Why do some traits skip generations while others appear in every one? Why can two brown-eyed parents have a blue-eyed child? The answers lie in how genes and alleles interact — and in a simple grid invented by a monk in the 1860s that still predicts inheritance today.
Think about your family or a family you know well. Consider a simple trait like whether earlobes are attached or detached.
Now answer: If both parents have detached earlobes but one of their children has attached earlobes, what does this tell you about how that trait is inherited? Can a "hidden" trait reappear in later generations? Explain your thinking.
A gene is not a fixed instruction — it is more like a recipe that comes in different versions. Each version is called an allele.
For example, the gene that influences eye colour in humans has several alleles: brown, blue, green and hazel. You inherit two alleles for each gene — one from each parent. These two alleles together determine what trait you actually show.
The key distinction at Stage 5 is between dominant and recessive alleles:
This is why two brown-eyed parents (both Bb) can have a blue-eyed child (bb) — each parent carries the hidden recessive allele and passes it on.
In the 1860s, Gregor Mendel grew thousands of pea plants in a monastery garden and deduced the basic rules of inheritance. Today, we use a simple grid called a Punnett square to apply his discoveries.
A Punnett square shows all possible combinations of alleles that offspring can inherit from two parents. Here is how it works for a single-trait cross:
The Punnett square does not tell you what will happen to a specific child — it tells you the probability of each outcome. Genetics is governed by chance, like flipping a coin.
Consider a cross between two heterozygous parents (Bb x Bb):
These ratios are averages. A family with four children might have all four showing the dominant trait, or three dominant and one recessive, or even (rarely) all four recessive. The Punnett square gives probabilities, not guarantees.
A homozygous genotype has two identical alleles (BB or bb). A heterozygous genotype has two different alleles (Bb). Heterozygous individuals are called carriers when the recessive allele they carry can cause disease if inherited by offspring.
Australian agricultural breeding programs demonstrate Mendelian genetics on an industrial scale. The Australian wool industry has selectively bred Merino sheep for over 200 years, concentrating alleles for fine wool fibre diameter. More recently, the Australian Wagyu cattle industry uses genetic testing to identify carriers of desirable alleles for marbling (intramuscular fat) and combines this with pedigree records to make breeding decisions. CSIRO scientists have also developed DNA markers for disease resistance in sheep and cattle, allowing farmers to select breeding stock based on genotype before the animal ever shows a phenotype. This is modern genetics applied to Australian primary production — proving that Mendel's pea plant discoveries power a multi-billion-dollar industry.
Not all inheritance follows the simple dominant-recessive pattern. Two important exceptions appear at Stage 5:
Incomplete dominance: The heterozygous phenotype is a blend of the two homozygous phenotypes. For example, in snapdragons, a cross between red (RR) and white (WW) flowers produces pink (RW) offspring. Neither allele is fully dominant.
Codominance: Both alleles are expressed equally in the heterozygote. The classic example is the ABO blood group system. A person with genotype IAIB has blood type AB because both A and B antigens are produced on red blood cells.
These patterns show that dominance is not an all-or-nothing rule. The relationship between alleles depends on the molecular function of the proteins they code for. At Stage 5, you need to recognise these patterns and predict offspring ratios, but you do not need to explain the biochemical mechanisms in detail.
Red hair is one of the most striking examples of Mendelian inheritance in human populations. It is caused by variants of the MC1R gene on chromosome 16. To have red hair, a person usually needs two recessive alleles of MC1R — one from each parent. The MC1R protein controls whether melanocytes produce eumelanin (brown/black pigment) or pheomelanin (red/yellow pigment). Non-functional MC1R alleles shift production toward pheomelanin, producing red hair, fair skin and freckles. Interestingly, red hair is most common in people of Celtic and Northern European ancestry, but it also appears in some Australian populations. Approximately 1-2% of the global population has red hair, but around 10-15% of people in Scotland and Ireland are red-haired. Because MC1R is recessive, two parents who are carriers (but do not have red hair themselves) have a 25% chance of having a red-haired child with each pregnancy.
Wrong: "Dominant alleles are more common than recessive ones."
Right: Dominance describes expression, not frequency. The recessive allele for not rolling your tongue is common in many populations. The dominant Huntington's disease allele is rare because it causes severe illness.
1 Cross: BB x Bb (B = black fur dominant, b = white fur recessive). What are the genotype and phenotype ratios?
2 Cross: Bb x bb. What percentage of offspring will show the recessive phenotype?
3 Two parents with brown eyes (both Bb) have four children. Explain why it is possible — though unlikely — that all four children could have blue eyes (bb).
1 In snapdragons, red flower colour (R) shows incomplete dominance over white (W). Predict the phenotype ratio from a cross between a red flower (RR) and a pink flower (RW).
2 A man with blood type A (genotype IAi) has a child with a woman with blood type B (genotype IBi). What are the possible blood types of their children, and what is the probability of each?
