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Schistosoma japonicum - Definitive and Intermediate Hosts

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Life Cycle Summary

1. Definitive hosts (sexual reproduction) - humans, cattle, dogs, rodents, and other mammals
2. Intermediate host (asexual replication) - Oncomelania snails (freshwater mollusk)

• Eggs excreted in feces hatch into miracidia in fresh water
• Miracidia penetrate Oncomelania snails and develop into cercariae
• Cercariae are released into water and directly penetrate human/animal skin
• No second intermediate host is involved

• Red Book 2021, p. 1034: "Animals play an important zoonotic role (as a source of eggs) in maintaining the life cycle of S. japonicum."
• Medical Microbiology 9e, p. 2424: "The free-swimming cercariae are released into the water, where they are immediately infectious for humans and other mammals."

Avery-MacLeod-McCarty Experiment (1944)

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The Experiment - Logic of Elimination

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Why the Other Options Are Wrong

• Protease - mice still died; proteins were ruled out
• Sulfuric acid - not a specific enzyme used; would destroy everything non-specifically
• Sodium hydroxide - same issue; non-specific chemical, not used in this experiment
• Lipase - lipids were already eliminated as candidates; mice still died
• Ribonuclease - mice still died; RNA ruled out

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The Significance

Characteristics of Generative (Germline) Mutations

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The Three Correct Answers:

1. They are inherited during sexual reproduction - germline mutations are transmitted via gametes (sperm/egg) to all cells of the offspring
2. Arise in the germ cells - they occur specifically in oocytes and spermatozoa (germline cells)
3. They are inherited through asexual reproduction - this is actually WRONG - germline mutations pass through sexual reproduction, NOT asexual

• They are inherited during sexual reproduction
• Arise in the germ cells

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Generative vs. Somatic Mutations - Key Comparison

Characteristics of Generative (Germline) Mutations

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Why each option is correct or incorrect:

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Key concept summary:

• Generative (germline) mutations - occur in germ cells (sperm/eggs), are heritable, transmitted to all cells of offspring via sexual reproduction
• Somatic mutations - occur in body cells, NOT heritable, affect only that tissue/cell clone

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📚 Study Guide: Mutation Types & Inheritance

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PART 1 - Classification of Mutations

By Location in the Body

Key exam point: Generative = germline = heritable. Somatic = NOT heritable unless it happens to occur in a germ cell by chance.

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By Molecular Change (Point Mutations)

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By Structural Change

Note: Frameshift mutations almost always lead to premature truncation and loss of protein function. Nonsense-mediated mRNA decay (NMD) often destroys the aberrant transcript before it can make a truncated protein.

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By Effect on Organism's Fitness

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PART 2 - Inheritance Patterns

Mendelian Inheritance Overview

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Autosomal Dominant (AD)

• Every affected person has an affected parent (unless de novo mutation)
• Heterozygote (Aa) is affected
• 50% of offspring of affected parent will be affected
• Both sexes equally affected; father-to-son transmission possible

     A     a
a   Aa    aa
a   Aa    aa

• Huntington disease (triplet repeat expansion in HTT)
• Achondroplasia (FGFR3 mutation)
• Marfan syndrome (FBN1 - fibrillin mutation)
• Neurofibromatosis type 1 (NF1)

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Autosomal Recessive (AR)

• Both parents typically unaffected (carriers: Aa)
• Must inherit 2 mutant copies (aa) to be affected
• 25% of offspring affected, 50% carriers, 25% normal
• Generations may be skipped
• Consanguinity increases risk

     A     a
A   AA    Aa
a   Aa    aa

• Cystic fibrosis (CFTR mutation)
• Phenylketonuria (phenylalanine hydroxylase deficiency)
• Sickle-cell disease (β-globin E6V)
• Albinism (melanocyte tyrosinase loss)

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X-linked Recessive (XLR)

• Males (XY) are affected (hemizygous - only one X copy)
• Females (XX) are usually carriers (heterozygous)
• No father-to-son transmission (fathers pass Y to sons)
• Carrier mothers pass mutation to 50% of sons (affected) and 50% of daughters (carriers)

• Hemophilia A (Factor VIII deficiency)
• Duchenne Muscular Dystrophy (DMD gene deletions)
• Red-green color blindness
• Ornithine transcarbamylase deficiency

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X-linked Dominant (XLD)

• Affected males pass to ALL daughters, no sons
• Affected females (heterozygous) pass to 50% of children of either sex
• Females may be more mildly affected than males

• Incontinentia pigmenti
• Rett syndrome (MECP2)

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Mitochondrial Inheritance

• Passed exclusively through mothers (maternal line only)
• ALL children of an affected mother are at risk
• Fathers NEVER pass to children
• Variable expression due to heteroplasmy (mix of normal and mutant mtDNA)

• MELAS (Mitochondrial Encephalomyopathy, Lactic Acidosis, Stroke-like episodes)
• Leber Hereditary Optic Neuropathy (LHON)

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PART 3 - Special Concepts

Lyon Hypothesis (X-Inactivation)

• In every female somatic cell, one X chromosome is randomly inactivated
• The condensed, inactivated X = Barr body
• Formula: Number of Barr bodies = Number of X chromosomes - 1
  • Normal female (XX): 1 Barr body
  • Klinefelter (XXY): 1 Barr body
  • Triple X (XXX): 2 Barr bodies

Dominant Negative Effect

• Mutant protein actively interferes with the function of the normal wild-type protein
• Common in proteins that form dimers/multimers

Penetrance vs. Expressivity

De Novo Mutations

• New mutations arising spontaneously, not inherited from either parent
• Important in dominant disorders where no family history exists
• More common with advanced paternal age

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PART 4 - Quick Comparison Table

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PART 5 - High-Yield Clinical Examples by Mutation Type

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