Difficulty: Beginner | Prerequisites: None. This is introductory-level genetics, suitable for a first biology course.
Genetics is the branch of biology that explains why organisms look and function the way they do, by tracing traits back to the genes inherited from each parent. This topic sits right at the start of most introductory biology courses and lays the groundwork for everything from evolution to biotechnology. You should already be comfortable with the idea that DNA carries hereditary information, but no maths or chemistry is needed here. If you missed earlier lectures, this is a good place to re-enter the course, since the concepts are self-contained and rely mostly on logic and pattern recognition.
Every person carries two copies of each gene, one from each parent. When both copies are the same the organism is homozygous; when they differ it is heterozygous. A dominant allele masks a recessive one, so a heterozygous individual shows the dominant trait. A Punnett square lets you map out every possible offspring genotype from two parents and read off the probability of each trait appearing.
Gene
A segment of DNA that codes for a particular trait. Genes come in pairs, one inherited from each parent.
Think of it as an instruction card. You get one card from your mum and one from your dad, and together they decide how a trait turns out.
Allele
A specific version of a gene. For any given gene, different alleles produce different variations of the trait.
In simple terms, if the gene is "earlobe shape," the alleles are the specific options: attached or unattached.
Dominant allele
An allele whose trait is expressed whenever it is present, even if only one copy exists. Represented by an uppercase letter (e.g. U).
Think of it as the louder voice in a pair. If one allele is dominant, its trait is the one you see.
Recessive allele
An allele whose trait is expressed only when two copies are present (homozygous recessive). Represented by a lowercase letter (e.g. u).
In simple terms, the recessive trait only shows up when there is no dominant allele around to override it.
Homozygous
Having two identical alleles for a given gene (e.g. UU or uu).
Think of it as a matched pair. Both instruction cards say the same thing.
Heterozygous
Having two different alleles for a given gene (e.g. Uu).
In simple terms, your two instruction cards disagree, and the dominant one wins.
Genotype
The genetic makeup of an organism, the combination of alleles it carries (e.g. UU, Uu, uu).
Think of it as what is written on the instruction cards, whether or not you can see the result.
Phenotype
The observable physical trait that results from the genotype (e.g. unattached earlobes or attached earlobes).
In simple terms, phenotype is what you actually see when you look at the organism.
Punnett square
A grid used to predict the possible genotypes of offspring from two parents. Each parent's alleles are placed along one axis, and the boxes show every possible combination.
Think of it as a simple 2 x 2 multiplication table, but for genes instead of numbers.
Genes occur in pairs. You inherit one allele from each parent.
The combination of alleles you carry (your genotype) determines the trait you display (your phenotype).
Siblings can look different even with the same parents because each child receives a different random combination of alleles.
A dominant allele only needs one copy to express its trait. Notation: uppercase letter (U).
A recessive allele needs two copies (one from each parent) to express its trait. Notation: lowercase letter (u).
If an organism is heterozygous (Uu), the dominant trait is the one you see. The recessive allele is still there, it is just masked.
Homozygous dominant (UU): Both alleles dominant. Phenotype: dominant trait (e.g. unattached earlobes).
Heterozygous (Uu): One dominant, one recessive. Phenotype: still the dominant trait, because one dominant allele is enough.
Homozygous recessive (uu): Both alleles recessive. Phenotype: recessive trait (e.g. attached earlobes). This is the only genotype that produces the recessive phenotype.
Place one parent's two alleles across the top (one per column).
Place the other parent's two alleles down the side (one per row).
Fill each box with the combination of the column allele and the row allele.
Count the results to find the ratio of possible genotypes and phenotypes.
Trial 1: Homozygous dominant x Homozygous recessive (UU x uu)
Parent 1 contributes: U or U.
Parent 2 contributes: u or u.
Every box in the Punnett square is Uu.
Result: 100% heterozygous. All offspring display the dominant trait (unattached earlobes).
This tells you which trait is dominant, because every offspring shows it despite carrying one recessive allele.
Trial 2: Heterozygous x Homozygous dominant (Uu x UU)
Parent 1 contributes: U or u.
Parent 2 contributes: U or U.
Possible offspring: UU, UU, Uu, Uu.
Result: 50% homozygous dominant, 50% heterozygous. All offspring display the dominant trait.
Chance of attached earlobes: 0%. Neither parent can contribute two recessive alleles.
