Genetics & Heredity
Genetics explains how traits are inherited from parents to offspring through DNA. Every organism's characteristics are encoded in genes.
DNA & Genes
DNA (deoxyribonucleic acid) is a double helix molecule made of nucleotides (A, T, G, C). A gene is a segment of DNA that codes for a protein. Humans have ~20,000–25,000 genes organized on 23 pairs of chromosomes.
Alleles: Dominant & Recessive
Alleles are versions of a gene. Dominant alleles (capital letter) are expressed when present. Recessive alleles (lowercase) only show when two copies are present.
B = brown eyes (dominant) | b = blue eyes (recessive)
BB = brown | Bb = brown | bb = blue
Punnett Squares
A grid predicting offspring probability. Cross two Bb parents:
B b B BB Bb b Bb bb
Result: 25% BB, 50% Bb, 25% bb → 75% brown, 25% blue eyes
Genotype vs Phenotype
Genotype = genetic makeup (BB, Bb, bb).
Phenotype = physical expression (brown eyes, blue eyes).
Mutations
Changes in DNA sequence. Can be spontaneous or caused by mutagens (UV radiation, chemicals). Most are neutral; some cause disease; rare ones are beneficial (basis of evolution).
FAQ
Can two brown-eyed parents have a blue-eyed child? Yes — if both parents are Bb (carriers). The child has a 25% chance of being bb.
Quick Quiz
Test what you just learned. Choose the best answer for each question.