Sex Determination & Chromosomes

Vocabulary flashcards covering sex chromosomes, primary and secondary sexual differentiation, various organismal sex-determination systems, aneuploidies, dosage compensation, and evolutionary sex-ratio mechanisms.

Sex Chromosomes

Chromosomes that differ in morphology and/or number between males and females, playing a primary role in sex determination.

Primary Sexual Differentiation

The developmental process that involves the formation of gonads where gametes are produced.

Secondary Sexual Differentiation

The developmental process that involves the overall phenotype, including accessory reproductive organs and external sexual characteristics.

Heterogametic Sex

The sex of a species that produces two different types of gametes with respect to sex chromosomes (e.g., $XY$ males or $ZW$ females).

Haldane's Rule

The evolutionary principle stating that if one sex is sterile, absent, or defective in species hybridizations, it is almost always the heterogametic sex.

Intersex

A condition describing individuals with intermediate or atypical combinations of male and female sexual characteristics.

Hermaphroditic

An organism that possesses functional reproductive organs of both sexes, either simultaneously or sequentially.

Isogamy

A reproductive state in which fusing gametes are morphologically identical in size and shape, governed by mating types such as $mt^+$ and $mt^-$ in Chlamydomonas.

Pistillate Mutation (Maize)

A loss-of-function mutation in a gene required for stamen development or male organ suppression that converts a monoecious maize plant into a unisexual female plant forming only female flowers.

X-to-Autosome Ratio ($X:A$) in Drosophila

The chromosomal balance system where sex is determined by the ratio of $X$ chromosomes to autosome sets ($1.0$ produces a female, $0.5$ produces a male), with the $Y$ chromosome functioning strictly as a fertility factor.

Sex Determination in Caenorhabditis elegans

A sex determination system governed by the total number of $X$ chromosomes, where $XX$ individuals develop as hermaphrodites and $XO$ individuals develop as males.

Bilateral Gynandromorph

An individual displaying male characteristics on one half of the body and female characteristics on the other half, resulting from mitotic nondisjunction or loss of an $X$ chromosome during the first embryonic cleavage division.

$ZZ/ZW$ Sex-Determination System

A chromosomal sex-determination mechanism where females are heterogametic ($ZW$) and males are homogametic ($ZZ$), commonly observed in birds and butterflies.

Four-Sexes Mating System (White-Throated Sparrow)

An evolutionarily transient mating system maintained by a large chromosomal inversion coupled with plumage morphs ($tan$ and $white$) that strictly enforces disassortative cross-morph mating.

Temperature-Dependent Sex Determination ($G \times E$)

A sex-determination system in which incubation temperature dictates sex ratios according to a thermal threshold that depends on the species' specific genetic background interacting with the environment.

Sequential Hermaphroditism

A life-history pattern where an individual changes its sex once during its lifetime, such as clownfish transitioning from male to female based on social hierarchy.

Simultaneous Hermaphroditism

A sexual condition in which an organism possesses functional reproductive organs of both male and female sexes at the same time.

Arrhenotokous Haplodiploidy

A sex-determination system characteristic of Hymenoptera in which fertilized diploid eggs develop into females and unfertilized haploid eggs develop into males, causing males to have no fathers and no heterozygous loci.

Platypus Sex Chromosome Chain

A multichromosome arrangement consisting of $10$ sex chromosomes ($5$ $X$ and $5$ $Y$) that form an alternating multivalency chain segregating together during meiosis, representing an exception to the law of independent assortment.

Turner Syndrome

A human condition caused by an $X$ ($XO$) monosomy karyotype, resulting in a sterile female phenotype with short stature.

Triple X Syndrome

A human condition caused by an $XXX$ karyotype, characterized by a female phenotype that is often normal with a statistically elevated risk of mild learning delays.

Jacobs Syndrome

A human condition caused by an $XYY$ karyotype, resulting in a fertile male phenotype typically associated with tall stature.

Klinefelter Syndrome

A human condition caused by an $XXY$ karyotype, resulting in a sterile male phenotype often displaying gynecomastia.

$Y$ Monosomy

A lethal human sex chromosome condition lacking an $X$ chromosome, which fails to develop because the $X$ chromosome carries essential housekeeping genes.

Incomplete Dosage Compensation

The phenomenon evidenced by increasing severity of intellectual disability and physical abnormalities in $XXXX$ and $XXXXX$ individuals, demonstrating that mammalian $X$-inactivation is not $100\%$ efficient.

Barr Body

A condensed, transcriptionally inactive $X$ chromosome formed in mammalian cells to equalize $X$-linked gene dosage between sexes.

Dosage Compensation Mechanisms Across Species

Species-specific strategies to balance $X$ gene expression: random $X$-inactivation in human females, $2\times$ hypertranscription of the $X$ in male Drosophila, and $0.5\times$ downregulation of both $X$ chromosomes in C. elegans hermaphrodites.

Calico Fur Mosaicism

A patchy black-and-orange coat phenotype in female cats produced by random and permanent embryonic $X$-chromosome inactivation in heterozygous lineages.

Male Calico Karyotype

An $XXY$ karyotype in male cats that provides the two different $X$-linked coat color alleles necessary for calico coloring, serving as an animal analog to human Klinefelter syndrome.

Bipotential Gonadal Ridges

Undifferentiated embryonic gonadal tissues that retain the structural capacity to develop into either testes or ovaries depending on genetic and hormonal signals.

SRY Gene

A transcription factor gene located on the $Y$ chromosome (Sex-determining Region Y) that serves as the master genetic switch specifying male development in mammals.

Secondary Sex Ratio

The male-to-female ratio at birth in humans, skewed toward males at approximately $105 - 108:100$ due to faster swimming of lighter $Y$-bearing sperm, offset by elevated male mortality during gestation and infancy.

Fisher's Principle

An evolutionary model showing that natural selection stabilizes population sex ratios near $1:1$ through frequency-dependent reproductive advantages favoring the rarer sex.