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).
Step by Step: Building a Punnett Square
A Punnett square is a bookkeeping tool. It lists every combination of alleles the offspring could inherit, which turns heredity into a counting problem.
Cross a homozygous dominant parent with a recessive one (BB × bb)
- Write one parent's alleles across the top: B and B.
- Write the other parent's alleles down the side: b and b.
- Fill each box by combining its row and column letters.
- Every box reads Bb.
Result: 100% Bb, so every child has brown eyes while still carrying a blue allele. This is why a trait can vanish for a generation and reappear later.
How DNA Becomes a Trait
Genes do not contain eye colour. They contain instructions for building proteins, and proteins do the work that produces the trait.
- Transcription: a gene's DNA sequence is copied into messenger RNA inside the nucleus.
- Translation: ribosomes read that RNA three letters at a time and assemble matching amino acids.
- The finished chain of amino acids folds into a protein — perhaps an enzyme that makes brown pigment.
A mutation changes a letter in the sequence, which can change an amino acid, which can change the protein's shape and stop it working.
When Traits Are Not Simply Dominant
- Incomplete dominance: the heterozygote is a blend. A red and a white snapdragon produce pink offspring.
- Codominance: both alleles show fully and separately, as in AB blood type.
- Multiple alleles: some genes have more than two versions in the population. Blood type has three: A, B, and O.
- Polygenic traits: height and skin colour depend on many genes at once, which is why they vary continuously instead of in categories.
Most human traits fall into these categories, not the neat dominant-recessive model used for practice problems.
Common Mistakes
- Assuming dominant means common. Dominant describes which allele is expressed, not how frequent it is — polydactyly is dominant and rare.
- Reading a 25% probability as a guarantee. Each child is an independent event; four children from a Bb × Bb cross can all have brown eyes.
- Confusing genotype with phenotype. BB and Bb look identical but are genetically different.
- Thinking mutations are always harmful. Most are neutral, and the rare beneficial ones are the raw material of evolution.
Summary
- Genes are DNA segments coding for proteins; alleles are different versions of a gene.
- Dominant alleles show whenever present; recessive ones need two copies.
- Punnett squares turn a cross into predicted probabilities.
- Genotype is the genetic code; phenotype is what you can observe.
- Incomplete dominance, codominance, and polygenic inheritance explain most real traits.
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.
Why do siblings look different if they share parents? Each parent passes on a random half of their alleles, so siblings receive different combinations. Only identical twins get the same set.
What is a carrier? Someone heterozygous for a recessive condition. They do not have the condition themselves but can pass the allele to a child.
How do sex chromosomes work? Humans have 23 pairs of chromosomes; the last pair determines sex, usually XX or XY. Genes on the X chromosome, such as the one for red-green colour blindness, produce sex-linked inheritance patterns.
Does environment affect traits? Considerably. Genes set a range of possibilities, and nutrition, activity, and other conditions determine where within that range someone lands.
Quick Quiz
Test what you just learned. Choose the best answer for each question.