Genetics Basics: DNA, Genes, Chromosomes, and Inheritance

Genetics Basics

DNA Structure and Function

DNA (deoxyribonucleic acid) is the fundamental unit of genetics, encoding the information necessary for the development and functioning of living organisms.

• DNA Bases: DNA is composed of four nucleotide bases: Adenine (A), Thymine (T), Guanine (G), and Cytosine (C).

• Base Pairing Rules (Chargaff's Rules): A pairs with T (2 hydrogen bonds), and G pairs with C (3 hydrogen bonds).

• The two strands of DNA are complementary and form a double helix structure.

• Genes: A gene is a stretch of DNA that contains the information to produce a functional product, usually a protein.

• Regulatory Elements: Sequences in DNA that control whether or not a gene is expressed.

Genes and Alleles

Genes exist in different forms called alleles. An allele is a gene variant, and in diploid cells, there are two alleles per gene (one from each parent).

• Allele: A specific version of a gene.

• Gene Variant: Another term for allele.

Example: Alleles vs. Genes

Chromosome	Gene	Allele Variant
Chromosome 1	Gene A	Allele A.1
Chromosome 1	Gene B	Allele B.1
Chromosome 1	Gene C	Allele C.1
Chromosome 2	Gene A	Allele A.2
Chromosome 2	Gene B	Allele B.1
Chromosome 2	Gene C	Allele C.2

From DNA to Protein

The process of gene expression involves two main steps:

• Transcription: The process of converting DNA into messenger RNA (mRNA). Other types of RNA produced include transfer RNA (tRNA) and ribosomal RNA (rRNA).

• Translation: The process of converting mRNA into proteins.

Note: DNA does not encode a protein in a 1:1 ratio; multiple codons can code for the same amino acid.

Chromosomal Fundamentals

Chromosomes and Ploidy

Chromosomes are structures that contain many genes. Organisms can be classified based on the number of chromosome sets they possess:

• Homologous Chromosomes: Chromosomes that exist in pairs and carry the same genes, though possibly different alleles.

• Diploid (2n): Organisms with two sets of chromosomes (one from each parent).

• Haploid (n): Organisms or cells with only one set of chromosomes.

Chromosomal Theory of Inheritance

This theory states that inherited traits are controlled by genes located on chromosomes, which are passed from parents to offspring through gametes (sex cells).

Cell Division: Mitosis and Meiosis

• Meiosis: The process of creating gametes (sperm and egg cells). In diploid individuals, meiosis reduces the chromosome number by half, producing four haploid cells from one diploid cell.

• Mitosis: The process of creating somatic (body) cells. Mitosis produces two identical diploid cells from one diploid cell.

Example: Meiosis and Chromosomes

During meiosis, homologous chromosomes separate, ensuring each gamete receives only one chromosome from each pair.

Genotype and Phenotype

Definitions

• Genotype: The set of alleles for a given trait in an organism (e.g., AA, Aa, or aa).

• Phenotype: The observable features or traits of an organism (e.g., yellow or green peas).

Example: Pea Color

Phenotype	Genotype
Yellow	AA or Aa
Green	aa

A = dominant allele, a = recessive allele

Types of Traits

• Morphological Traits: Affect the appearance of the organism.

• Physiological Traits: Affect the ability of an organism to function properly.

• Behavioral Traits: Affect the way an organism responds to its environment.

Divisions of Genetics

• Transmission Genetics: Studies the ability to pass traits onto the next generation.

• Molecular Genetics: Studies gene activity at the molecular level (DNA replication, transcription, translation, etc.).

• Population Genetics: Studies the genetic composition of populations and how it changes over time.

Practice Questions

1. True or False: In a diploid cell, the number of alleles is the same as the number of genes. Answer: True

2. What is an allele? Answer: Different variants of a gene.