Mechanisms and Evolution of Sex Determination

Comprehensive vocabulary flashcards detailing the genetic, environmental, and evolutionary mechanisms of sex determination discussed in the lecture.

Chromosomes

Rod-shaped packages of genetic material responsible for carrying hereditary traits from parent to offspring.

Gametes

Specialized reproductive cells—specifically eggs and sperm—that typically contain a half copy of a parent's genome and are genetically unique.

Aristotle's Hypothesis of Sex Determination

The ancient belief that males were formed by excess heat and dryness from the father during intercourse, whereas females resulted from excess cold and wetness from the mother.

Nettie Stevens

A scientist at Bryn Mawr College who published a breakthrough paper in 1905 demonstrating that a specific small chromosome (later designated the Y chromosome) determined sex in mealworms.

Mealworm Chromosomal Distribution

The pattern identified by Nettie Stevens in 1905 where female cells contain 20 large chromosomes (eggs having 10 large), while male cells contain 19 large chromosomes and one small chromosome (sperm having either 10 large, or 9 large and one small).

XX/XY Sex-Determination System

A chromosomal system used by humans, most mammals, and many insects where females inherit two X chromosomes ($\text{XX}$) and males inherit one X and one Y chromosome ($\text{XY}$), making the sperm the determinant of biological sex.

ZW Sex-Determination System

A chromosomal system found in some birds and reptiles where males have two Z chromosomes ($\text{ZZ}$) and females have one Z and one smaller W chromosome ($\text{ZW}$), meaning the egg determines the sex of the offspring.

Platypus Sex Chromosomes

A unique mammalian system consisting of 10 sex chromosomes—ten X chromosomes in females and five pairs of Xs and Ys in males—which form a chain during sperm production to yield sperm with either five Xs or five Ys.

Temperature-Dependent Sex Determination

An environmental mechanism seen in crocodilians, sea turtles, and certain fish where the ambient incubation temperature during development determines whether male or female genes are expressed.

European Pond Turtle

A reptile species whose sex is governed by incubation temperature: egg incubation above 30^\text{ }^\frown\text{C} (read as $30^\text{ }\text{degrees Celsius}$) produces all females, below $25^\text{ }\text{degrees Celsius}$ produces all males, and intermediate temperatures produce an equal sex ratio.

Marine Worm Sex Determination

An environmental sex-determining strategy where released larvae become females if they settle on the sea floor, but develop into sperm-producing males if they re-enter the mother's body.

Haplodiploidy

A sex-determination system utilized by over $200{,}000$ species of ants, bees, and wasps where unfertilized eggs develop into males, and fertilized eggs containing paternal genetic material develop into females.

Master Sex-Determining Gene

A pivotal gene carried on a sex chromosome (such as Y or W) that regulates the development of reproductive traits in an organism.

SRY Gene

The master sex-determining gene located on the human Y chromosome that regulates the development of male reproductive traits.

Meiosis

The specialized cell division process that produces gametes, involving chromosome duplication followed by chromosome pairing and genetic recombination.

Sex Chromosome Degradation

The evolutionary process wherein a chromosome carrying a master sex-determining gene stops recombining during meiosis, leading it to accumulate mutations, shed genetic material, and shrink over generations.

Mole Voles

A group of rodents whose genomes lack a Y chromosome entirely, with both sexes typically carrying only a single X chromosome while other genomic regions govern sex determination.