Lecture slides Chapter 5

Sex Determination and Sex Chromosomes

Introduction to Reproductive Modes

Organisms exhibit a wide range of reproductive strategies and life cycles, which are fundamental to genetic diversity and evolution. Understanding these modes is essential for grasping the mechanisms of sex determination.

• Asexual reproduction: Some organisms reproduce without the involvement of gametes, resulting in offspring genetically identical to the parent.

• Sexual reproduction: Involves the fusion of gametes from two parents, increasing genetic variation.

• Alternation of modes: Certain species alternate between sexual and asexual reproduction depending on environmental conditions.

• Phenotypic dimorphism: In complex organisms, sexual differentiation is often visible as distinct male and female forms.

Sex Chromosomes and Sex Determination

Sex determination is the process by which organisms develop as male or female. In many species, this is governed by specific chromosomes known as sex chromosomes.

• Heteromorphic chromosomes: Chromosomes that differ in size and shape between sexes (e.g., XY in mammals).

• Sex chromosomes: Labeled as X and Y in many animals; these carry genes that influence sexual development.

• Genetic basis: Ultimately, genes located on sex chromosomes determine sex, not the chromosomes themselves.

Discovery of X and Y Chromosomes

The link between X and Y chromosomes and sex determination was established in the early twentieth century through studies in insects and other organisms.

• X-body: Nuclear structure observed in insect sperm, later identified as the X chromosome.

• Example (Butterfly Protenor):

  • Female somatic cells: 14 chromosomes (including two X chromosomes).

  • Male somatic cells: 13 chromosomes (one X chromosome).

  • Fertilization by X-bearing sperm produces female offspring; X-deficient sperm produces male offspring.

The Y Chromosome Determines Maleness in Humans

In humans, the presence of the Y chromosome is the primary determinant of male development.

• Human karyotype: 46 chromosomes (diploid number), with one pair differing between males and females.

• Sex chromosome configuration:

  • Females: XX

  • Males: XY

• Y chromosome: Its presence triggers male development.

Sex Chromosome Aneuploidies

Abnormal numbers of sex chromosomes can lead to distinct syndromes with characteristic phenotypes.

• Klinefelter Syndrome (47,XXY):

  • Individuals have two or more X chromosomes and one Y chromosome.

  • Features: Tall stature, long limbs, rudimentary testes, sterility, possible breast development.

• Turner Syndrome (45,X):

  • Individuals have only one X chromosome.

  • Features: Short stature, webbed neck, underdeveloped ovaries, normal intelligence.

• 47,XXX Syndrome (Triplo-X):

  • Three X chromosomes; usually female phenotype.

  • Most are phenotypically normal, but some may have developmental issues.

• 47,XYY Condition:

  • Extra Y chromosome; males are typically taller, may have subnormal intelligence.

Syndrome	Karyotype	Main Features
Klinefelter	47,XXY	Tall, sterile, some female traits
Turner	45,X	Short, webbed neck, underdeveloped ovaries
Triplo-X	47,XXX	Female, usually normal
XYY	47,XYY	Tall male, possible learning difficulties

Mechanisms of Sex Determination in Humans

Sexual differentiation in humans is a complex process involving genetic signals and embryonic development.

• Bipotential gonads: Early embryonic gonadal tissue can develop into either testes or ovaries.

• SRY gene: Located on the Y chromosome, encodes the testis-determining factor (TDF) protein, which initiates male development.

• Developmental pathway:

  • XY embryos: Medulla of gonadal ridge develops into testes.

  • Absence of Y: Cortex develops into ovaries; Müllerian ducts form female reproductive structures.

Structure and Function of the Y Chromosome

The Y chromosome contains regions critical for male development and shares homology with the X chromosome at its ends.

• Pseudoautosomal regions (PAR): Present at both ends of the Y chromosome; allow pairing and recombination with the X chromosome during meiosis.

• Male-specific region of Y (MSY): Non-recombining portion containing genes unique to males.

• SRY gene: Located within MSY; essential for initiating male sex determination.

Dosage Compensation and Barr Bodies

Dosage compensation ensures equal expression of X-linked genes in males and females, despite differences in X chromosome number.

• Barr body: Inactive X chromosome in female somatic cells, visible as condensed chromatin near the nuclear envelope.

• Lyon hypothesis: X inactivation is random in each cell; all descendant cells maintain the same inactive X.

• Mechanism: Inactivation involves chemical modification of DNA and histones, creating a heritable silenced state.

• X-inactivation center (Xic): Region on X chromosome containing the XIST gene, which is crucial for initiating inactivation.

Karyotype	Number of Barr Bodies
XX	1
XY	0
XXX	2
XXY	1
XYY	0

Formula for Barr bodies:

$\text{Number of Barr bodies} = \text{Number of X chromosomes} - 1$

Sex Determination in Drosophila and C. elegans

In some species, sex is determined by the ratio of X chromosomes to sets of autosomes, rather than the presence of a Y chromosome.

• Drosophila melanogaster: Sex determined by X:A ratio.

• C. elegans: Two sexual phenotypes: males (XO) and hermaphrodites (XX).

X Chromosomes	Autosome Sets	X:A Ratio	Sex
2	2	1.0	Female
1	2	0.5	Male
3	2	1.5	Metafemale
2	3	0.67	Intersex
3	3	1.0	Female

Formula for X:A ratio:

$\text{X:A ratio} = \frac{\text{Number of X chromosomes}}{\text{Number of sets of autosomes}}$

Temperature-Dependent Sex Determination (TSD) in Reptiles

In some reptiles, environmental temperature during embryonic development determines sex.

• TSD patterns:

  • Case I: Low temperatures produce females; high temperatures produce males.

  • Case II: Opposite of Case I.

  • Case III: Both low and high temperatures produce females; intermediate temperatures produce males.

• Aromatase: Enzyme that converts androgens to estrogens; its activity is influenced by temperature.

• Implications: TSD is characteristic of many nonmammalian vertebrates, such as turtles, crocodiles, and some lizards.

Case	Temperature	Sex Produced
I	Low	Female
I	High	Male
II	Low	Male
II	High	Female
III	Low/High	Female
III	Intermediate	Male

Example: In some turtle species, eggs incubated at lower temperatures develop into females, while those at higher temperatures develop into males.

Additional info: The notes have been expanded to include definitions, formulas, and tables for clarity and completeness, as well as academic context for mechanisms and examples.