Genetics Basics
Discover the molecular basis of inheritance — what DNA, genes, and chromosomes are, how genetic information is passed from parents to offspring, and why siblings can look both similar and different.
DNA, Genes, and Chromosomes
DNA (deoxyribonucleic acid) is the molecule that carries the genetic instructions for the development, function, growth, and reproduction of all known organisms. It is shaped like a double helix — two strands twisted around each other like a spiral staircase.
From DNA to Chromosome — The Hierarchy
Nucleotide
Basic building block of DNA (sugar + phosphate + base)
DNA strand
Long chain of nucleotides in double helix
Gene
Segment of DNA coding for one protein/trait
Chromosome
Tightly coiled DNA + protein. Found in nucleus.
DNA (Deoxyribonucleic Acid)
A long, double-stranded molecule shaped like a double helix. The two strands are made of sequences of four chemical bases: Adenine (A), Thymine (T), Guanine (G), and Cytosine (C). A pairs with T, and G pairs with C. The order of these bases is the genetic "code."
Gene
A specific section of DNA that contains the instructions for making a particular protein, which determines a specific trait (e.g. eye colour, blood type, enzyme production). The human genome contains approximately 20,000–25,000 genes.
Chromosome
A tightly coiled structure of DNA wrapped around proteins (histones). Humans have 46 chromosomes in 23 pairs in most body cells. Chromosomes are found in the nucleus of eukaryotic cells and are only visible under a microscope during cell division.
Inheritance and Alleles
Traits are inherited from parents through sexual reproduction. Each parent contributes half of their genetic material (via sex cells — sperm and eggs). This means offspring inherit one copy of each gene from each parent — giving them two alleles for most genes.
Key Genetic Terms
- Allele: One version of a gene. E.g. the gene for eye colour has alleles for blue, brown, green.
- Dominant allele: The allele whose effect is shown when present. Written with a capital letter (B).
- Recessive allele: The allele whose effect is hidden when a dominant allele is present. Written with a lowercase letter (b).
- Genotype: The genetic make-up of an organism (e.g. BB, Bb, or bb).
- Phenotype: The physical characteristic that shows (e.g. brown eyes, blue eyes).
Dominant vs Recessive
If an organism inherits two copies of the same allele, it is homozygous. If it inherits two different alleles, it is heterozygous.
| Genotype | Phenotype (brown=dominant) |
|---|---|
| BB | Brown eyes (homozygous dominant) |
| Bb | Brown eyes (heterozygous) |
| bb | Blue eyes (homozygous recessive) |
Punnett Squares
A Punnett square is a diagram used to predict the genotypes of offspring when two parents with known genotypes reproduce. It was named after Reginald Crundall Punnett.
Example: Both parents are heterozygous (Bb × Bb) for eye colour (B = brown dominant)
| B | b | |
|---|---|---|
| B | BB | Bb |
| b | Bb | bb |
Results: 1 BB (brown) : 2 Bb (brown) : 1 bb (blue) = 3 brown : 1 blue ratio (75% brown, 25% blue)
Variation: Genetic and Environmental
Even within a species, individuals show variation — differences in characteristics. Variation can be caused by genetic factors (differences in DNA), environmental factors (diet, climate, experiences), or a combination of both.
Genetic Variation
Caused by differences in the alleles inherited from parents. These traits are inherited.
- Blood type (A, B, AB, O)
- Attached or free earlobes
- Eye colour
- Genetic diseases (e.g. cystic fibrosis)
- Natural hair texture
Environmental Variation
Caused by environmental factors. These traits are not inherited.
- Skin tan from sun exposure
- Body mass (diet and exercise)
- Scars and tattoos
- Language spoken
- Academic skills
Nature vs Nurture: Many traits — such as height and intelligence — are influenced by both genes and environment. A person may have genes for being tall, but poor nutrition in childhood can reduce their final height. Genetics sets a range; the environment determines where within that range you end up.
Key Vocabulary
Gene
A section of DNA that carries the instructions for making a protein, which determines a particular characteristic. Located at a specific position (locus) on a chromosome.
Allele
One of the different versions of a gene. For example, the gene controlling tongue-rolling has a "roller" allele and a "non-roller" allele. Organisms inherit two alleles — one from each parent.
Genotype
The set of alleles an organism has for a particular gene (e.g. BB, Bb, or bb). The genotype is the actual genetic code, not what you can see.
Phenotype
The observable physical characteristic produced by the genotype in its environment (e.g. brown eyes, tall). The phenotype is what you can see or measure.
Worked Examples
In pea plants, tall (T) is dominant over short (t). A tall plant with genotype Tt is crossed with a short plant (tt). Predict the offspring using a Punnett square.
Set up the Punnett square: Parent 1 (Tt) gives T or t; Parent 2 (tt) gives t or t.
| T | t | |
|---|---|---|
| t | Tt | tt |
| t | Tt | tt |
Result: 2 Tt (tall) : 2 tt (short) = 50% tall : 50% short. Half the offspring will be tall (heterozygous) and half will be short (homozygous recessive).
How many chromosomes does a human have, and where do they come from?
Humans have 46 chromosomes arranged in 23 pairs in most body (somatic) cells.
23 chromosomes come from the mother (via the egg) and 23 chromosomes come from the father (via the sperm).
The sex chromosomes determine biological sex: females have XX and males have XY. All eggs carry an X chromosome; sperm can carry either X or Y — the father determines biological sex.
Sex cells (gametes) — eggs and sperm — have only 23 chromosomes (half), so when they combine at fertilisation, the full complement of 46 is restored.
A person has a genotype of Bb for tongue-rolling ability (B = roller, dominant). What is their phenotype, and are they homozygous or heterozygous?
Genotype: Bb — they have one dominant allele (B) and one recessive allele (b).
Phenotype: Because B is dominant, the person can roll their tongue. The recessive allele (b) is masked.
Homozygous or heterozygous? They have two different alleles (Bb), so they are heterozygous. They are a "carrier" of the non-roller allele.
Knowledge Check
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Key Concepts Summary
- ✓ DNA is a double helix molecule made of nucleotides containing the bases A, T, G, and C.
- ✓ A gene is a segment of DNA that codes for a protein determining a trait. Humans have ~20,000–25,000 genes.
- ✓ Chromosomes are coiled DNA structures. Humans have 46 (23 pairs) — 23 from each parent.
- ✓ Alleles are versions of a gene. Dominant alleles (uppercase) mask recessive alleles (lowercase).
- ✓ Genotype is the actual genetic code; phenotype is the observable trait.
- ✓ A Punnett square predicts the probability of different genotypes/phenotypes in offspring.
- ✓ Variation can be genetic (inherited) or environmental (due to the organism's surroundings and experiences).