Chapter 7: Sex Determination and Sex Chromosomes

Comprehensive vocabulary flashcards covering the genetic, chromosomal, and environmental mechanisms of sex determination, sex-chromosome aneuploidies, dosage compensation, and X-inactivation.

Biological Sex

The anatomical and physiological sexual phenotype of an organism, fundamentally characterized in most organisms by gamete size, where males produce small gametes and females produce relatively large gametes.

Gender

A category assigned by an individual or others based on cultural practices and behavior, which does not necessarily coincide with biological sex.

Monoecious

An organism that possesses both male and female reproductive structures within the same individual (exhibiting hermaphroditism).

Dioecious

An organism that possesses either male or female reproductive structures individually, requiring an individual of the opposite sex to reproduce.

Heteromorphic Chromosomes

Chromosome pairs that are visibly dissimilar in size, structure, and genetic composition (such as the X and Y sex chromosomes), unlike autosomes.

Protenor Mode of Sex Determination

An $XX/XO$ mechanism where females possess two X chromosomes ($XX$) and males possess only one X chromosome ($XO$), depending on the random segregation of the single X chromosome into half of the male gametes.

Lygaeus Mode of Sex Determination

An $XX/XY$ sex-determining system where female gametes uniformly carry an X chromosome and male gametes carry either an X or a Y chromosome, yielding $XX$ females and $XY$ males.

Homogametic Sex

The sex that produces gametes carrying identical sex chromosomes, such as $XX$ human females or $ZZ$ male birds.

Heterogametic Sex

The sex that produces gametes carrying unlike sex chromosomes, such as $XY$ human males or $ZW$ female birds.

Genic Sex Determination

A system of sex determination observed in some plants, fungi, protozoans, and fishes (such as Danio rerio) where sex is dictated by multiple genes on undifferentiated chromosomes rather than morphologically distinct sex chromosomes.

Environmental Sex Determination

A system where an organism's biological sex is dictated by non-genetic external factors, such as physical position within a stack (e.g., Crepidula fornicata) or incubation temperature.

Klinefelter Syndrome

A human genetic disorder caused by nondisjunction in meiosis, resulting in individuals with male genitalia, rudimentary testes that fail to produce sperm, tall stature, enlarged breasts, and a karyotype containing more than one X chromosome (most commonly $47,XXY$).

Turner Syndrome

A human genetic condition caused by nondisjunction in meiosis, resulting in phenotypically female individuals with a $45,X$ karyotype, characterized by rudimentary ovaries, underdeveloped breasts, short stature, and sterility.

Triplo-X Syndrome

A condition in females possessing three X chromosomes ($47,XXX$) with a normal set of autosomes, frequently resulting in a normal phenotype with taller-than-average stature, or occasionally secondary sex characteristic underdevelopment.

Jacob's Syndrome

A human condition characterized by a $47,XYY$ karyotype, where males are consistently over $6\text{ feet}$ tall and have an elevated risk for comorbidities such as speech/learning disabilities and seizure disorders.

Pseudoautosomal Regions (PARs)

Homologous regions located at both ends of the human X and Y chromosomes that pair, synapse, and recombine during male meiosis, which is essential for proper chromosome segregation.

Male-Specific Region of the Y (MSY)

The largely non-recombining portion of the human Y chromosome (comprising $23\text{ million}$ base pairs) divided into X-transposed ($15\%$), X-degenerative ($20\%$), and ampliconic ($30\%$) regions.

Sex-Determining Region Y (SRY)

A gene located in the X-degenerative region of the MSY adjacent to the short arm PAR that triggers male sexual development by encoding testis-determining factor.

Testis-Determining Factor (TDF)

A protein encoded by the $SRY$ gene that activates at $6\text{--}8\text{ weeks}$ of embryonic development in $XY$ embryos, directing the bipotential gonad to develop into testes which secrete testosterone and anti-Müllerian hormone.

Bipotential Gonads

Embryonic gonadal ridges associated with embryonic kidneys that arise by the fifth week of human gestation and have the capacity to develop into either ovaries (from the outer cortex) or testes (from the inner medulla).

Müllerian Ducts

Embryonic ducts that persist and differentiate into the female reproductive tract (oviducts, uterus, and upper vagina) in the absence of a Y chromosome, but degenerate in males due to embryonic testicular hormones.

Wolffian Ducts

Embryonic ducts that differentiate into male internal structures (epididymis, vas deferens, and seminal vesicles) in response to testosterone, but degenerate in females.

Androgen-Insensitivity Syndrome (AIS)

An X-linked condition in genetically male ($46,XY$) individuals caused by a defective androgen receptor, resulting in female external secondary sex characteristics and a blind-ending vagina despite possessing intra-abdominal testes producing normal male testosterone levels.

Paternal Age Effects (PAE)

The collective increased risk of offspring congenital disorders, cancers, autism, and schizophrenia linked to advanced paternal age, driven by age-dependent mutations and microdeletions that are positively selected in male sperm.

Primary Sex Ratio (PSR)

The proportion of males to females conceived at fertilization in a population, determined experimentally to be equal ($1.0$).

Secondary Sex Ratio

The proportion of males to females actually born in a population, which exhibits a male bias due to higher female embryonic and fetal mortality.

Dosage Compensation

A mechanism that balances the expression levels of X-linked genes between males ($XY$) and females ($XX$) to prevent excessive protein production in the homogametic sex.

Barr Body

A densely condensed, transcriptionally inactive sex chromatin body formed by an inactivated X chromosome in mammalian somatic cell nuclei, following the $N - 1$ rule where $N$ is the total number of X chromosomes.

Lyon Hypothesis

The principle formulated by Mary Lyon stating that one of the two X chromosomes in mammalian female somatic cells is randomly, permanently inactivated early in embryonic development, creating a mosaic expression pattern.

X-Inactivation Center (Xic)

A $1\,\text{Mb}$ major regulatory region located near the centromere of the mammalian X chromosome that controls the counting, choice, and execution of X-chromosome inactivation.

X-Inactive Specific Transcript (XIST)

A gene within the Xic locus that produces a structural long noncoding RNA (lncRNA) that physically coats and epigenetically silences the inactive X chromosome.

Genetic Balance Theory

A mode of sex determination in Drosophila melanogaster stating that sexual differentiation is dictated by the ratio of X chromosomes to sets of autosomes ($X:A$), where a ratio of $1.0$ produces females and $0.5$ produces males, independent of the Y chromosome.

Sex-Lethal (Sxl)

The master switch gene in Drosophila that controls four autosomal genes to induce female differentiation when activated by an $X:A$ ratio of $1.0$.

Temperature-Dependent Sex Determination (TSD)

An environmental mechanism seen in crocodilians, turtles, and lizards where the incubation temperature of eggs during embryonic development determines the sexual phenotype of the hatchlings.

Pivotal Temperature ($T_p$)

The narrow incubation temperature threshold range (varying by $1\,^\circ\text{C}$ to $5\,^\circ\text{C}$) at which a $1:1$ ($50\%$) sex ratio of males to females is produced in reptiles with temperature-dependent sex determination.