TERMINOLOGY IN GENETICS

These keywords provide a good foundation for understanding the basics of genetics.

• Gene – A unit of heredity that carries information from generation to generation, controlling traits like eye color, hair texture, etc.

• DNA (Deoxyribonucleic Acid) – The molecule that contains the genetic instructions for the development and functioning of living organisms. It's structured in a double helix.

• Chromosomes – Structures in the cell nucleus that are made up of tightly packed DNA. Humans typically have 46 chromosomes, organized in 23 pairs.

• Sex Chromosomes – The chromosomes (X and Y) that determine an individual’s sex. Females have two X chromosomes (XX), and males have one X and Y chromosome (XY).

• Alleles – Different gene versions that can produce variations in a trait, such as different eye colors.

• Mutations – Changes or errors in the DNA sequence that can result in new traits or sometimes cause genetic disorders.

• Inheritance – The process by which traits are passed from parents to offspring through their genes.

• RNA (Ribonucleic Acid) – A molecule involved in translating the instructions from DNA to make proteins, which are essential for the body to function.

• Double Helix – The spiral structure of DNA, consisting of two strands that coil around each other.

• Dominant and Recessive Traits – Dominant traits only need one allele to be expressed, while recessive traits need two alleles for the trait to appear.

• Genome – The complete set of an organism’s genetic material, including all its genes.

• Genotype – An individual's genetic makeup may not always be visible in appearance.

• Phenotype – The physical expression of an individual's genes, such as height, eye color, or behavior.

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

DNA, GENES, AND CHROMOSOMES: THE BLUEPRINT OF LIFE

DNA (Deoxyribonucleic Acid) is a molecule that contains the genetic instructions necessary for building and maintaining an organism. These instructions are encoded by sequences of four chemical bases: adenine (A), thymine (T), cytosine (C), and guanine (G). DNA forms the blueprint for how living organisms develop and function.

Genes are specific segments of DNA that carry the instructions for traits. Each gene codes for a particular function or characteristic, such as hair colour, eye colour, or how your cells process sugar. Think of genes as individual recipes within the larger cookbook of DNA.

Chromosomes are long strands of DNA wrapped around proteins, organised into 46 chromosomes in humans (23 pairs). Each chromosome contains many genes, and you inherit one set of chromosomes from each parent.

ALLELES AND MUTATIONS: THE ROLE OF VARIATION

Alleles are different versions of the same gene. For example, a gene for eye colour can have a "brown" allele and a "blue" allele. Each person inherits one allele from each parent for every gene. Alleles can be either dominant or recessive:

• Dominant alleles need just one copy to express the trait. For example, suppose a person inherits one dominant allele for brown eyes and one recessive allele for blue eyes. In that case, they will have brown eyes because the brown allele "overpowers" the blue one.

• Recessive alleles require two copies to express the trait (one from each parent). If both alleles are for blue eyes, the person will have blue eyes.

Mutations are changes or errors in the DNA sequence of a gene. Mutations can occur naturally during DNA replication or be triggered by environmental factors like UV radiation. Mutations can have three different effects:

• Neutral mutations: No visible impact on the organism.

• Harmful mutations: Can disrupt normal gene function, potentially causing diseases or disorders.

• Beneficial mutations: Rarely, they may lead to advantageous traits, such as resistance to certain diseases.

For example, sickle cell anaemia is caused by a mutation in the gene for haemoglobin, the protein that carries oxygen in red blood cells. This mutation causes red blood cells to become sickle-shaped, leading to health problems.

MONOGENIC AND POLYGENIC DISORDERS

MENDELIAN GENETICS

Gregor Mendel, a 19th-century scientist, studied how traits are passed from parents to offspring through experiments with pea plants. He observed that traits (such as flower colour) followed specific inheritance patterns, leading to the principles of Mendelian Genetics. These rules explain how genes, located on pairs of chromosomes, determine traits. Each organism inherits two copies of a gene—one from each parent—and these copies can be dominant or recessive. For example, a plant with one dominant gene for purple flowers and one recessive gene for white flowers will display purple flowers as the dominant gene "wins out."

