Ch. 7: Sex Determination and Sex Chromosomes

Primary sexual differentiation

The differentiation that involves only the gonads, where gametes are produced.

Secondary sexual differentiation

The differentiation that involves the overall appearance of the organism.

Bisexual (Monoecious or Hermaphroditic)

Individuals that contain both male and female reproductive organs and can produce both male and female gametes.

Unisexual (Dioecious or Gonochoric)

Individuals that contain only male or female reproductive organs.

Heteromorphic chromosomes

Dissimilar chromosomes, exemplified by the sex chromosomes X and Y

Protenor mode of sex determination (XX/XO)

Sex determination mode (e.g., in butterflies) where two X chromosomes result in a female offspring and one X chromosome results in a male offspring, depending on the random distribution of the X chromosome into half of the male gametes.

Lygaeus mode of sex determination (XX/XY)

Sex determination mode (e.g., in the milkweed bug) where female gametes have one X chromosome, and male gametes have either an X or Y chromosome.

Homogametic sex

The sex producing like chromosomes (e.g., XX individuals resulting in female offspring in the Lygaeus mode). In chickens, the male is the homogametic sex (ZZ).

Heterogametic sex

The sex producing unlike chromosomes (e.g., XY individuals resulting in male offspring in the Lygaeus mode). In chickens, the female is the heterogametic sex (ZW).

Nondisjunction (Sex Chromosome)

The failure of X chromosomes to segregate during meiosis, which causes human abnormalities like Klinefelter and Turner syndromes.

Klinefelter Syndrome (47,XXY)

An abnormality resulting from nondisjunction, where individuals have male genitalia but more than one X chromosome. Phenotype includes being tall, long arms/legs, large hands/feet, rudimentary testes (fail to produce sperm), and often enlarged breasts/rounded hips (feminine development).

Turner Syndrome (45,X)

An abnormality resulting from nondisjunction, where individuals are phenotypically female (X_). Phenotype includes rudimentary ovaries, underdeveloped breasts, short stature, and cognitive impairment.

Jacob’s Syndrome (47,XYY)

The 47,XYY condition; the only consistently shared characteristic is that males are over 6 feet tall, sometimes associated with subnormal intelligence and personality disorders.

Y Chromosome: SRY (Sex-Determining Region of the Y)

A single gene within the Y chromosome that encodes the testis-determining factor (TDF), a protein that triggers testes formation, active at 6–8 weeks of development. The Y chromosome determines maleness in humans.

Y Chromosome: PARs (Pseudoautosomal Regions)

Present on both ends of the Y chromosome, sharing homology with regions on the x chromosome, allowing them to synapse and recombine with the X during meiosis

Y Chromosome: MSY (Male-Specific Region of the Y)

Contains the SRY gene and consists of three regions: the X-transposed region (15%), X-degenerative region (20%), and the Ampliconic region (30%), which encodes proteins specific to testis development and function.

Sex Ratio

The actual proportion of male to female offspring.

Primary Sex Ratio

Reflects the proportion of males to females conceived in a population.

Secondary Sex Ratio

Reflects the proportion of each sex born. The ratio in humans is not 1.0 (1:1); worldwide census data suggests more males are conceived than females (e.g., 1.06 ratio in US Caucasians).

Dosage Compensation

A genetic mechanism that balances the dose of X chromosome gene expression in males and females, preventing excessive expression of X-linked genes.

Barr Bodies

An inactive X chromosome that is highly condensed and observed as a darkly stained body in interphase nerve cells. Dosage compensation follows the N − 1 rule (N = total number of X chromosomes).

Lyon Hypothesis

States that X-inactivation occurs randomly in somatic cells early in embryonic development. Once deactivated, all descendant cells maintain the same X-inactivation. The Calico cat pattern is often used as an example of this random permanent inactivation.

Temperature-Dependent Sex Determination (TSD)

Environmental control of sex determination in many reptiles (e.g., crocodiles, most turtles, and some lizards), where the incubation temperature of eggs during embryonic development determines the sex.