# Genetics

Genetics is the **study of how characteristics determined by genes are passed down from parents to their offspring**. It is also known as transmission genetics or Mendelian genetics, in recognition of Gregor Mendel's pioneering work. This field of biology helps to understand the **inheritance of traits and the variation within species**.

#### Key Genetic Terms

* **Gene**: A **sequence of DNA bases** that codes for either a **polypeptide** (which forms the primary structure of a protein) or **functional RNA**. Different polypeptides have varying numbers and orders of amino acids, and this order is determined by the base order in the gene. A gene is considered a **unit of inheritance**.
* **Allele**: **Different versions of the same gene**. Alleles code for different variations of a characteristic, such as blonde or brown hair. Most plants and animals, including humans, carry two alleles for each gene, one from each parent.
* **Locus (plural: loci)**: The **fixed position of a gene on a chromosome**. Diploid organisms have two copies of each chromosome (one from each parent), which is why they have two alleles at each locus.
* **Genome**: The **complete set of genes in a cell**. It includes all the DNA present in an organism. For eukaryotes, this encompasses nuclear, mitochondrial, and for plants, chloroplast DNA.
* **Proteome**: The **full range of proteins that a cell is able to produce**.
* **Genotype**: An organism's **genetic constitution**, referring to the specific combination of alleles it possesses.
* **Phenotype**: The **observable characteristics** of an organism, resulting from the expression of its genetic constitution and its **interaction with the environment**.
* **Homozygous**: Describes a diploid organism with **two identical alleles** for a gene at the same locus.
* **Heterozygous**: Describes a diploid organism with **two different alleles** for a gene at the same locus.
* **Dominant Allele**: An allele whose characteristic **always appears in the phenotype** when present, even if there's only one copy. It masks the expression of a recessive allele.
* **Recessive Allele**: An allele whose characteristic is **only shown in the phenotype if two copies are present**.
* **Codominant Alleles**: Both alleles are **expressed in the phenotype** because neither is recessive. An example is the human ABO blood group system, where Iᴬ and Iᴮ are codominant.
* **Multiple Alleles**: When there are **more than two possible alleles for a single gene**.
* **Carrier**: A person carrying an allele that is **not expressed in the phenotype** but can be passed on to offspring, often associated with recessive genetic diseases.

#### Patterns of Inheritance

Genetic diagrams, including Punnett squares, are used to **predict the genotypes and phenotypes of offspring** resulting from crosses between parents.

* **Monohybrid Crosses**: Involve the **inheritance of a single gene**. A typical phenotypic ratio for a monohybrid cross between two heterozygous parents is **3:1** (dominant:recessive). For codominant alleles, the ratio is typically **1:2:1** (homozygous for one allele: heterozygous: homozygous for the other allele).
* **Dihybrid Crosses**: Involve the **inheritance of two characteristics controlled by different genes**. The typical phenotypic ratio for a dihybrid cross between two heterozygous parents is **9:3:3:1**.
* **Sex Linkage**: Occurs when the gene is located on the **sex chromosomes** (e.g., X or Y). X-linked disorders are more common in males because they only have one X chromosome, so a recessive allele on it will be expressed, unlike females who need two copies of the recessive allele. Examples include red/green colour blindness and haemophilia.
* **Epistasis**: When the **allele of one gene masks the expression of the alleles of other genes**. Different types of epistasis (recessive or dominant) result in altered phenotypic ratios.
* **Linkage**: Genes located on the **same chromosome** are said to be linked and tend to be inherited together. The closer together the loci of two genes, the more likely they are to stay linked, as crossing over is less likely to split them up.

#### Production of Genetic Variation

Genetic variation is crucial for natural selection and evolution. It arises from:

* **Meiosis**: A **reduction division** where a diploid cell divides to form **four haploid cells that are genetically different**. It involves two key events:
  * **Crossing Over (Recombination)**: During **prophase I**, homologous chromosomes pair up, and **chromatids exchange segments of genetic material**. This creates **new combinations of alleles** on the chromatids.
  * **Independent Segregation (Assortment)**: During **meiosis I**, homologous pairs of chromosomes separate **randomly** into daughter cells. This leads to different combinations of maternal and paternal chromosomes in the gametes.
* **Random Fertilization**: The **random fusion of any male gamete with any female gamete**. This ensures that each zygote, and thus each new individual, has a **unique combination of genetic material**.
* **Mutations**: The **primary source of genetic variation**, creating new alleles. Mutations are **random changes in the DNA base sequence**. They can occur spontaneously during DNA replication or be induced by mutagens like UV radiation or chemicals.
  * **Gene Mutations**: Involve changes in the DNA base sequence. Types include:
    * **Substitution**: One base is replaced by another.
    * **Deletion/Insertion**: One or more bases are removed or added, respectively, often leading to a **frameshift** where the entire downstream sequence is altered.
  * **Chromosome Mutations**: Involve changes in the **number or structure of chromosomes**, such as non-disjunction during meiosis leading to an abnormal chromosome count (e.g., in Edwards' syndrome).

#### Genetic Diversity

**Genetic diversity** is the **number of different alleles of genes in a species or population**. A large number of different alleles indicates high genetic diversity and provides a wide variety of characteristics. High genetic diversity is important because it allows a population to **adapt to changes in the environment**, enabling natural selection and evolution. Genetic diversity can also be influenced by **gene flow** (introduction of new alleles through migration) and phenomena like the **founder effect** (where a new population starts with a small, unrepresentative gene pool).

#### Genetic Technologies

Genetics also encompasses technologies that allow for the direct study and manipulation of genetic material.

* **Genetic Engineering (Recombinant DNA Technology)**: Involves **transferring a fragment of DNA from one organism to another** to modify specific characteristics. This is possible due to the **universal nature of the genetic code**. Transferred DNA can be expressed in the recipient organism, leading to new phenotypes or the production of valuable proteins like insulin.
* **Genetic Screening**: Testing individuals (embryos, fetuses, or adults) for the presence of particular alleles, often for heritable conditions or health risks.
* **Gene Therapy**: Involves **altering defective genes inside cells to treat genetic disorders**.
* **Genetic Fingerprinting**: Analyzes unique patterns of Variable Number Tandem Repeats (VNTRs) in DNA to determine genetic relationships or identify individuals.
* **DNA Sequencing**: Determines the complete nucleotide sequence of an organism's genome.

Overall, genetics is the study that underpins how biological information is stored, passed on, and varied across generations, forming the basis for **heredity and evolution**.