Genes, Alleles and Inheritance Patterns
1 in 25 Australians unknowingly carries the cystic fibrosis allele, and two carriers have a 25% chance of an affected child, predicted by a grid invented in 1905.
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Q1 · What does it mean for a trait to be 'dominant'? What does 'recessive' mean to you?
Think about whether a dominant trait must be common, or whether a recessive trait can be hidden in some people.
Q2 · Two parents are both carriers for a recessive genetic condition. What are the chances their child will have the condition? Explain your reasoning.
Consider how many copies of a recessive allele are needed for the trait to appear, and how many each parent can pass on.
● Know
- That genes are segments of DNA that code for traits
- That alleles are different versions of the same gene
- The difference between dominant and recessive alleles
- The definitions of genotype and phenotype
● Understand
- How dominant and recessive alleles interact to produce traits
- How Punnett squares predict offspring ratios
- Why two heterozygous parents can produce homozygous offspring
● Can do
- Construct and interpret a Punnett square for single-trait inheritance
- Distinguish between homozygous and heterozygous genotypes
- Predict genotype and phenotype ratios from a genetic cross
Two brown-eyed parents are puzzled when their newborn has blue eyes, but draw one simple grid and the answer becomes obvious. A Punnett square is that grid, used to predict the genetic makeup of offspring from a particular cross. It works by listing the possible alleles from one parent along the top and the possible alleles from the other parent along the side. Each box inside the square represents one possible combination of alleles in the offspring.
When both parents are heterozygous for a trait, meaning they carry one dominant allele and one recessive allele, the Punnett square reveals something surprising: there is a 25% chance that any child will inherit two recessive alleles and display the recessive trait. This is exactly how two brown-eyed parents can have a blue-eyed child. The brown-eye allele (B) is dominant, so both parents have brown eyes despite carrying a hidden blue-eye allele (b).
Cross two heterozygous parents (Bb x Bb). The Punnett square shows: 25% BB (brown eyes), 50% Bb (brown eyes, carrier), and 25% bb (blue eyes). Out of four children on average, one will have blue eyes, not magic, just probability.
Australian context: Cattle farmers in northern Australia use Punnett squares to predict the chance of calves inheriting tolerance to cattle ticks, a trait controlled by dominant alleles in some breeds. By selecting breeding pairs carefully, they can improve herd health without chemical treatments.
Many students think dominant alleles are more common or 'stronger' in a biological sense. Dominance only means that the allele is expressed when paired with a recessive allele. A dominant allele can be extremely rare in a population, while a recessive allele can be very common. Frequency and dominance are completely separate concepts.
Complete this Punnett square for two heterozygous brown-eyed parents (Bb x Bb).
Parent 2 (Bb) can pass on allele or .
The chance of a blue-eyed (bb) child is in 4, or %.
Complete dominance is the simplest inheritance pattern: the dominant allele completely masks the recessive one in heterozygotes. However, not all traits follow this rule. In incomplete dominance, the heterozygote shows a blended phenotype, red and white flower alleles produce pink flowers. In codominance, both alleles are expressed equally, such as in AB blood type, where both A and B antigens appear on red blood cells.
Understanding these patterns is essential because many real traits do not fit the simple dominant-recessive model. Human height, skin colour and intelligence are all influenced by multiple genes (polygenic inheritance) as well as environment. Even eye colour, while often taught as a single-gene trait, is actually controlled by several genes with complex interactions.
Sickle cell trait is codominant. A person with one sickle allele and one normal allele produces both normal and sickle-shaped red blood cells. This heterozygote usually has no severe symptoms and gains resistance to malaria, a powerful example of how genetic variation can be advantageous in specific environments.
Australian health: The Royal Children's Hospital in Melbourne provides genetic counselling for families with inherited conditions like cystic fibrosis. Counsellors use Punnett squares and probability to help parents understand the chances of passing on recessive disorders, empowering informed family planning decisions.
If a brown-eyed parent (Bb) has a child with a blue-eyed parent (bb), what percentage of their children will have blue eyes? Predict before revealing.
How close was your prediction?
Nice calibration, your intuition is good for this kind of problem.
