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SCI EOT1

09/04/2025

Heya, print this out as well. It’s abt pedigrees.

Last-Minute Prep: Units 2.3 & 2.4 (Genetics)

Unit 2.3: Patterns of Inheritance

Core Concepts

  • Gene: DNA section coding for a trait.
  • Allele: Different versions of a gene (B vs b).
  • Genotype: Allele combination (BB, Bb, bb).
  • Phenotype: Physical trait expressed (Brown eyes, Blue eyes).
  • Homozygous: Identical alleles (BB or bb).
  • Heterozygous: Different alleles (Bb).
  • Dominant: Allele expressed in heterozygote (B in Bb).
  • Recessive: Allele only expressed when homozygous (b in bb).

Key Tools & Patterns

  1. Punnett Squares:

    • Purpose: Predict offspring genotype/phenotype probabilities.
    • Example: Bb (Het.) x bb (Homo. Rec.) 
        |  B |  b |
--|----|----|
 b| Bb | bb |
 b| Bb | bb |
    • Result: 50% Bb (Dominant Phenotype), 50% bb (Recessive Phenotype)

  2. Other Inheritance Patterns:

    • Codominance: Both alleles fully expressed (e.g., AB blood type - IᴬIᴮ).
    • Incomplete Dominance: Heterozygote is a blend (e.g., Red RR x White WW -> Pink RW).
    • Sex-Linked Traits: Genes on X chromosome. Key: Recessive traits show up more in males (XY) because they only have one X.

  3. Pedigrees:

    • Purpose: Track traits in families.
    • Symbols: ○ = female, □ = male; Shaded = affected, Half/Dot = carrier.
    • Quick Analysis:
      • Dominant: Appears every generation.
      • Recessive: Can skip generations (unaffected parents -> affected child = parents are carriers).
      • X-linked Recessive: Affects males >> females; never father-to-son.

Unit 2.4: Changes in DNA

Mutations: The Basics

  • Mutation: Any change in DNA sequence.
  • Causes: Spontaneous (replication errors) or Induced (mutagens like UV, chemicals).
  • Types: Germline (in sperm/egg, heritable) vs. Somatic (body cells, not heritable).

Types of Mutations

  1. Point Mutations (Gene Level):

    • Substitution: One base swapped for another.
      • Silent: No amino acid change.
      • Missense: Changes one amino acid (e.g., Sickle Cell).
      • Nonsense: Creates early STOP codon -> short protein.
    • Insertion/Deletion (Indels): Bases added or removed.
      • Frameshift: If not multiple of 3, shifts reading frame -> massive amino acid changes downstream -> usually non-functional protein.

  2. Chromosomal Mutations (Large Scale):

    • Structure Changes:
      • Deletion: Segment lost.
      • Duplication: Segment repeated.
      • Inversion: Segment flipped.
      • Translocation: Segment moved to another non-homologous chromosome.
    • Number Changes (Aneuploidy): Incorrect chromosome number, caused by Non-disjunction (failure to separate in meiosis).
      • Trisomy: 3 copies of a chromosome (e.g., Trisomy 21 = Down Syndrome).
      • Monosomy: 1 copy of a chromosome (e.g., Turner Syndrome = XO).

Gene Technology & Ethics

  • Genetic Testing: Screening for mutations/conditions (prenatal, carrier).
  • Gene Therapy: Aiming to fix faulty genes.
  • CRISPR: Precise DNA editing tool.
  • GMOs: Organisms with artificially altered DNA (e.g., pest-resistant crops).
  • Ethics: Key issues = Privacy, discrimination, consent, potential unforeseen consequences.

Practice Questions (Quick Check)

  1. Punnett: Cross Aa x aa. Genotypic Ratio?
    • Ans: 1 Aa : 1 aa (50% : 50%)
  2. Mutation ID: AUG (Met) -> UUG (Leu). Type?
    • Ans: Substitution (specifically Missense).
  3. Pedigree: Unaffected parents have an affected child. Dominant or Recessive?
    • Ans: Recessive (parents must be carriers Aa).
  4. Cause: Extra chromosome 21 (Down Syndrome) is caused by…?
    • Ans: Non-disjunction during meiosis.

