Genes - idexcel igcse biology

1. What Are Genes?
  • Definition: A gene is a length of DNA that codes for a particular protein or characteristic.

  • Genes are the basic units of inheritance and are found on chromosomes.

  • DNA (Deoxyribonucleic acid) is the molecule that carries genetic information.

  • DNA is composed of nucleotides (A, T, C, G) which form the genetic code.

2. Chromosomes
  • Chromosomes are structures made of DNA and proteins found in the nucleus of a cell.

  • Humans have 46 chromosomes (23 pairs).

  • Sex chromosomes: One pair determines the sex of the individual:

    • Female: XX

    • Male: XY

3. Alleles
  • Alleles are different versions of a gene that can occupy the same position on a chromosome.

  • There are dominant and recessive alleles:

    • Dominant allele: The allele that expresses its trait even if only one copy is present (e.g., A).

    • Recessive allele: The allele that only expresses its trait when two copies are present (e.g., a).

4. Genotype and Phenotype
  • Genotype: The genetic makeup of an individual (the combination of alleles, e.g., AA, Aa, or aa).

  • Phenotype: The physical expression or characteristic resulting from the genotype (e.g., blue eyes, tall height).

  • Homozygous: Having two identical alleles for a gene (AA or aa).

  • Heterozygous: Having two different alleles for a gene (Aa).

5. Mendelian Inheritance
  • Gregor Mendel studied inheritance in pea plants and established the laws of inheritance.

  • Law of Segregation: Each parent passes one of their two alleles for each gene to their offspring.

  • Law of Independent Assortment: Genes for different traits are inherited independently of each other.

6. Monohybrid Crosses
  • A monohybrid cross studies the inheritance of one gene with two alleles.

  • Punnett square diagrams are used to predict the probability of offspring inheriting different allele combinations.

    Example:

    • Cross between homozygous dominant (AA) and homozygous recessive (aa).

    • All offspring (F1 generation) will be heterozygous (Aa), showing the dominant trait.

7. Co-dominance and Incomplete Dominance
  • Co-dominance: Both alleles in a heterozygous individual contribute to the phenotype.

    • Example: Red and white flower alleles produce a flower with both red and white spots.

  • Incomplete Dominance: The heterozygous phenotype is an intermediate between the two homozygous phenotypes.

    • Example: Red and white flowers producing pink flowers.

8. Mutations
  • Mutation: A change in the sequence of DNA that can alter the function of a gene.

  • Mutations can be beneficial, neutral, or harmful.

  • Some mutations may lead to genetic disorders (e.g., cystic fibrosis, sickle cell anemia).

9. Sex Determination
  • In humans, gender is determined by sex chromosomes:

    • Female: XX (two X chromosomes)

    • Male: XY (one X and one Y chromosome)

  • The Y chromosome carries the gene for male development (the SRY gene).

10. Genetic Disorders
  • Cystic Fibrosis: A genetic disorder caused by a recessive allele (cc). Individuals with two copies of the mutated allele have thick mucus that affects the lungs and digestive system.

  • Sickle Cell Anemia: A genetic condition caused by a mutation in the hemoglobin gene. Individuals with the homozygous recessive allele (ss) have sickle-shaped red blood cells, leading to various health issues.

  • Huntington's Disease: A neurodegenerative genetic disorder caused by a dominant allele (H). Symptoms appear later in life, often in adulthood.

11. Genetic Screening
  • Genetic screening involves testing individuals for genetic disorders or mutations.

  • It can be used for early detection of conditions like cystic fibrosis or Huntington's disease.

12. Applications of Genetics
  • Genetic Engineering: The manipulation of an organism’s genome using biotechnology.

    • GM Crops: Genetically modified crops can have improved resistance to pests or diseases, or increased nutritional value.

    • Gene Therapy: Inserting, altering, or removing genes within a person’s cells to treat disease.

13. Sex Linkage
  • Some genes are carried on the sex chromosomes (X and Y).

  • X-linked genes: More commonly affect males because they have only one X chromosome.

    • Example: Haemophilia and Red-green color blindness are X-linked recessive traits.


Key Diagrams to Remember

  • Punnett Square: Used to predict the outcome of genetic crosses.

  • Pedigree Chart: A diagram used to track inheritance of traits within a family.


These notes summarize the key ideas around genes and inheritance in IGCSE Ed

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