Sex Determination

Genetic Sex Determination Overview

Lecture Title: Genes to Genomes

Version: 1.25.S 2025Designers: J. Fonta 2024, Revised by M. J. Michelsohn 2025

Review of Previous Material (Unit I Slide 126)

• Incomplete Dominance/Codominance: Genetic phenomena where heterozygous phenotypes exhibit a blend or both traits respectively.

• Multiple Allelism: Presence of more than two alleles at a genetic locus, contributing to diversity in traits (e.g., ABO blood types).

• Lethal Alleles: Certain combinations of alleles that can prevent the survival of an organism, influencing population genetics.

• Epistasis: Interactions between genes that result in the masking or altering of the expression of other genes.

• Polygenic Traits: Traits controlled by multiple genes, leading to a range of phenotypes (e.g., skin color, height).

• Pleiotropism: Single genes affecting multiple traits, demonstrating the complexity of genetic function.

• Homozygous sex-linkage: Situations where certain traits are directly linked to sex chromosomes, influencing inheritance patterns.

• Extranuclear Inheritance: Transmission of genes located outside the nucleus, such as those in mitochondria or chloroplasts.

• Gene × Environment Interactions: How environmental factors influence the expression of genetic traits.

Overview of New Material (Unit I Slide 127)

Topics Covering:

• Evolution of Sexual Reproduction: Examines how sexual reproduction has evolved as a strategy for genetic diversity and adaptability in changing environments.

• Environmental Systems: Studies how different environments can alter sex determination mechanisms across species.

• Chromosomal Systems: Overview of chromosomal structures that determine sex, including variations and anomalies.

• Aneuploidy: The implications of having an abnormal number of chromosomes and its consequences in development.

• Polyploidy: Discusses the prevalence of polyploid organisms in nature and their evolutionary advantages.

• Chromosomal Mutations: Types of chromosomal mutations and their impact on genetic diversity and evolution.

The Paradox of Sex (Unit I Slide 128)

• Cost of Sexual Reproduction: 50% of offspring (typically males) cannot directly reproduce, raising questions about the efficiency of sexual reproduction.

• Asexual Lineages: Theoretically, they could double their population growth rate compared to sexual lineages, prompting investigation into evolutionary advantages.

• Relatedness: Offspring from sexual lineages share only half the genetic material from each parent compared to asexual reproducing offspring, impacting evolutionary dynamics.

• Muller’s Ratchet: A theory illustrating the accumulation of mutations in asexual populations leading to a decline in fitness over generations.

• Recombination Advantage: Sexual reproduction may reduce the accumulation of harmful mutations, enhancing population resilience and adaptability.

Evolution of Sexual Differentiation (Unit I Slide 129)

• Isogamy: The ancestral reproductive state characterized by uniform gamete size.

• Anisogamy: Unequal-sized gametes evolved to maximize reproductive success; males produce smaller gametes, while females produce larger ones.

• Oogamy: A specific type of anisogamy where the female gamete is non-motile, and the male gamete is motile, prevalent in both animals and certain plant groups.

• Primary Sexual Characteristics: Traits associated directly with gamete production, such as the development of ovaries in females or testes in males.

Environmental Systems (Unit I Slide 130)

• Hormonal Influence on Development: Hormones and environmental factors shape sexual phenotype via gene-environment interactions.

• Temperature-dependent Sex Determination: Example seen in the Green Sea Turtle (Chelonia mydas), where temperature during incubation influences hatchling sex ratios.

• Behavior-dependent Sex Determination: For instance, in the Red Sea Clownfish (Amphiprion bicinctus), social structures and behaviors dictate sex roles and transitions.

• Parasite-dependent Sex Determination: In species like Tribolium madens (Black Flour Beetle), the presence of the Wolbachia parasite can skew sex ratios through host manipulation.

Chromosomal Systems XX/XY Sex (Unit I Slide 131)

• Homogametic Sex (XX): Produces larger, non-motile ova or eggs.

• Heterogametic Sex (XY): Produces smaller, motile sperm.

• Y Chromosome Role in Homo sapiens: Presence triggers male differentiation by initiating hormones leading to male-typical gamete formation.

• Absence of Y Chromosome: Results in female-sex traits being activated through hormonal cascades.

The SRY Gene (Unit I Slide 132)

• Function: Encodes for testis-determining factor (TDF), critical for male sex differentiation during early embryonic development.

• Impact of SRY Levels: Elevated TDF levels correlate with the development of male phenotype, while lower levels correlate with female phenotype development.

• Translocation of SRY: Can lead to the development of a male phenotype in individuals who are genetically female (XX).

Chromosomal Systems ZW/ZZ Sex (Unit I Slide 133)

• ZW System: In species like birds (Aves), the ZW configuration defines the heterogametic sex (females) producing large eggs.

• ZZ System: Males are ZZ and produce smaller gametes.

Chromosomal Systems XX/X0 and ZZ/Z0 (Unit I Slide 134)

• XX/X0 System: Presence of two X chromosomes (XX) leads to female development, while a solitary X (X) leads to male development.

• ZZ/Z0 System: Homologous chromosomes (ZZ) yield male offspring, while a single Z (Z) leads to female development.

Chromosomal Systems Haplodiploidy (Unit I Slide 135)

• Diploid Offspring: Develop into females (worker bees), while haploid offspring develop into males (drones).

• Environmental Influence: Some diploid offspring can develop in response to environmental stimuli, such as nutrition (e.g., royal jelly for queen bees).