Good, being surprised is the point. This answer is worth remembering.
While Mendelian genetics gives us a powerful starting framework, most traits in the real world are more complex. Polygenic inheritance occurs when multiple genes contribute to a single trait. Human skin colour, for example, is influenced by at least three different genes, each with multiple alleles. This produces the continuous range of skin tones we see across human populations.
Sex-linked inheritance is another important pattern. Genes located on the X chromosome (such as those for red-green colour blindness and haemophilia) show different inheritance patterns in males and females because males have only one X chromosome. A male needs only one recessive allele on his X chromosome to express the trait, while a female needs two, making these conditions far more common in males.
Red-green colour blindness affects about 8% of males but only about 0.5% of females. The gene is on the X chromosome. A male with the recessive allele on his single X chromosome will be colour blind. A female must inherit the recessive allele on both X chromosomes, a much rarer event because she would need a colour-blind father and a carrier mother.
Australian research: Scientists at the Murdoch Children's Research Institute in Melbourne study how multiple genes interact with prenatal environment to influence birth weight and later health. This polygenic approach is replacing older single-gene models and leading to better predictions of disease risk.
Students often say that dominant alleles 'destroy' or 'overwrite' recessive alleles. This is false. The recessive allele remains intact in the DNA of a heterozygote and can be passed to the next generation. Dominance is about expression, not destruction. The recessive allele is still there, just silent in the phenotype.
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.
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 whether an allele is expressed in the phenotype, not how common it is in the population. Recessive alleles can be very common.
Wrong: 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.
Right: Dominance refers to expression, not frequency. Many recessive alleles are common, while some dominant alleles (like Huntington's) are rare.
Punnett Square Practice
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).
Inheritance in the Real World
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?
Copy Into Your Book
▼Genes and Alleles
- Gene = DNA segment for one trait
- Allele = version of a gene
- Dominant = expressed when one copy present
- Recessive = only expressed with two copies
Genotype vs Phenotype
- Genotype = genetic makeup (letters)
- Phenotype = observable trait
- BB and Bb = same phenotype if B dominant
- bb = recessive phenotype only
Punnett Squares
- Shows all possible offspring genotypes
- Gives probabilities, not guarantees
- Bb x Bb = 1:2:1 genotype ratio
- Bb x Bb = 3:1 phenotype ratio
Beyond Simple Dominance
- Incomplete dominance = blended phenotype
- Codominance = both alleles expressed
- ABO blood type = codominance example
At the start of this lesson you encountered the striking fact that one in 25 Australians carries a hidden recessive allele for cystic fibrosis, giving two healthy carriers a 1-in-4 chance of having an affected child. That probability probably felt abstract before, now that you have worked with Punnett squares and inheritance patterns, revisit it.
Can you now draw out the cross that produces that 1-in-4 ratio and explain why it happens? What was the most important thing the Punnett square revealed to you about how alleles behave?
Q1. Distinguish between genotype and phenotype. Use an example involving flower colour to illustrate your answer. 3 MARKS
Q2. 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
Q3. 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
Revisit Your Initial Thinking
Go back to your Think First responses at the top of the lesson.
- Did you correctly identify that a child with attached earlobes from detached-earlobe parents indicates a recessive trait?
- Did you recognise that recessive alleles can be carried across generations without showing?
- Write one sentence explaining why Punnett squares give probabilities, not certainties.
Model answers (click to reveal)
Comprehensive Answers
▼Activity 1, Punnett Square Practice
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].
Activity 2, Inheritance in the Real World
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].
Multiple Choice
1. CA gene is a DNA segment for a trait; an allele is a version of that gene.
2. BIn a heterozygote (Bb), the dominant allele is expressed and masks the recessive allele.
3. DBb x Bb produces 25% bb offspring, which show the recessive phenotype.
4. ARW x RW with incomplete dominance gives 1 RR (red) : 2 RW (pink) : 1 WW (white).
5. CCarriers are heterozygous and appear normal (dominant phenotype) but can pass the recessive allele to offspring.
Short Answer Model Answers
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].
Jump Through Inheritance!
Climb platforms using your knowledge of genes, alleles and Punnett squares. Pool: Lesson 4.