Genes - idexcel igcse biology

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.

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

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).

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).

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.

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.

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.

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).

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).

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.

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.

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.

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.

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