Chromosomal Systems Polygenic Sex (Unit I Slide 136)

• Danio rerio (Zebrafish): Involvement of dmrt1 and cyp21a2 genes, demonstrating complex genetic pathways in sex determination.

• dmrt1 Gene: Functions as a transcription factor, influencing sexual development across various taxa.

• cyp21a2 Gene: Regulates the synthesis of corticosteroid hormones, further influencing sex differentiation.

Chromosomal Systems Mating Types (Unit I Slide 137)

• Mating-Type Genes: Determine the types of gametes produced; numerous alleles contribute to high genetic variation.

• Fusion Requirement: Successful fusion occurs only between gametes of differing mating types (heterozygous zygote).

• Example: In Schizophyllum commune, three mating-type genes can produce over 23,000 distinct mating types, enhancing genetic diversity.

Dosage Compensation (Unit I Slide 138)

• Reduced Transcription in Drosophila melanogaster: Females (XX) experience a halved transcription rate of the X chromosome compared to males (XY).

• Random Chromosome Inactivation in Homo sapiens: X-inactivated chromosomes form Barr bodies to balance expression between sexes.

• Partial Silencing in Gallus gallus domesticus: Portions of the ZZ chromosome may undergo silencing to achieve dosage balance.

Aneuploidy (Unit I Slide 139)

• Definition: Pertains to the presence of an abnormal number of chromosomes affecting normal development and functioning.

• Nondisjunction Mechanism: The failure of homologous chromosomes to separate during meiosis leads to the creation of gametes with atypical chromosome numbers.

• Zygote Results: Nondisjunction can create monosomic (2n-1) or trisomic (2n+1) conditions, significantly affecting viability and health.

Down Syndrome in Homo sapiens (Unit I Slide 140)

• Condition: Result of Trisomy 21, where the triple presence of chromosome 21 alters gene expression and phenotype, leading to various health issues.

• Parental Age Correlation: Increased probabilistic odds of nondisjunction occur with advancing maternal age due to shuffling mechanisms during meiosis.

Polyploidy (Unit I Slide 141)

• Definition of Euploidy: Refers to a complete set of chromosomes; used to categorize organisms based on chromosome number.

• Polyploidy Significance: The condition of having multiple complete sets can facilitate hybrid vigor and speciation.

• Types: Autopolyploidy involves duplication of chromosomes within the same species while allopolyploidy results from hybridizations between different species.

Autopolyploidy in Hyla versicolor (Unit I Slide 142)

• Gray Treefrog: Normally, 2n Hyla chrysoscelis produces 1n gametes, yet nondisjunction events can yield 2n gametes.

• Result: Formation of tetraploid (4n) lineages as a consequence of diploid gamete fusion.

Allopolyploidy in Triticum (Unit I Slide 143)

• Wheat Hybridization: Occurs as hybridization of wild diploids produces tetraploid lineages with increased genetic variability.

• Further Hybridization: Tetraploids may undergo additional hybridizations yielding hexaploid species; these lead to resilience and adaptability in agricultural environments.

Partial Chromosomal Mutations (Unit I Slide 144)

• Types of Mutations:

  • Deletion: Complete loss of genetic material, impacting gene function.

  • Duplication: Addition of genetic material, which can lead to genetic redundancy or novel functions.

  • Translocation: Movement of genetic regions to novel locations affecting gene expression patterns.

  • Inversion: Rearrangement of gene order which can alter genetic regulation.

Study Review (Unit I Slide 146)

Key Vocabulary:

• Homogametic sex: The sex that produces identical gametes (e.g., XX in human females, ZZ in male birds).

• Heterogametic sex: The sex that produces two different types of gametes (e.g., XY in human males, ZW in female birds).

• SRY (Sex-determining Region Y): A gene on the Y chromosome that triggers male development in mammals.

• Dosage compensation: A mechanism to equalize gene expression between sexes despite differences in sex chromosome number (e.g., X-inactivation in mammals).

• Chromosome inactivation: The silencing of an entire chromosome, such as X-inactivation in female mammals to balance X-linked gene expression.

• Aneuploidy: The presence of an abnormal number of chromosomes, such as trisomy 21 (Down syndrome) or monosomy X (Turner syndrome).

• Nondisjunction: A failure of chromosomes to separate properly during meiosis or mitosis, leading to aneuploidy.

• Polyploidy: The condition of having more than two sets of chromosomes (common in plants, rare in animals).

• Autopolyploidy: Polyploidy resulting from chromosome duplication within a single species.

• Allopolyploidy: Polyploidy resulting from hybridization between two different species, leading to a new viable species.

Review Questions:

• Does the heterogametic sex always produce a smaller gamete than the homogametic sex?

The heterogametic sex ZW produces a large gamete, female

The homogametic sex ZZ produces a small gamete

The ZW-ZZ system is prevalent in Aves (Birds)

• What phenotype usually results from the presence of SRY gene products?

The SRY gene triggers testis development, leading to a male phenotype in mammals. It activates genes that direct gonads to become testes, which then produce testosterone and anti-Müllerian hormone (AMH), promoting male traits and suppressing female reproductive structures.

• How can chromosome inactivation lead to a mosaic phenotype?

In female mammals, one X chromosome is randomly inactivated in each cell (X-inactivation). This creates a mosaic pattern because some cells express genes from the maternal X and others from the paternal X.

Example: Calico cats have orange and black fur patches because the X-linked coat color geneis expressed differently in different cells.

• Describe the differences between autopolyploidy and allopolyploidy.