MONOGENIC DISORDERS

Mutations in a single gene cause monogenic disorders. These disorders often profoundly impact an individual’s quality of life and can lead to birth defects or intellectual, sensory, and motor disabilities. The inheritance pattern can be dominant or recessive, meaning a person may inherit the mutation without displaying symptoms (asymptomatic carriers) but can still pass it on to their children.

Examples of Monogenic Disorders:

• Sickle Cell Anaemia: A recessive disorder caused by mutations in a single gene that affects haemoglobin.

• Huntington's Disease: A dominant disorder caused by mutations in a single gene, leading to the degeneration of nerve cells in the brain.

• Fragile X Syndrome: A monogenic disorder caused by mutations in the FMR1 gene, leading to intellectual disability and behavioural disorders.

Monogenic disorders follow simple inheritance patterns, with dominant or recessive genes determining whether the trait or disorder is expressed.

POLYGENIC INHERITANCE

Polygenic traits are influenced by multiple genes working together. These traits do not follow simple dominant or recessive rules, as many genes contribute to the overall risk or expression of the trait. Environmental factors also play a role, leading to complex patterns of inheritance.

Examples of Polygenic Traits/Disorders:

• Height and Skin Colour: These traits are determined by the combined effects of many genes.

• Obsessive-Compulsive Disorder (OCD): Multiple genes contribute to OCD risk, and both genetic and environmental factors play a role.

• Schizophrenia: A polygenic disorder influenced by numerous genes, along with environmental factors like prenatal stress or drug use.

EPIGENETICS: HOW ENVIRONMENT AFFECTS GENE EXPRESSION

Epigenetics refers to changes in how genes are expressed without altering the underlying DNA sequence. Environmental factors, such as diet, stress, or exposure to toxins, can modify gene expression through epigenetic mechanisms. This process can turn genes on or off, affecting an individual’s phenotype without changing their genotype.

Examples of Epigenetics:

• Nutrition: Poor nutrition during pregnancy can lead to epigenetic changes in the developing foetus, increasing the risk of diseases such as obesity, heart disease, or diabetes later in life.

• Stress: Long-term stress can trigger epigenetic changes that affect brain function, potentially increasing the risk of depression or anxiety.

• Smoking: Smoking not only causes mutations but also leads to epigenetic changes in lung cells, increasing the likelihood of developing cancer.

Epigenetics explains why individuals with the same genetic makeup (genotype), such as identical twins, can have different outcomes. For example, if one twin experiences significant stress while the other does not, epigenetic changes could result in only one twin developing a mental health disorder.

GENOTYPE, PHENOTYPE, AND EPIGENETICS

• Genotype: The genetic makeup of an individual, representing the specific combination of alleles they carry.

• Phenotype: An individual's observable traits or characteristics, influenced by both genotype and environmental factors.

Epigenetics bridges the gap between genotype and phenotype by showing how external factors can modify gene expression. For example, two individuals with the same genetic predisposition to diabetes may have different outcomes due to environmental influences, such as diet and exercise.

Example: Identical twins with the same genotype may have different phenotypes if one twin adopts a healthy lifestyle while the other does not. Epigenetic changes resulting from diet and exercise can influence how genes associated with diabetes are expressed, potentially preventing the condition in the healthier twin.

POLYGENIC SCORE (PGS)

A polygenic score (PGS) summarises the estimated effect of many genetic variants on an individual's phenotype. It is often used to predict the likelihood of developing certain traits or diseases based solely on genetics. The PGS is typically calculated as a weighted sum of trait-associated alleles and is commonly used in assessing disease risk.

CONCLUSION: A UNIFIED UNDERSTANDING OF GENETICS

1. DNA, genes, and chromosomes form the foundation of inheritance.

2. Mutations can introduce changes to genes, leading to disorders or new traits.

3. Mutations in a single gene cause monogenic disorders, while polygenic disorders involve multiple genes.

4. Dominant and recessive alleles govern how traits are inherited in simple Mendelian patterns.

5. Epigenetics shows how environmental factors can alter gene expression, influencing the phenotype without changing the genotype.

6. Genotype and phenotype are linked, but the environment can modify gene expression through epigenetic changes.

Understanding these concepts helps explain how both genetics and environmental factors shape who we are, influencing everything from physical characteristics to disease susceptibility.

FURTHER READING AND RESEARCH

• Monogenic vs. Polygenic: Traits, Examples, and Disorders (scienceabc.com)