Pros of Genetic Testing

Genetic testing offers crucial advantages, including early detection and prevention of genetic disorders, enabling timely medical interventions. It promotes personalized medicine, enhancing treatment efficacy and reducing side effects. Additionally, it aids in family planning, helping individuals make informed reproductive decisions. This testing also provides psychological clarity, alleviating uncertainty and offering peace of mind for those who test negative.

Cons of Genetic Testing

Despite its benefits, genetic testing poses significant challenges. Privacy concerns arise from potential data misuse, and the emotional impact of learning about genetic predispositions can be profound. Risks of discrimination in employment and insurance further complicate its use. Ethical dilemmas, such as selective pregnancy termination, spark debates. High costs and ambiguous results can lead to confusion, highlighting the need for cautious implementation and regulation.

  • DISTINGUISH Point (Substitution, Frameshift) vs. Chromosomal (Deletion, Non-disjunction etc.) mutations.
  • READ Pedigrees: Look for skipped generations (recessive) or father-to-son transmission (rules out X-linked). Males affected more by X-linked Recessive.

Genetics Q&A

Remembering

  • Q: How many copies of each gene do we have, and where do they come from?
    A: Two copies: one from the mother, one from the father.

  • Q: Who is the father of genetics?
    A: Gregor Mendel.

  • Q: Give examples of codominance and incomplete dominance.
    A:

    • Codominance: Cows with red/white fur produce offspring with red and white patches.
    • Incomplete dominance: Red + white flowers produce pink offspring.

Understanding

  • Q: What is the sex of individual II-5 in the pedigree chart?
    A: Male.

  • Q: Is the disease in Figure 2.37 dominant or recessive? Explain.
    A: Dominant. Unaffected parents (II-5 and II-6) produced unaffected offspring, which is impossible with a recessive trait.

  • Q: What is the genotype of individual II-5? Explain.
    A: Heterozygous (e.g., Dd). To produce unaffected offspring, both parents must carry a recessive allele.

Applying

  • Q: What are pedigrees useful for?
    A: To study inheritance patterns and determine if traits are dominant/recessive.

  • Q: How is sex linkage different from other inheritance types?
    A: Genes are on the X/Y chromosomes, leading to different inheritance patterns in males vs. females.

Analysing

  • Q: Contrast codominance and incomplete dominance.
    A:

    • Codominance: Both alleles expressed (e.g., red + white fur).
    • Incomplete dominance: Blend of alleles (e.g., pink flowers).

  • Q: Why is haemophilia recessive (using Figure 2.38)?
    A: Unaffected parents (II-2 and II-3) produced affected children (III-3), indicating the mother was a carrier (XHXh).

Evaluating

  • Q: Propose how to determine if a black-furred mouse is homozygous or heterozygous.
    A: Use a test cross with a homozygous recessive (brown) mouse:

    • Homozygous (BB): All offspring black.
    • Heterozygous (Bb): 50% black, 50% brown.

  • Q: Does all blue-eyed offspring mean blue eyes are dominant?
    A: No. A heterozygous parent could theoretically produce all blue-eyed offspring by chance.

Mutations & Genetic Engineering

Remembering

  • Q: Where must a mutation occur to be inherited?
    A: In germline cells (sperm/ova).

  • Q: Define “mutagen” and “GMO.”
    A:

    • Mutagen: Agent causing DNA mutations.
    • GMO: Organism with engineered genome.

Applying

  • Q: What happens if a nucleotide is deleted in a gene?
    A: A frameshift mutation occurs, altering all subsequent amino acids.

  • Q: Why are not all mutations harmful?
    A: Some mutations confer advantages (e.g., antibiotic resistance) or are neutral.

Analysing

  • Q: Contrast GMOs and transgenic organisms.
    A:
    • GMO: Any engineered organism.
    • Transgenic: Contains DNA from another species.

Evaluating

  • Q: Why is a substitution mutation less critical than insertion/deletion?
    A: Substitutions may not change the amino acid (silent mutation) or only alter one amino acid.

  • Q: Link ozone layer thinning to skin cancer.
    A: Increased UV radiation (a mutagen) raises mutation rates in skin cells.