Inheritance Edexcel IGCSE Biology Study Notes

The Genome & Genes

• The Genome: The entire DNA sequence of an organism. It contains all the information needed to build and maintain that organism.

• Gene: A specific sequence of nucleotides in a DNA molecule that codes for a particular polypeptide (protein). It is the basic unit of heredity.

• Function of Genes:

  • Genes control characteristics by providing instructions for the synthesis of proteins.

  • Proteins determine the phenotype: structural proteins (collagen, keratin), enzymes (amylase), and hormones (insulin).

• Chromosomes: Long, coiled molecules of DNA found in the nucleus.

  • Human Sets: Humans have $46$ chromosomes in total arranged in $23$ pairs. This is known as the diploid number ($2n$).

  • Homologous Pairs: Each pair consists of one chromosome from the mother and one from the father, carrying the same genes at the same loci (positions), although they may carry different alleles.

• Complexity: The number of chromosomes is not an indicator of biological complexity (e.g., chickens have $78$ chromosomes).

DNA Structure

• Chemical Composition: DNA is a polymer made of repeating units called nucleotides. Each nucleotide consists of:

  • A phosphate group

  • A deoxyribose sugar

  • A nitrogenous base

• The Double Helix: Two polynucleotide strands run anti-parallel to each other and are twisted into a spiral shape.

• Complementary Base Pairing: The strands are held together by hydrogen bonds between specific bases:

  • Adenine (A) always pairs with Thymine (T) via 2 hydrogen bonds.

  • Guanine (G) always pairs with Cytosine (C) via 3 hydrogen bonds.

RNA Structure

• Key Characteristics: RNA differs from DNA in three main ways:

  1. It is usually single-stranded.

  2. It contains a ribose sugar instead of deoxyribose.

  3. It contains the base Uracil (U) instead of Thymine (T).

• Types of RNA:

  • mRNA (Messenger): Transports the genetic code from the DNA in the nucleus to the ribosomes.

  • tRNA (Transfer): Transports specific amino acids to the ribosome; it has an anticodon that matches the mRNA codon.

  • rRNA (Ribosomal): Structural component of the ribosome.

Stages of Protein Synthesis

1. Transcription (Nucleus):

   • The DNA helix unzips around the specific gene.

   • RNA polymerase binds to the DNA and uses one strand as a template.

   • Free mRNA nucleotides pair with the DNA bases ($U$ replaces $T$).

   • The completed mRNA strand detaches and leaves the nucleus via a nuclear pore.

2. Translation (Cytoplasm/Ribosome):

   • The mRNA attaches to a ribosome.

   • The ribosome reads the mRNA in triplets called codons.

   • tRNA molecules with complementary anticodons bring specific amino acids to the ribosome.

   • Amino acids are joined by peptide bonds to form a polypeptide chain.

   • The process stops when a 'stop codon' is reached.

Alleles and Key Terms

• Alleles: Different versions of the same gene. For example, the gene for eye color has alleles for blue and brown.

• Genotype: The combination of alleles an organism has (e.g., $Bb$).

• Phenotype: The physical expression of the genotype (e.g., Brown eyes).

• Dominant: An allele that is always expressed even if only one copy is present (represented by a capital letter).

• Recessive: An allele that is only expressed if two copies are present ($homozygous$), represented by a lowercase letter.

• Homozygous: Having two of the same alleles ($BB$ or $bb$).

• Heterozygous: Having two different alleles ($Bb$).

Codominance

• Definition: A situation where both alleles contribute to the phenotype; neither is dominant over the other.

• Blood Groups: The alleles for blood type are $I^A$, $I^B$, and $i$.

  • $I^A$ and $I^B$ are codominant ($I^AI^B$ results in type AB blood).

  • $i$ is recessive ($ii$ results in type O blood).

Polygenic Inheritance

• Definition: Most phenotypic traits aren't controlled by a single gene but by the interaction of multiple genes.

