GENETICS

TERMINOLOGY

Mendelian Genetics – The set of theories proposed by Gregor Mendel, which attempt to explain the inheritance patterns of genetic characteristics based on simple breeding experiments involving single genes on chromosome pairs.

• Genotype – The genetic makeup of an individual organism.

• Phenotype – The physical and biological characteristics expressed in an individual as determined by their genotype.

• Epistasis – The interactions between separate genes, in which one masks the effect of another

MONOGENTETIC

Monogenic diseases are caused by a mutation in a single gene within a person's DNA. These conditions can significantly impact an individual's quality of life and lifespan, often leading to birth defects, intellectual, sensory, or motor disabilities. Unfortunately, there is currently no cure or effective treatment for monogenic diseases.

These genetic disorders are hereditary, meaning they can be passed down from parents to their children. Depending on the specific disease, the inheritance pattern can be either dominant or recessive. This means that parents can carry the gene mutation without showing symptoms themselves.

However, even if the parents are unaffected carriers, they can still conceive a child with the disease. To prevent this, the only available option is In Vitro Fertilization (IVF) combined with Preimplantation Genetic Diagnosis (PGD).

Polygenic Inheritance: Simplified

Definition
Polygenic inheritance, also known as quantitative inheritance, is when a single trait is controlled by two or more genes.

Unlike Mendelian genetics, where one gene determines a trait, polygenic traits show a range of possible outcomes because multiple genes and their interactions influence the trait.

In polygenic inheritance, traits do not follow simple patterns of dominant and recessive genes. Instead, the traits show incomplete dominance, where the offspring's traits are a mix of the parents' traits. Each gene contributing to a polygenic trait adds to the final outcome.

Traits controlled by polygenic inheritance, such as hair color, height, and skin color, show a continuous range of variations. Non-visible traits like blood pressure and intelligence also follow this pattern.

Key Points

• Polygenic vs. Multiple Alleles: Polygenic inheritance involves multiple genes influencing a trait. In contrast, multiple alleles refer to different forms of a gene at a single location on a chromosome (e.g., blood types A, B, and O).

• Probability: Polygenic traits usually fit into a normal distribution, where most offspring show a blend of their parents' traits.

Examples

1. Skin Color:
   Skin color is influenced by several genes. For simplicity, let's say three genes (A, B, and C) contribute to melanin production. Each gene has two forms: one that contributes to dark skin (A, B, C) and one that doesn’t (a, b, c). When two parents are heterozygous for these genes (AaBbCc x AaBbCc), various combinations produce different skin tones.

   In this example, there are 64 possible combinations, resulting in seven different skin tones. The lightest (aabbcc) and darkest (AABBCC) are the least common, while intermediate tones are more likely.

2. Height:
   Human height is controlled by over 400 genes, making it complex to predict. Two short parents can have a tall child, and two tall parents can have a short child. Height is also a multifactorial trait, meaning it's influenced by both genetics and environment (e.g., nutrition, health)

POLYGENETIC SCORE (PGS)

In genetics, a polygenic score (PGS) is a number that summarizes the estimated effect of many genetic variants on an individual's phenotype. The PGS is also called the polygenic index (PGI) or genome-wide score; in the context of disease risk, it is called a polygenic risk score (PRS or PR score[1]) or genetic risk score. The score reflects an individual's estimated genetic predisposition for a given trait and can be used as a predictor for that trait.[ It gives an estimate of how likely an individual is to have a given trait based only on genetics, without taking environmental factors into account; and it is typically calculated as a weighted sum of trait-associated alleles.

FURTHER READING AND RESEARCH

Monogenic Vs Polygenic: Traits, Examples, And Disorders (scienceabc.com)