheritance is also a factor here: most chromosomal variants are spontaneous events and don’t run in families. Gene mutations are usually inherited variants: there is often a family/community history of the variant type.
- New mutations do occur spontaneously, but it’s rare. Most gene variants are inherited from the parents.
5-alpha Reductase Deficiency (5-ARD)
- 5-alpha reductase is the enzyme that converts testosterone into DHT. If both copies of the gene that makes this enzyme are defective, the person has 5-ARD.
- Recall that DHT is responsible for the development of male external genitalia
- At birth, people with 5-ARD have undescended testes and male ducts (with no female ducts), but genitalia that appear somewhere between female and ambiguous, including a a very small penis with hypospadias (which appears to be an enlarged clitoris), and a short vagina. Often raised as girls
- At puberty, the increase in testosterone is large enough that some DHT gets made, and they develop a male appearance: the testes descend, the penis enlarges, facial hair appears, the voice deepens, muscles develop.
- Large group in the Dominican Republic: maybe 1 in 90 men. Called Guevedoces, a corruption of “huevos a los doce” (eggs--testicles- at age 12). Raised as girls, they easily switch to the male role.
- Other groups found in Malta, Jordan, Pakistan, New Guinea
Also, a character on the TV show
Nip and Tuck has this condition
- Incidence about 1 in 20,000 births
- Used to be called “testicular feminization”. 46,XY with normal (undescended) testes. The testes secrete testosterone, but the cells lack a receptor for it. No receptor = no response to the hormone. Complete androgen insensitivity, CAIS.
- As a result, the male ducts (vas deferens, epididymus, seminal vesicles) are not present. However, the testes secrete MIS, which causes the female ducts (uterus, fallopian tubes, upper vagina) to degenerate.
- External genitalia develop as male if DHT is present, but testosterone and DHT use the same receptor. So, female external genitalia, including the lower 2/3 of the vagina.
- At puberty, the testes again secrete testosterone. The enzyme aromatase converts it into estradiol. Thus, female secondary sexual characteristics develop. Often “voluptuously feminine”. No menstruation of course: no ovaries and no uterus. Pubic and armpit hair is usually scant or absent.
- Occasionally, the undescended testes can become cancerous, so they are often surgically removed after puberty is complete (so as to get normal female development).
Partial Androgen Insensitivity
- Sometimes, the testosterone receptors work inefficiently, due to less drastic mutations than in CAIS. In these cases, the body cells respond in a variable manner to testosterone, leading a a wide variety of ambiguous genitalia. PAIS = partial androgen insensitivity. Also called Reifenstein syndrome.
- there is also mild androgen insensitivity (MAIS), which leads to completely male appearance internally and externally, but with some impairment of masculinization at puberty.
- Variable symptoms: can be predominantly male (with hypospadia, abnormal scrotum, small penis), predominantly female (with enlarged clitoris, fused labia, separate vaginal and urethral openings), or ambiguous genitalia (microphallus--less than 1 cm long), labia-like scrotum, hypospadia, gynecomastia.
- Similar variability in male internal ducts; females ducts are usually absent due to MIS secretion.
- Sometimes people with PAIS change gender identity after puberty, in either direction.
Congenital Adrenal Hyperplasia
- The adrenal glands sit on top of the kidneys and secrete a variety of steroid hormones, including cortisone (stress response), aldosterone (salt balance) and androgens (male sex hormones).
- Steroid hormones are made from cholesterol through a series of biochemical steps. Any one of these steps can be inactivated by mutation. However, about 95% of CAH cases involve defects in the enzyme 21-hydroxylase.
- 21-hydroxylase is needed to make cortisol and aldosterone (but not androgens). Cortisol is secreted in response to the pituitary hormone ACTH, in a feedback loop. So, if there isn’t enough cortisol being made, more ACTH is made, and this causes the adrenal gland to grow larger (hyperplasia).
- And, all of those steroid molecules that were destined to become cortisol and aldosterone get diverted into male sex hormones (androstendione and testosterone), which don’t need the 21-hydroxylase.
- Very little effect on male fetus, which is already making testosterone, except that after birth the lack of salt regulation can lead to death from excess salt secretion (salt-wasting).
- Female fetuses with 21-hydroxlase deficiency have some problems due to the flood of androgens released by the adrenal gland. The ovaries are normal, and the female (Mullerian) ducts are also normal (since no MIS is made).
- Main effects are on the external genitalia: enlarged clitoris, sometimes with an enclosed urethra (i.e. like the penis), labia can fuse and become scrotum-like, vaginal opening can be partly or completely closed.
- Appearance at birth varies a lot. Some appear to be normal male with undescended (because non-existent) testes. However, the chromosomes are XX, the gonads are ovaries, and the uterus and fallopian tubes are usually intact.
- Normally, very little androgen is made in childhood. CAH causes excess androgens throughout life, leading to rapid growth, but an early closure of the bone growth plates: a very short adult. Also: early puberty, with menstrual problems (and poor sperm production in males).
- The other hormones, aldosterone and cortisol, need to be replaced. The cortisol replacement calms the ACTH activity, leading to less androgen production.
CAH is the most frequent cause
of non-standard genitals in
genetically female (XX) children.
Some Environmental Causes
- Progestin-induced virilization. Progestin was used to prevent miscarriages in the 1950’s and 60’s. Related to this is the use of androgens to treat endometriosis during that time period, and occasional accidental use of androgens. 160 known cases.
- XX fetuses develop as normal females with functioning ovaries, but they may develop some male secondary characteristics and often have enlarged clitorises. Effects are very similar to CAH.
- Freemartin: usually seen in cattle: female and male twins, with testosterone from male leaking over to the female due to a shared placenta. Normal female appearance, but undeveloped ovaries and masculinized behavior. Rare or unknown in humans.
- Aldous Huxley’s book Brave New World has human freemartins created by hormone treatment of fetuses.
And Lots More…
- We have covered the main causes of variation in human biological sex. However, there are many other, rarer conditions that also affect this.
- As always with biology, there are exceptions to every rule, exceptions to everything I said here today.
- What any individual feels about their body, and how society reacts to these variations, is more a matter for psychology and sociology than for biology.