L26 2025 - Genetics VI - Tagged

Page 1: Attendance

• Attendance record was taken.

Page 2: Overview of Lecture 2

• **Objectives: **

  • Complete discussion on the benefits of sexual reproduction.

  • Explore mammalian sex determination in developmental biology.

Page 3: Significance of Sexual Reproduction

• Genetic Diversity:

  • Sexual reproduction promotes genetic diversity, important for evolution.

  • Eukaryotic sex evolved once during the Last Eukaryotic Common Ancestor (LECA).

  • Asexual reproduction has emerged multiple times but leads to evolutionary dead-ends, lacking species diversification.

  • Query: Are there other benefits of sexual reproduction aside from evolutionary advantages?

Page 4: Benefits of Sex Beyond Speciation

• Short-term Advantages of Sexual Reproduction:

  • Offspring from sexual species exhibit genetic differences, affecting their susceptibility to diseases.

  • Varying susceptibility can lead to some individuals surviving plagues.

  • Example: Commercial bananas (Cavendish) are all clones, making them susceptible to Panama disease, which is affecting them globally.

Page 5: Developmental Biology of Sex

• Phases of Sexual Development:

  1. Sex Determination: Initial decision phase for male or female development.

  2. Sexual Differentiation: Actual execution of the determined sex developmental state.

Page 6: Examples of Sex Determination Systems

• Chromosomal Systems:

  1. XY System (Mammals): XX=female; XY=male. Heterogametic is male.

  2. ZW System (Birds): ZZ=male; ZW=female. Heterogametic is female.

  3. X0 System (Insects): XX=female; X=male. Heterogametic is male.

  4. Haploid/Diploid: Seen in bees.

• Environmental Systems:

  • Temperature-dependent sex determination in alligators and turtles.

  • Population density effects seen in certain fish and nematodes.

• Evolution of Sex Determination:

  • Sexual reproduction is an ancient, conserved mechanism; thus, multiple systems exist.

Page 7: Mammalian Sex Chromosomes

• Males are heterogametic (XY); females are homogametic (XX).

• Variations in autosomes exist among different mammal species.

Page 8: Structure of Mammalian Sex Chromosomes

• X and Y Chromosome Traits:

  • XX = female; XY = male.

  • Males have both X and Y chromosomes, while females have two X chromosomes.

  • The existence of this chromosomal difference sparks the creation of two distinct sexes.

Page 9: Function of the XY System

• Mechanism of XY Sex Determination:

  1. Klinefelter Syndrome (XXY): Phenotypically appears as male but is infertile.

  2. Turner Syndrome (X0): Phenotypically appears female but is infertile.

  • One X chromosome suffices for female development; one Y chromosome induces male characteristics.

  • Hypothesis: A dominant gene on Y may be responsible for male determination.

Page 10: Identification of Sex-Determination Genes

• Chromosomal Mutations:

  • Understanding chromosomal mutations tied to sex determination; includes deletions, translocations, etc.

  • Identified mutations can link to issues like infertility.

  • Not the entire Y chromosome that determines maleness, but specific regions appear necessary.

  • Examples of rare conditions: XY females (due to Y deletions) and XX males (due to Y translocations).

Page 11: Gene Mapping in Sex Determination

• Techniques to map deletions and translocations in sex chromosomes can identify critical genes.

• SRY Gene:

  • Identified in common regions for XY females and XX males; known as the Sex Determining Region of Y (previously TDF).

Page 12: Proving SRY Gene Function

• Experimentation with SRY:

  • Transgenic XX mice with the SRY gene develop as males; absence leads to female development.

Page 13: SRY Protein Role

• SRY Characteristics:

  • Encodes a transcription factor that regulates other gene expressions.

  • SRY functions in determining gonad type and ultimately sex.

  • SRY interaction with DNA — creating a molecular switch involving multiple genes

Page 14: Pre-Male and Pre-Female States

• Development Stages Controlled by SRY:

  • Under SRY's influence, certain genes lead to male gonad (testis) development;

  • Without SRY, female pathways are activated, leading to ovary development.

Page 15: Gonadal Development Differences

• **Male Gonads (Testes) Development: **

  • SRY promotes SOX9, leading to Sertoli cells and testosterone production.

• Female Gonads (Ovaries) Development:

  • The active WNT4 gene directs the formation of female structures.

  • Steroid hormones like estradiol and progesterone support the female phenotype through their absence and presence.

Page 16: Sex Steroid Hormones Role

• Impact of Gonads on Embryo Development:

  • Sex-specific hormones guide anatomical differentiation and physiological variations in mammals, leading to distinct genital and reproductive systems and even behaviors.

Page 17: Introduction to Steroid Hormones

• Basics of Steroid Hormones:

  • Derived from cholesterol; enzymes catalyze their synthesis.

Page 18: Complexity of Steroid Hormone Synthesis

• dont need to remember all.

• Overview of numerous steroid hormones:

  • Includes androgens (male) and estrogens (female).

  • Notable hormonal implications from adrenal glands as well.

Page 19: Disorders of Sexual Development (DSDs)

• Conditions linked to steroid hormone receptor mutations.

• Mutations in sex determination genes can lead to diverse reproductive abnormalities.

Page 20: Development of Sex-Specific Anatomy in Humans

• Expanding understanding of how male and female anatomy develops distinctly, based on genetic and hormonal influences.

Page 21: The Indifferent Gonad

• Characteristics of the indifferent gonad before sex determination.

  • Development features appear indistinct until the SRY gene activity indicates future sex.

Page 22: Genital Ridge and Germ Cell Migration

• Germ cell migration supporting the development of gonads.

  • Forms Indifferent Gonads containing precursor cells.

Page 23: Development of the Indifferent Gonad

• Emphasis on the key anatomical structures arising during indifferent stages:

  • Metanephric kidney, mesonephros, and the cloaca, which later subdivides.

Page 24: Male vs. Female Duct Development

• Clear distinctions between male (Wolffian duct) and female (Mullerian duct) pathways during development.

Page 25: Summary of Indifferent Gonad and Genitals

• Recap on the classical development in early embryonic stages.

Page 26: Principles of Development

• Recognizing similarities in male and female developmental paths through the indifferent stage.

Page 27: Male and Female Genital Development

• Differences in genital formations influenced mainly by testosterone and other hormones.