HEREDITY – LIFE SCIENCE

DNA and Chromosomes

A cell uses the instructions in its deoxyribonucleic acid (DNA) to make substances called proteins. Proteins are responsible for producing physical traits such as eye color.

Genes: A gene is a segment of DNA that codes for a specific protein. DNA itself is made up of pairs of four chemical bases. The specific sequence of these bases in a gene determines which protein is produced.

The diagram of DNA shows how these base pairs are arranged in the famous double helix structure.

From DNA to Protein

The process of using a gene to make a protein happens in two major steps:

•  Transcription: The gene’s DNA sequence is transcribed into a molecule called ribonucleic acid (RNA). RNA acts as a messenger, carrying the genetic instructions out of the nucleus.
•  Translation: The RNA travels to a cell structure called a ribosome. At the ribosome, the sequence is translated into a specific protein.
• This precise flow of information—from DNA to RNA to Protein—is so fundamental to life that it is known as the central dogma of biology (or central dogma of molecular biology).

Inheritance of Traits

Children typically exhibit a mixture of traits from both of their parents. For example, you might have your mother’s eye color and your father’s face shape. When organisms reproduce, they pass on these traits to their offspring through their DNA.

Chromosomes: Reproductive cells (egg and sperm) contain DNA packaged into structures called chromosomes.

The diagram shows how chromosomes carry genetic information.

When an egg and sperm unite during fertilization, they create a new organism with a complete set of chromosomes—half from the mother and half from the father. This combination of genetic material results in your unique mixture of traits, determined by the specific genes you inherit from each parent. In humans, normal body cells contain 46 chromosomes. Reproductive cells (egg and sperm) each contain 23 chromosomes. Chromosomes are stored inside the nucleus of the cell.

A chromosome consists of two identical chromatids attached at a region called the centromere. Each chromosome can carry hundreds or even thousands of genes.

Meiosis and Chromatids: During meiosis, chromosomes split apart into individual, but identical, chromatids. This ensures that reproductive cells (egg or sperm) each receive one chromatid, containing one copy of every gene.

Alleles and Traits

Alleles are different versions of the same gene. Most genes have two alleles, but some have more.

Examples: A human eye-color gene can have two alleles: brown and blue. The human blood-type gene has three alleles: A, B, and O.

Mutations create new alleles. A mutation is a random change in the base sequence of a gene.

 The diagram shows different types of mutations that can lead to new alleles.

Assortment of Alleles

At the start of meiosis, a cell has two copies of each chromosome. Before the chromosomes are separated into new cells, they undergo crossing over.

> Diagram Note: During crossing over, arms from each homologous chromosome literally cross over and exchange segments.

For example, the chromosomes may trade alleles of the “B” gene. After crossing over: Each chromosome has one original chromatid and one chromatid with a new combination of alleles. When meiosis completes, the four resulting reproductive cells each have a unique combination of alleles. This genetic variation ensures no two reproductive cells (sperm or eggs) are exactly the same.

Environmental Influence on Traits: Some traits are determined entirely by the alleles you inherit. For example: Blood type is genetically determined and cannot be changed by the environment. Other traits result from a combination of your genes and environmental factors.

> Table Note: The following table (shown in the text) describes three examples of traits that are linked to specific genes but can be altered by environmental conditions.

Expression of Traits

When a gene is actively used to make a protein, it is said to be turned on or expressed.

Some genes are always expressed because their proteins are constantly needed.

Other genes are turned on only when a specific protein is required.

The study of how and when genes are turned on or off is called epigenetics.

Environmental Effects on Gene Expression

Environmental factors, such as temperature, can directly affect which genes are expressed. A classic example is the coat color of Siamese cats: Hair on warm parts of the body stays light-colored. Hair on cool parts of the body (face, ears, feet, and tail) becomes dark-colored. This happens because the hair color gene is expressed only in areas below a certain temperature.

Genotype and Phenotype

Remember: You have two copies of every gene—one from your mother and one from your father. Your genotype is your combination of alleles (gene versions) for a specific trait. Your phenotype is your physical appearance for that trait. Your genotype determines your phenotype.

Simple Inheritance: Dominant and Recessive Allele

In simple inheritance, a gene has two alleles (versions). These alleles are often represented by capital and lowercase letters:

The dominant allele is represented by a capital letter. The recessive allele is represented by a lowercase letter. If a person has at least one copy of the dominant allele, they will have the dominant phenotype. The recessive phenotype only appears if the person has two copies of the recessive allele.

> Example (from diagram): Brown eye color = dominant, Blue eye color = recessive

Even though human eye color involves multiple genes, this simple model focuses only on the blue/brown gene.

Probability of Inheriting Traits

A Punnett square is a chart used to calculate the probability that parents will pass on a specific trait to their child.

> How to use a Punnett square: One parent’s alleles go along the top. The other parent’s alleles go along the left side. The boxes inside the square show all possible allele combinations (genotypes) the child could inherit. Each box represents a 25% chance if all combinations are equally likely.

Example (from diagram):

Brown-eyed father: Bb            Blue-eyed mother: bb

Results:     50% chance of brown eyes (Bb)

                   50% chance of blue eyes (bb)

Pedigree Charts

A pedigree chart is used to trace the inheritance of a trait through multiple generations of a family. Pedigrees are especially useful for studying genetic disorders, such as colorblindness.

> Diagram Note: Symbols represent family members and their traits.

A half-shaded symbol indicates a carrier: someone with one copy of the recessive disorder allele and one normal (dominant) allele.

Example: Colorblindness is a recessive trait. A carrier does not have colorblindness but can pass the allele to their children.