3 A breeding program wants to eliminate a recessive genetic disease in cattle. Why is it difficult to identify and remove all carriers (heterozygotes) from the herd?
1. What is the difference between a gene and an allele?
2. An organism has genotype Bb, where B is dominant. What is its phenotype?
3. In a Punnett square cross of Bb x Bb, what is the probability of offspring with the recessive phenotype?
4. Two pink snapdragons (RW) are crossed. What phenotype ratio is expected if flower colour shows incomplete dominance?
5. A cattle breeder wants to eliminate a recessive genetic disorder. Why is testing the phenotype alone insufficient to remove all affected alleles from the herd?
6. Distinguish between genotype and phenotype. Use an example involving flower colour to illustrate your answer. 3 MARKS
7. Two heterozygous parents (Bb) have four children. One child has the recessive phenotype, and the other three have the dominant phenotype. A student claims this "proves" the 3:1 ratio. Evaluate this claim. 4 MARKS
8. Explain why understanding inheritance patterns is important for Australian agricultural breeding programs. In your answer, refer to at least two applications: disease resistance and production traits (such as wool quality or meat marbling). 5 MARKS
Go back to your Think First responses at the top of the lesson.
1. BB x Bb: Genotype ratio = 1 BB : 1 Bb [1 mark]. Phenotype ratio = 100% dominant (black fur) [1 mark]. All offspring inherit at least one dominant B allele.
2. Bb x bb: Genotype ratio = 1 Bb : 1 bb [1 mark]. Phenotype ratio = 50% dominant : 50% recessive [1 mark]. 50% of offspring show the recessive phenotype.
3. Each child has a 25% chance of being bb [1 mark]. The probability of all four being bb is (0.25)4 = 0.39% — very unlikely but not impossible [1 mark]. Each pregnancy is an independent event [1 mark].
1. RR x RW: Genotype ratio = 1 RR : 1 RW [1 mark]. Phenotype ratio = 1 red : 1 pink [1 mark]. There are no white offspring because the white allele (W) is not present in both parents.
2. IAi x IBi: Possible blood types: A (IAi), B (IBi), AB (IAIB), O (ii) [1 mark]. Each has a 25% probability [1 mark]. This demonstrates codominance (IA and IB together) and recessive inheritance (ii) [1 mark].
3. Carriers (heterozygotes) have the normal dominant phenotype [1 mark] but carry one recessive disease allele [1 mark]. When two carriers breed, there is a 25% chance of an affected offspring [1 mark]. DNA testing is needed to identify carriers that phenotype screening cannot detect [1 mark].
1. C — A gene is a DNA segment for a trait; an allele is a version of that gene.
2. B — In a heterozygote (Bb), the dominant allele is expressed and masks the recessive allele.
3. D — Bb x Bb produces 25% bb offspring, which show the recessive phenotype.
4. A — RW x RW with incomplete dominance gives 1 RR (red) : 2 RW (pink) : 1 WW (white).
5. C — Carriers are heterozygous and appear normal (dominant phenotype) but can pass the recessive allele to offspring.
Q6 (3 marks): Genotype refers to the genetic makeup of an organism — the alleles it carries (e.g., BB, Bb or bb) [1 mark]. Phenotype refers to the observable physical trait (e.g., red flowers, pink flowers, white flowers) [1 mark]. For example, a snapdragon with genotype RR has red flowers (phenotype), while one with genotype RW has pink flowers because of incomplete dominance [1 mark].
Q7 (4 marks): The student's claim is partially correct but overstated [1 mark]. While 3 dominant : 1 recessive matches the expected phenotype ratio for a Bb x Bb cross, a single family of four children is too small a sample to "prove" anything [1 mark]. The Punnett square gives probabilities, not guarantees [1 mark]. With only four offspring, random chance could easily produce 4:0, 2:2 or even 0:4 ratios [1 mark]. Larger sample sizes are needed to approach theoretical ratios.
Q8 (5 marks): Understanding inheritance patterns allows Australian farmers to make evidence-based breeding decisions [1 mark]. For disease resistance, if resistance is controlled by a dominant allele, breeders can select animals with the resistant phenotype and test their genotype to identify carriers [1 mark]. For production traits such as wool fineness in Merinos or meat marbling in Wagyu cattle, breeders track pedigrees and use DNA markers to identify animals carrying desirable alleles [1 mark]. This accelerates genetic improvement compared to selecting on phenotype alone [1 mark]. Australian breeding programs demonstrate how Mendelian genetics, combined with modern DNA technology, improves agricultural productivity and animal welfare [1 mark].
Test your knowledge of genes, alleles, Punnett squares and inheritance patterns in this fast-paced quiz battle. Correct answers power your attacks!
Climb platforms using your knowledge of genes, alleles and Punnett squares. Pool: Lesson 4.
Tick when you have finished all activities and checked your answers.