Difficulty: Introductory | Prerequisites: None (first genetics unit)
This is the opening unit of introductory genetics, covering the experiments and reasoning that launched the entire field. Gregor Mendel's work with garden peas in the 1800s established the basic rules of inheritance that underpin everything from selective breeding to modern gene therapy. You do not need any prior genetics knowledge here, but a rough sense of what DNA is (a molecule that carries instructions for building an organism) will help. Everything that follows in the course, from Punnett squares to molecular genetics, builds directly on the vocabulary and logic introduced in this unit.
Gregor Mendel crossed garden peas and discovered that traits are controlled by discrete units (genes) that come in pairs (alleles), with one version (dominant) masking the other (recessive) in the organism's appearance. The physical appearance is the phenotype; the underlying pair of alleles is the genotype. If both alleles match, the genotype is homozygous; if they differ, it is heterozygous.
Trait
Any characteristic that can be passed from parent to offspring via DNA.
In simple terms, a trait is any feature an organism can inherit, such as eye colour or plant height.
Gene
A segment of DNA (on a chromosome) containing the instructions for building a protein, which in turn produces a trait.
Think of a gene as a single recipe in the cookbook of your DNA.
Allele
A specific version of a gene. Each organism carries two alleles per gene, one from each parent.
In simple terms, alleles are the different "flavours" a gene can come in, for example, brown-eye allele vs. blue-eye allele.
Dominant allele
An allele that is expressed whenever it is present, whether in one copy or two. Written as a capital letter (e.g. B).
If a dominant allele is in the pair, you will see its trait.
Recessive allele
An allele that is expressed only when two copies are present (no dominant allele to mask it). Written as a lowercase letter (e.g. b).
A recessive trait stays hidden unless the organism inherits the recessive allele from both parents.
Genotype
The actual pair of alleles an organism carries for a given gene. Written as two letters (e.g. BB, Bb, bb).
Think of the genotype as the behind-the-scenes genetic code.
Phenotype
The physical, observable characteristic that results from the expression of the genotype.
The phenotype is what you can actually see: brown eyes, tall plant, roller tongue.
Homozygous
Having two identical alleles for a particular gene (e.g. BB or bb).
Both copies match.
Heterozygous
Having two different alleles for a particular gene (e.g. Bb).
The two copies differ, and the dominant one determines the phenotype.
True-breeding (purebred)
Organisms that produce offspring identical to themselves for a given trait, generation after generation. They carry only one version of the allele (homozygous).
If you cross two true-breeding tall pea plants, every offspring is tall.
Hybrid
The offspring of two different true-breeding (purebred) parents. Hybrids are heterozygous for the trait in question.
A hybrid inherits one allele from each purebred parent, so the dominant trait shows.
Gregor Mendel is considered the Father of Modern Genetics (work began c. 1856, published 1866; born 1822).
He chose garden peas (Pisum sativum) because they produce many varieties, usually self-pollinate (making true-breeding lines easy to maintain), and have a short generation time with many offspring.
He tracked seven distinct traits: seed shape, seed colour, seed coat colour, pod shape, pod colour, flower position, and plant height.
By carefully crossing true-breeding lines, he could observe how traits passed from one generation to the next.
Each trait is controlled by a gene, and each gene comes in variant forms called alleles.
Every organism carries two alleles per gene (one from each parent).
A dominant allele is always expressed when present; a recessive allele is expressed only when no dominant allele is there to mask it.
Convention: dominant alleles are written as capital letters (B), recessive as lowercase (b).
Genotype = the pair of alleles an organism actually has (BB, Bb, or bb).
Phenotype = what you observe (brown eyes, blue eyes, tall plant).
A heterozygous individual (Bb) shows the dominant phenotype, so the phenotype alone does not always reveal the full genotype.
Homozygous: both alleles are the same (BB or bb). The organism is a purebred for that trait.
