Genetics is the study of heredity, the process by which specific characteristics are passed from parents to offspring. This field explores the variation among individuals within a species, explaining why no two organisms are exactly alike, except for identical twins. Genetics seeks to understand the fundamental instructions that orchestrate life, including development, function, and behavior.
The Blueprint Molecule
The physical representation of genetic information is the Deoxyribonucleic Acid (DNA) molecule. DNA has a double helix structure, resembling a twisted ladder consisting of two long strands that coil around a central axis. The backbone of each strand is a repeating chain of sugar (deoxyribose) and phosphate groups.
The “rungs” of the ladder are formed by pairs of nitrogenous bases: adenine (A), guanine (G), cytosine (C), and thymine (T). They follow a complementary pairing rule, connecting the two strands: A always pairs with T, and G always pairs with C.
A gene is a specific segment of this DNA sequence that provides instructions for building a functional product, typically a protein. The sequence of these base pairs contains the coded instructions for life, determining the structure and function of the resulting protein.
Organizing the Genetic Material
DNA must be compactly organized to fit inside the cell’s nucleus. This massive length is managed by wrapping the DNA tightly around specialized proteins called histones. This complex forms chromatin, a thread-like material.
Chromatin condenses further into distinct structures called chromosomes during cell division. Humans possess 46 chromosomes, arranged in 23 pairs. One chromosome in each pair comes from each parent, forming homologous pairs.
The first 22 pairs are autosomes, carrying most genetic information. The final pair consists of the sex chromosomes (XX for females, XY for males), which determine biological sex. A karyotype is the visual arrangement of all these chromosomes, allowing inspection for structural abnormalities.
The Flow of Information
The Central Dogma of molecular biology describes how information stored in DNA is converted into the functional components of the cell. This framework explains a directional flow of information, moving from DNA to RNA to protein. This conversion happens in two stages: transcription and translation.
Transcription is the process where a specific gene segment on the DNA is copied into a messenger RNA (mRNA) molecule. An enzyme called RNA polymerase reads the DNA sequence and synthesizes a complementary RNA strand, substituting uracil (U) for thymine (T). In eukaryotic cells, this mRNA then leaves the nucleus and travels to the cytoplasm.
Translation occurs on cellular structures called ribosomes. The mRNA sequence is “read” in three-base segments, each called a codon. Transfer RNA (tRNA) molecules bring the correct amino acid to the ribosome based on the codon sequence. The ribosome links these amino acids together to build a polypeptide chain, which folds into a functional protein.
Predicting Traits
Mendelian genetics focuses on how traits are passed down and predicted across generations. Traits are controlled by versions of a gene called alleles. Since organisms inherit one set of chromosomes from each parent, they possess two alleles for every gene.
An organism is homozygous if it has two identical alleles for a specific trait, and heterozygous if the two inherited alleles are different. Dominant traits are expressed even if only one copy of that allele is present. A recessive trait will only be expressed if an individual inherits two copies of the recessive allele.
The Punnett square is a visual tool used to predict the probability of offspring inheriting specific genotypes. This chart summarizes all possible combinations of parental alleles in a grid format. Each box represents a 25% chance of a particular genotype occurring, allowing for the calculation of expected inheritance ratios.