Genetics VI
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:
Sex Determination: Initial decision phase for male or female development.
Sexual Differentiation: Actual execution of the determined sex developmental state.
Page 6: Examples of Sex Determination Systems
Chromosomal Systems:
XY System (Mammals): XX=female; XY=male. Heterogametic is male.
ZW System (Birds): ZZ=male; ZW=female. Heterogametic is female.
X0 System (Insects): XX=female; X=male. Heterogametic is male.
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:
Klinefelter Syndrome (XXY): Phenotypically appears as male but is infertile.
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.