Heterozygous: the two alleles differ (Bb). The organism is a hybrid for that trait.
Crossing two different purebreds produces hybrid offspring that express the dominant trait.
The following are treated as simple dominant/recessive in this course, though in reality most involve more complex inheritance:
Dark hair colour: dominant over light hair
Red hair colour: dominant over non-red hair
Brown eye colour: dominant over blue
Green eye colour: dominant over blue
Tongue rolling: roller is dominant over non-roller
Widow's peak: peak is dominant over no peak
Ear lobes: free is dominant over attached
Mendelian inheritance is the basis of selective breeding in agriculture (crossing plant varieties to combine desirable traits) and of genetic counselling in medicine (predicting the likelihood that a child will inherit a recessive disorder such as cystic fibrosis). Every time a breeder or a clinician draws a Punnett square, they are using Mendel's logic.
Students often think "dominant" means "more common in the population." It does not. Dominant simply means the allele is expressed when present, regardless of how frequently it occurs.
Students sometimes assume that a dominant trait will always appear in 75% of offspring. That ratio (3:1) only holds for a cross between two heterozygous parents, not for every possible cross.
Students confuse genotype with phenotype. Two organisms can share the same phenotype (e.g. brown eyes) but have different genotypes (BB vs. Bb).
Many real human traits are presented in intro courses as simple dominant/recessive (tongue rolling, widow's peak) when they are polygenic or more complex. The simplified model is a teaching tool, not the full picture.
⚠️ Know the difference between genotype and phenotype cold. Expect a question that gives you a phenotype and asks for the possible genotype(s).
⚠️ Be able to define and distinguish homozygous dominant (BB), homozygous recessive (bb), and heterozygous (Bb).
⚠️ Understand why Mendel chose peas: self-pollination, many varieties, fast reproduction. This is a common short-answer prompt.
⚠️ Remember the convention: dominant alleles are capitalised, recessive are lowercase. Mixing this up will cost marks on notation questions.
True or False: A recessive allele is never expressed. (False: it is expressed when two copies are present.)
Fill in the blank: The physical appearance of an organism is called its __________. (Phenotype.)
True or False: An organism with the genotype Bb is homozygous. (False: Bb is heterozygous.)
Fill in the blank: Mendel chose garden peas because they self-__________, produce many __________, and have a short generation time. (Pollinate; varieties/offspring.)
True or False: "Dominant" means the allele is more common in the population. (False: dominant means the allele is expressed when present.)
Q: What is the difference between a gene and an allele?
A: A gene is a segment of DNA that codes for a particular trait. An allele is one specific version of that gene. Each organism has two alleles per gene.
Q: If a pea plant has the genotype Tt for height (T = tall, t = short), what is its phenotype?
A: Tall. T is dominant, so the plant expresses the tall phenotype.
Q: An organism with the genotype bb for eye colour shows blue eyes. Is this organism homozygous or heterozygous? Dominant or recessive?
A: Homozygous recessive. Both alleles are identical (b and b), and both are the recessive form.
Q: Why are garden peas a good model organism for studying inheritance?
A: They produce many varieties, usually self-pollinate (which makes true-breeding lines easy to establish), and produce many offspring in a short time.
Q: Two true-breeding pea plants are crossed: one with purple flowers (PP) and one with white flowers (pp). What genotype and phenotype do the offspring (F1 generation) have?
A: Genotype: Pp (heterozygous). Phenotype: purple flowers, because P (purple) is dominant over p (white).
Q: A person has the phenotype "non-roller" for tongue rolling. What is their genotype?
A: tt (homozygous recessive). Non-rolling is the recessive phenotype, so both alleles must be the recessive form.
This material connects directly to Punnett squares and probability, which formalise the crosses Mendel performed. It also lays the groundwork for understanding incomplete dominance and codominance (where the simple dominant/recessive model breaks down) and for sex-linked inheritance, where alleles sit on the X chromosome and follow slightly different rules.
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