• Continuous Variation: Traits like height, skin color, and mass show continuous variation rather than distinct categories.

Mitosis vs Meiosis

• Mitosis:

  • Product: Two genetically identical diploid ($2n$) daughter cells.

  • Purpose: Growth, tissue repair, and asexual reproduction.

• Meiosis:

  • Product: Four genetically different haploid ($n$) daughter cells (gametes).

  • Purpose: Sexual reproduction.

  • Variation: Occurs through independent assortment and crossing over.

Genetic Variation & Natural Selection

• Sources of Variation:

  • Genetic: Mutations, meiosis, and random fertilization of gametes.

  • Environmental: Factors like sunlight, diet, and climate.

• Natural Selection (Darwin's Theory):

  1. Variation: Exists within a population due to mutation.

  2. Competition: Organisms produce more offspring than can survive (Struggle for existence).

  3. Survival of the Fittest: Individuals with advantageous alleles are more likely to survive selection pressures.

  4. Reproduction: Surviving individuals pass on their advantageous alleles to their offspring.

  5. Evolution: Over generations, the frequency of these alleles increases in the population.

Mutation

• Definition: A random change in the DNA base sequence. Mutations can be spontaneous or caused by mutagens (UV radiation, X-rays, tobacco chemicals).

• Types:

  • Substitution: One base is replaced by another. May only change one amino acid.

  • Insertion/Deletion: A base is added or removed, causing a frameshift. This changes every subsequent codon and usually results in a non-functional protein.

• Antibiotic Resistance: A mutation in bacteria can allow survival against antibiotics. Because bacteria reproduce rapidly, the resistant strain quickly becomes the dominant population through natural selection.

Genetic Diagrams

• Monohybrid Cross: A genetic cross focusing on a single trait, often used with a Punnett Square to predict offspring ratios.

• Test Cross: Used to determine the genotype of an individual expressing a dominant phenotype (e.g., crossing with a known homozygous recessive individual) by breeding them with a known homozygous recessive individual.

Sex Linkage

• Traits determined by genes located on the sex chromosomes ($X$ or $Y$).

• $X$-linked inheritance: Genes are typically carried on the $X$ chromosome because the $Y$ chromosome is much smaller and carries fewer genes.

• Recessive $X$-linked disorders: Affect males more frequently (e.g., color blindness, hemophilia) because they only have one $X$ chromosome ($XY$), meaning one copy of the recessive allele expresses the disorder. Females ($XX$) require two recessive alleles to express the disorder.

Family Pedigrees

• Definition: A chart showing the inheritance pattern of a trait or disorder within a family across several generations.

• Symbols:

  • Square: Male.

  • Circle: Female.

  • Shaded/Filled Symbol: Individual expresses the trait/disorder (affected).

  • Unshaded/Open Symbol: Individual does not express the trait/disorder (unaffected).

• Analyzing Pedigrees: Helps determine if a disorder is dominant, recessive, autosomal, or sex-linked.

Genetic Engineering (Modification)

• Definition: The process of changing the genetic material of an organism by removing, inserting, or modifying individual genes.

• Key Tools:

  1. Restriction Enzymes: Cut DNA at specific recognition sequences, creating 'sticky ends' (unpaired bases).

  2. DNA Ligase: Joins DNA fragments (e.g., the desired gene inserted into a plasmid) by forming phosphodiester bonds.

  3. Vector (Plasmid): Small, circular piece of DNA from bacteria used to carry the gene into the host cell.

• Steps (e.g., making human insulin in bacteria):

  1. Isolate the human insulin gene (using restriction enzymes).

  2. Isolate a bacterial plasmid (using the same restriction enzymes to create complementary sticky ends).

  3. Join the insulin gene and the plasmid using DNA Ligase to form a recombinant plasmid.

  4. Insert the recombinant plasmid into a host bacterium (transformation).

  5. The bacteria multiply, expressing the human insulin protein, which can then be purified.