For a monohybrid cross (one gene, two alleles), the square is a 2 x 2 grid:
Parent 1: Allele A | Parent 1: Allele B | |
|---|---|---|
Parent 2: Allele C | AC | BC |
Parent 2: Allele D | AD | BD |
Each cell represents one equally likely offspring genotype. With four cells, each outcome has a baseline probability of 25% (1 in 4).
To find the probability of a particular phenotype, count the number of boxes showing that phenotype and divide by 4. For example, in a Uu x Uu cross, one box out of four is uu (homozygous recessive), so the probability of the recessive phenotype is 25%.
Genetic counsellors use Punnett squares to advise prospective parents on the likelihood of their children inheriting conditions such as sickle cell anaemia or cystic fibrosis, both of which follow a recessive inheritance pattern. Agricultural breeders apply the same logic when crossing plant or animal lines to select for desirable traits like disease resistance or higher yield.
Students often think "dominant" means "more common in the population." It does not. Dominant only means the allele expresses its trait when at least one copy is present. A dominant allele can be rare.
Students sometimes assume that a recessive trait will disappear over generations. It will not, because heterozygous carriers continue to pass the recessive allele to their offspring without showing it.
Students frequently confuse genotype and phenotype. Two organisms can look the same (same phenotype) but carry different alleles (different genotype). For instance, UU and Uu both produce unattached earlobes.
Students occasionally think a Punnett square predicts the exact outcome of a single offspring. It does not. It shows probabilities across many offspring. One child is one roll of the dice, not a guaranteed ratio.
⚠️ You will almost certainly be asked to fill in a Punnett square and state the genotype and phenotype ratios of the offspring. Practise until it is automatic.
⚠️ Know the difference between homozygous dominant, heterozygous, and homozygous recessive, and be able to identify each from a given pair of alleles.
⚠️ Be ready to explain how you know which trait is dominant from experimental or simulation results. The key indicator: if crossing two homozygous parents (one dominant, one recessive) produces offspring that all show the same trait, that trait is the dominant one.
⚠️ Expect questions that give you two parents' genotypes and ask for the probability of a specific phenotype in their offspring. Count the Punnett square boxes and express it as a fraction, percentage, or ratio.
True or False: A heterozygous individual always shows the recessive trait.
Fill in the blank: An organism with the genotype uu is described as homozygous __________.
True or False: If both parents are homozygous dominant (UU), some offspring can be homozygous recessive (uu).
Fill in the blank: A Punnett square for a monohybrid cross has __________ boxes.
True or False: Two organisms with the same phenotype must have the same genotype.
Answers: 1. False (it shows the dominant trait). 2. Recessive. 3. False (all offspring will be UU). 4. Four. 5. False (UU and Uu both produce the dominant phenotype).
Q: If you cross a heterozygous parent (Uu) with a homozygous recessive parent (uu), what are the possible genotypes of the offspring and in what ratio?
A: The possible genotypes are Uu and uu, in a 1:1 ratio (50% each). Half the offspring will be heterozygous (dominant phenotype) and half will be homozygous recessive (recessive phenotype).
Q: Two parents both have unattached earlobes. They have a child with attached earlobes. What must both parents' genotypes be?
A: Both parents must be heterozygous (Uu). Only Uu x Uu can produce a uu offspring. If either parent were UU, no child could receive two recessive alleles.
Q: In a cross between UU and uu, what percentage of offspring will show the dominant phenotype?
A: 100%. Every offspring will be Uu (heterozygous), and since U is dominant, all of them display the dominant trait.
Q: What is the difference between genotype and phenotype? Give an example where two different genotypes produce the same phenotype.
A: Genotype is the pair of alleles an organism carries; phenotype is the observable trait. UU and Uu are different genotypes, but both produce the same phenotype (unattached earlobes), because the dominant allele U is present in both.
Q: A Punnett square for Uu x Uu gives the genotype ratio 1 UU : 2 Uu : 1 uu. What is the phenotype ratio?
A: 3:1 (three showing the dominant trait to one showing the recessive trait). UU and Uu both express the dominant phenotype, so the three dominant-phenotype boxes outnumber the single recessive-phenotype box.
This material connects directly to Mendelian genetics and the laws of segregation and independent assortment, which extend the Punnett square to multi-gene crosses. It also feeds into the study of incomplete dominance, codominance, and polygenic inheritance, where the simple dominant/recessive model breaks down. Understanding alleles and genotype ratios is essential groundwork for population genetics and the Hardy-Weinberg equilibrium, which you are likely to encounter later in the course.
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