Overview of Basic Genetic Science

Dr. Mike DoughertyDepartment of Biology

Hampden-Sydney College

What is science and how does it differ from other ways of knowing the world?

Scientific understanding relies on:

Logic

Evidence

Naturalism

Tentativeness and falsifiability

Science rejects:

Supernatural explanations

Arguments from authority

Methodological Materialism

What are “genes” and how do they work?

DNA is an information molecule with two functions:

1. storage and copying (i.e., inheritance)

2. expression (i.e., physiology, health, disease)

Each DNA strand serves as a template for its own replication, ensuring continuity of information.

Pedigrees illustrate how information can be passed on to the next generation of family members through inheritance.

Gene expression

Linear and causal links exist between DNA and proteins.

Proteins are the workhorses of cells and tissues.

Step 1:

Recognizing a gene.

Step 2:

Production of an RNA, a molecule that bridges the chemical gap between DNA and protein.

Step 3:

Translating the language of RNA (nucleic acid) into the language of proteins (amino acids).

The Genetic Code

Why are proteins so important?

Chain of causation:

DNA sequence

RNA sequence

amino acid sequence

folded structure

function

If proteins carry out the functions of cells and tissues, does that mean they control traits, such as height, heart disease, and happiness?

NOT by themselves!

It is more accurate to say that “genes influence traits” to varying degrees?

Phenotypes (traits) exist on a continuum:

Weak genetic Strong genetic

influence influence

Personality Major depression

Heart disease Phenylketonuria

Strong environmental Weak environmental influence influence

Genetic variation

Humans have the same genes,

but those genes exist in

different versions.

These differences help explain why many people with the same disease manifest symptoms in unique ways.

These differences also reveal evolutionary connections among related species.

How does DNA sequence variation arise?

Through environmental insults to DNA and mistakes during copying.

Mutations can alter proteins, which may lead to normal trait variations or disease.

But . . . in many cases the mutations are harmless.

Can we predict a person’s traits if we know their DNA sequence?

Only in rare cases . . .

If a genotype is highly penetrant, we can often predict accurately that a person will develop certain traits (usually diseases), but the degree of the trait is highly variable and unpredictable.

(e.g., Huntington’s disease)

As it turns out, most genotypes . . .

. . . are NOT highly penetrant.

e.g., familial colon cancer

65% of persons having the mutated gene will develop cancerous polyps, but we cannot predict whether any given individual will be affected or disease-free.

Even worse (for prediction), most traits are not strongly influenced by a single gene. They are polygenic and multifactorial.

e.g., height, skin color, personality traits, heart disease, diabetes.

Such traits are said to show continuous variation.