Heredity and Genetics Notes

Objectives

1. Discovery of DNA’s structure

2. How DNA, RNA, & Proteins are replicated/produced

3. Understanding DNA as the code for life

4. Source of biological variation

1. DNA: The Blueprint of Life

• DNA (Deoxyribonucleic Acid) is the molecule carrying genetic instructions.

• Structure:

  • Composed of two chains of nucleotides in a double helix.

  • Each nucleotide contains:

    • Deoxyribose sugar

    • Phosphate group

    • Nitrogenous base (Adenine, Guanine, Thymine, Cytosine)

• Base Pairing Rules:

  • A = T (Adenine - Thymine)

  • C = G (Cytosine - Guanine)

• Classification of Bases:

  • Purines: Adenine (A) & Guanine (G) - "Pure silver" (Ag)

  • Pyrimidines: Cytosine (C) & Thymine (T) - "Pyramids in CT"

2. Discovery of DNA’s Structure

• 1953: James Watson, Francis Crick, Maurice Wilkins, & Rosalind Franklin discovered DNA's double-helix structure.

• Rosalind Franklin's X-ray diffraction images revealed the helical nature of DNA.

• Watson, Crick, & Wilkins won the Nobel Prize in 1962. Franklin was not credited properly and passed away in 1958.

3. DNA Replication

• Process of copying DNA before cell division:

  1. Enzymes break hydrogen bonds between nitrogenous bases.

  2. Exposed bases attract complementary bases.

  3. Two identical DNA molecules are formed:

     • Each consists of one old strand and one new strand.

4. DNA vs. RNA

• Most of DNA remains in the nucleus.

• RNA transports genetic instructions to ribosomes.

5. Transcription: How DNA Codes for Proteins

• DNA → mRNA (Messenger RNA)

• Steps:

  1. Enzymes break hydrogen bonds, exposing bases.

  2. RNA nucleotides pair with exposed DNA bases.

  3. mRNA strand is formed and exits the nucleus to the ribosomes.

  4. Stops at terminator region.

6. Exons vs. Introns

• Exons: Expressed sequences (code for proteins).

• Introns: Non-coding sequences (removed before translation).

• Alternative Splicing: Different combinations of exons can create different proteins.

7. Translation: mRNA to Proteins

• Occurs at ribosomes

• Steps:

  1. mRNA is read in triplets (codons).

  2. Each codon codes for a specific amino acid.

  3. Transfer RNA (tRNA) brings amino acids to ribosomes.

  4. 20 different amino acids form proteins.

8. The Central Dogma of Molecular Biology

DNA → RNA → Protein

• The genetic code is redundant, meaning multiple codons can code for the same amino acid.

• Example: Sickle Cell Anemia is caused by a point mutation.

9. DNA Terminology

• Genome: Entire DNA set of an organism.

  • Coding DNA: 2% (codes for proteins).

  • Non-Coding DNA: 98% (introns, regulatory elements).

• Chromosome: DNA-protein structures containing genes.

• Gene: A DNA segment coding for a protein.

• Alleles: Different versions of a gene.

• Mutations: Changes in DNA sequence causing variation.

10. Regulatory Genes

• Control the activity of other genes.

• Homeobox (Hox) Genes: Highly conserved genes that shape body development.

11. Cell Types

• Karyotype: Chromosomal layout of an organism.

12. Mitosis (Somatic Cell Division)

• Purpose: Growth & repair.

• Produces: 2 genetically identical diploid cells (2n = 46).

• Process:

  1. DNA replicates.

  2. Cell divides once.

  3. Daughter cells are identical.

13. Meiosis (Gamete Formation)

• Purpose: Genetic diversity in sexual reproduction.

• Produces: 4 genetically unique haploid cells (n = 23).

• Key Features:

  • Recombination (Crossing Over): Homologous chromosomes exchange DNA.

  • Two divisions occur:

    1. First division (homologous chromosomes separate).

    2. Second division (sister chromatids separate).

14. Errors in Meiosis: Nondisjunction

• Nondisjunction: Chromosomes fail to separate properly.

  • If occurs in Meiosis I → All gametes are abnormal.

  • If occurs in Meiosis II → Half of gametes are normal.

• Leads to genetic disorders (e.g., Down Syndrome - Trisomy 21).

15. Why This Matters?

• Natural selection acts on genetic variation.

• Sources of Variation:

  • Mutations create new alleles.

  • Recombination reshuffles genetic material.

• Sexual reproduction = Evolutionary advantage.

16. Applications of DNA Technology

Forensics: DNA Fingerprinting

• Short Tandem Repeat (STR) Analysis:

  • Unique DNA patterns used for identifying individuals.

  • Crime scene investigations use STR to match suspects.

Medicine: Recombinant DNA Technology

• Example: Human insulin production in bacteria.

  • Avoids allergic reactions from animal insulin.

  • More efficient & ethical.

Evolutionary Biology: Genome Sequencing

• Polymerase Chain Reaction (PCR): Amplifies DNA.

