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- Sex Determination Mechanisms:
- Chromosomal Systems: Mammals (XX females, XY males; SRY gene on Y is key), Birds (ZW females, ZZ males), Insects (XO males, XX females, e.g., crickets).
- Haplo-diploidy: Males are haploid (from unfertilized eggs) and females are diploid (from fertilized eggs) (e.g., bees).
- Environmental Factors: Temperature-dependent sex determination (e.g., turtles, alligators) and positional/social influences (e.g., Crepidula fornicata).
- Sex Chromosomes and Associated Conditions:
- X and Y Chromosomes: Not fully homologous but share pseudoautosomal regions (PARs) crucial for meiotic pairing.
- SRY Gene: Sex-determining Region Y, expressed in Sertoli cells to initiate male development, including anti-Müllerian hormone secretion and Leydig cell stimulation (testosterone production).
- Y Chromosome Microdeletions: Linked to male infertility, specifically in AZFa, AZFb, AZFc regions.
- Sex Chromosome Aneuploidies:
- Turner Syndrome (45, X0): Females with a single X chromosome.
- Klinefelter Syndrome (47, XXY): Males with an extra X chromosome.
- Poly X Females (e.g., 47, XXX): Generally taller, often fertile; intellectual disability risk increases with more extra X chromosomes.
- Dosage Compensation (X-Inactivation): In mammalian females, one X chromosome is randomly inactivated in each somatic cell, forming a Barr body (condensed, transcriptionally inactive X) to balance gene dosage. This leads to mosaic phenotypes, as seen in calico cats.
- Species-Specific Compensation: Mammals (X-inactivation), Drosophila (male X gene expression doubled), C. elegans (female X gene expression halved).
- Y-Linked Traits: Exclusively present in males, passed from father to all sons, useful for tracking paternal lineages.
- Genetic Mutations:
- Types of Gene Mutations (DNA Level):
- Base Substitutions: Transitions (purine-to-purine or pyrimidine-to-pyrimidine) and Transversions (purine-to-pyrimidine or vice versa).
- Frameshift Mutations: Insertions or deletions of nucleotides (not multiples of three) altering the reading frame.
- Consequences on Protein Function:
- Missense Mutation: Changes an amino acid (e.g., sickle cell anemia from Glu to Val). Can be *neutral* if the new amino acid has similar chemical properties.
- Nonsense Mutation: Introduces a premature stop codon, leading to a truncated protein.
- Silent (Samesense) Mutation: Changes a codon but results in the same amino acid due to the degeneracy of the genetic code.
- Causes of Mutations:
- Spontaneous: Deamination (e.g., cytosine to uracil), tautomeric shifts, and wobble base pairing.
- Induced (Mutagens): Base analogs (e.g., 5-Bromouracil), chemical modifiers (e.g., EMS, nitrous acid, hydroxylamine), intercalating agents (e.g., proflavine, acridine orange), and radiation (ionizing like X-rays causing breaks, non-ionizing like UV forming pyrimidine dimers).
- DNA Repair Mechanisms: Systems like methyltransferases actively repair damaged bases (e.g., O6-Methylguanine).
- Genetic Disorders from Mutations:
- Trinucleotide Repeat Expansion Disorders: Caused by abnormal increases in repetitive DNA sequences (e.g., Fragile X Syndrome, Huntington's Disease, Friedreich's Ataxia, Myotonic Dystrophy).
- Xeroderma Pigmentosum: An autosomal recessive disorder due to defective DNA repair, particularly nucleotide excision repair, leading to extreme photosensitivity and high skin cancer risk.
- Types of Gene Mutations (DNA Level):
- Classification of Mutations (General): Mutations are classified as point mutations, chromosomal aberrations (affecting number or structure of chromosomes, e.g., deletions, duplications, inversions, translocations), and genomic mutations (changes in chromosome sets, e.g., euploidy, aneuploidy).
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