BIOL80A Final | Sex Determination & Development

what are sex chromosomes?

the x and y chromsomes that generally determine mammalian sex

how many distinct chromosomes do humans technically have?

24 (22 autosomes, one X, and one Y).

what does a haploid genome (sperm or egg) contain?

a complete set of 22 autosomes and either an X or a Y chromosome (23 total)

what does a diploid genome contain?

a pair of each autosome and a pair of “sex chromosomes” (46 total)

what sex are individuals with an XX chromosome pair?

female

what sex are individuals with an XY chromosome pair?

male

what is a nondisjunction?

a rare meiotic event that can produce atypical chromosomal combinations (chromosomal intersex)

what is XXY (klinefelter syndrome)?

phenotypically male; can present with small testicles, hypogonadism, infertility, and neurocognitive impacts

what is XYY (Jacobs syndrome)?

phenotypically male; often mild/undiagnosed; can present with tall stature, large testes/head, and higher risk for conditions like asthma, autism, and seizures

what is XO (turner syndrome)?

phenotypically female; can cause fetal developmental issues, short stature, webbed neck, forearm deformity, ocular issues, and increased risk for autoimmune disorders

what are sex chromosomes?

one, often simplistic, way to assign biological sex

what is biological sex?

a spectrum that is less binary than many believe or understand

what are intersex conditions?

often categorized as disorders (or differences) or sexual development (DSDs).

what are DSDs (disorders/differences of sexual development)?

conditions that can lead to an individual’s genetic, anatomical and hormonal phenotype not aligning with binary biological male or female characteristics

how common is it for people to have intersex conditions?

depending on the definition, could be up to 2% of the population

what are the associated issues of DSDs?

can be associated with other health issues in individuals

what is a complete androgen insensitivity syndrome (CAIS)?

a condition in a genetically XY individual where the body is completely insensitive to androgens (like testosterone) due to genetic mutations in androgen receptors

what are the characteristics of a genetically XY individual with CAIS?

their external genitals appear female, but female sex organs (ovaries, fallopian tubes, or uterus) do not develop. often diagnosed after puberty due to amenorrhea (lack of menstruation)

what is partial androgen insensitivity syndrome (PAIS)?

a condition with variable symptoms, where genitals may appear neither fully male nor female, or more male, and individuals are often infertile

what is congenital adrenal hyperplasia (CAH)?

a condition where the adrenal glands cannot properly synthesize hormones (cortisol, aldosterone, androgens) due to genetic mutations in an important enzyme

what does CAH in XX individuals look like?

results in the production of excessive testosterone. individuals are born with ambiguous genitalia (enlarged clitoris, fused labia) but have typical female internal reproductive organs

what are secondary sex characteristics in XX individuals with CAH like?

they can develop male secondary sex characteristics at puberty, such as facial hair and a deepening voice

what does a milder form of CAH look like?

can lead to male-like facial and body hair, irregular periods, fertility problems, or sometimes no obvious symptoms at all, highlighting variability

what is the cause of 5-alpha reductase deficiency?

a genetic mutation that leads to very low levels of dihydrotestosterone (DHT)

what is the function of DHT?

it is important for the detailed development of external genitalia in XY individuals

what is the function of the 5-alpha reductase enzyme?

it helps convert testosterone to DHT

what are the physical characteristics of 5-alpha reductase deficiency at birth?

underdeveloped or ambiguous external genitalia (in severe cases, appearing female), with most internal reproductive structures appearing male

what physical changes occur during puberty for individuals with 5-alpha reductase deficiency?

increased testosterone drives voice deepening, muscle mass increase, penis enlargement, and partial descending of the testes

in which country is there a community with a high incidence of this deficiency?

the dominican republic (incidence of 1 in 50 births).

what is the local name for individuals in the dominic republic with this condition?

guevedoces (aka “penis at 12"; or locally as machihembras)

what is the colloquial name for the condition in the dominican republic

“penis at 12 syndrome,” where girls appear to “turn into boys” during adolescence

what are primordial gonads?

structures that begin to develop about one month after fertilization and can develop into either testes or ovaries

when do the primordial gonads begin to develop?

approximately one month after fertilization of the egg.

what is bipotential?

the state where male and female reproductive tissues develop similarly in utero (until about the seventh week of gestation) and have the potential to become either male or female structures

when does the reproductive system start to differentiate into male or female sex organs?

around the seventh week of gestation, based on the presence or absence of specific chemical prompts

what determines whether the primordial gonads develop into testes?

the presence and release of testosterone from the gonads at around seven weeks of development

what determines whether the primordial gonads develop into ovaries?

the absence of testosterone (or other chemical prompting)

without any chemical prompting, into which sex would all fertilized eggs develop?

females

what is the SRY gene (sex-determining region of the Y chromosome)?

a single gene located on the Y chromosome responsible for the development of a male

what is the function of the SRY gene?

it acts as a transcription factor, activating other genes that promote the development of the testes and testosterone production, while suppressing genes for female development

why do females not develop testes?

females do not have a Y chromosome and therefore lack the SRY gene, which is required to initiate male development

when does reproductive development show little change?

between infancy and puberty, there is little change in the reproductive system for both males and females

what is puberty?

the stage of development at which individuals become sexually mature

do males and females have different outcomes of puberty?

yes, the physical outcomes differ, but the hormonal control mechanisms are similar

what is the h-p-g axis?

the hypothalamo-pituitary-gonadal axis— a coordinated release of hormones from the hypothalamus, anterior pituitary, and gonads that drives reproductive maturation and secondary sex characteristics

what does the h-p-g axis promote?

maturation of the reproductive systems and the development of secondary sex characteristics

what is GnRH?

gonadotropin-releasing hormone; released by the hypothalamus to stimulate the pituitary

what is LH?

luteinizing hormone; stimulates sex hormone production and ovulation/sperm production processes

what is FSH?

follicle-stimulating hormone; promotes gamete development (follicles in ovaries, sperm in testes).

what is estrogen?

a primary female sex hormone involved in reproductive development and secondary sex characteristics

what is testosterone?

a primary male sex hormone involved in reproductive development and secondary sex characteristics

what factors can affect the age of onset of puberty?

genetics, environment, and psychological stress

what is typically the first sign of puberty in males?

enlargement of the testes, growth and pigmentation of the scrotum, and growth of the penis

what is usually the first sign of puberty in females?

development of breast tissue

what is menarche?

the onset of menstruation

male secondary sex characteristic: voice changes

increased larynx size and deepening of the voice

male secondary sex characteristic: muscular changes

increased muscular development

male secondary sex characteristic: hair growth

growth of facial, axillary, and pubic hair, and increased overall body hair

female secondary sex characteristic: fat distribution

deposition of fat predominantly in the breasts and hips

female secondary sex characteristic: breast development

growth and maturation of breast tissue

female secondary sex characteristic: skeletal changes

broadening of the pelvis

female secondary sex characteristic: hair growth

growth of axillary and pubic hair