• Applications:

  • Understanding evolutionary relationships.

  • Tracking ancient migrations.

17. Key Takeaways

• DNA is the blueprint for all life.

• RNA helps translate DNA into proteins.

• Genetic variation fuels evolution.

• Mitosis = identical cells; Meiosis = genetic diversity.

• DNA technology has vast real-world applications.

2. What is Heredity?

• Heredity: The transmission of genetic traits from parents to offspring.

• Heritable Trait: A characteristic that can be passed down through generations.

• Question: What traits do you think you inherited from your parents?

3. Gregor Mendel: The Father of Genetics

• Lived 1822–1884, a Bohemian monk.

• Studied pea plants to uncover laws of inheritance.

• Published in 1866, but his work was ignored until 1900.

Why Pea Plants?

✅ Many distinct traits
✅ Can self-fertilize OR cross-fertilize
✅ Easy to control genetic crosses

4. Mendel’s Three Principles of Inheritance

1. Principle of Segregation

• Each individual has two alleles for a trait (one from each parent).

• Alleles separate (segregate) during gamete formation and reunite at fertilization.

• No blending inheritance (traits remain distinct).

• Example: A tall (T) and short (t) plant cross → Tall offspring (Tt), no medium height.

2. Principle of Dominance

• Some alleles are dominant, some are recessive.

  • Dominant allele (T) → Expressed when present.

  • Recessive allele (t) → Expressed only if no dominant allele is present (tt).

• Example: T = Tall, t = Short

  • TT = Tall

  • Tt = Tall (dominant allele masks recessive)

  • tt = Short

3. Principle of Independent Assortment

• Genes for different traits are inherited independently (if on different chromosomes).

• Example: Height & seed color in pea plants inherited separately.

• Exception: If genes are on the same chromosome, they tend to stay together (linked genes).

5. Dominant vs. Recessive Traits

6. Punnett Squares: Predicting Inheritance

• Tool to determine possible genetic outcomes of offspring.

• Example:

  T = Tall, t = Short

  	T	t
  T	TT	Tt
  t	Tt	tt

• Results:

  • 75% tall (TT, Tt)

  • 25% short (tt)

7. Other Forms of Trait Dominance

1. Complete Dominance

• Classic Mendelian inheritance.

• One allele completely masks the other.

• Example: Tall (T) is completely dominant over short (t).

2. Incomplete Dominance

• Blended traits – neither allele is fully dominant.

• Example: Red (RR) × White (WW) → Pink (RW).

3. Co-Dominance

• Both alleles are expressed simultaneously.

• Example: Blood Type AB (A & B are equally expressed).

• Example: Roan cattle (red & white hairs both present).

8. Mendelian vs. Non-Mendelian Traits

Mendelian Traits

✅ Single gene controls the trait
✅ Little environmental influence
✅ Discrete (either you have it or you don’t)
✅ Example: Blood type, albinism

Non-Mendelian Traits

✅ Multiple genes influence the trait (polygenic)
✅ Environment affects expression
✅ Continuous variation (e.g., height, skin color)
✅ Example: Eye color, height, skin tone

9. Mendelian Patterns of Inheritance

Autosomal Inheritance

• Autosomal Dominant (One copy = Trait)

  • Example: Achondroplasia (Dwarfism)

  • Does not skip generations.

• Autosomal Recessive (Two copies = Trait)

  • Example: Albinism, Sickle Cell Anemia

  • Can skip generations (hidden in carriers).

Sex-Linked Inheritance

• X-Linked Recessive

  • More common in males (XY, only one X copy).

  • Examples: Hemophilia, color blindness.

• X-Linked Dominant

  • Both males & females affected (only one X copy needed).

• Y-Linked

  • Rare; only passed from father to son.

• Mitochondrial Inheritance

  • Passed only through the mother (egg provides mitochondria).

10. Genetic Testing: PTC Tasting

• PTC Paper Test → Some people taste bitterness, others don’t.

• Controlled by a single gene.

• TT (strong tasters), Tt (mild tasters), tt (non-tasters).

• Example of Mendelian inheritance in humans.

11. Epigenetics: The Influence of Environment on Genes

• Epigenetics: Study of how environmental factors influence gene expression without changing DNA sequences.

• Examples of Epigenetic Effects:

  • Nutrition & Stress during pregnancy can affect offspring health.

  • Chemical marks on DNA can turn genes "on" or "off."

  • Some epigenetic changes are heritable across generations.

12. Summary

• Mendelian Genetics follows simple inheritance rules (dominant/recessive traits).

• Non-Mendelian Genetics involves polygenic traits, environmental factors, and epigenetics.

• Understanding inheritance helps explain genetic disorders, evolution, and human diversity. In conclusion, the study of heredity and genetics not only enhances our knowledge of biological processes but also informs medical research and practices aimed at treating genetic conditions.

• Genetic variation is crucial for adaptation and survival of species, influencing natural selection.