Not every inheritance pattern fits a simple dominant-recessive model. Some traits show both alleles together, some show an intermediate phenotype, and some genes have more than two alleles in the population.
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A student says, "If red flower colour is dominant and white flower colour is recessive, then crossing a red flower with a white flower should always give red offspring. So pink flowers prove the experiment went wrong."
Before reading on, explain why that reasoning is weak. What inheritance pattern could produce pink offspring, and why does that not mean the alleles have disappeared or blended permanently?
Wrong: Homeostasis means the body stays exactly the same all the time.
Right: Homeostasis involves dynamic equilibrium — constant small adjustments around a set point.
Simple dominant-recessive inheritance is only one inheritance pattern. The effect of two alleles in a heterozygote depends on how the gene product is expressed.
In Lesson 14, the heterozygous genotype often showed the dominant phenotype. In this lesson, the heterozygous phenotype may be intermediate, or it may show both allele products together. That is why some crosses do not produce the expected 3:1 phenotypic ratio.
One allele masks the other in the heterozygote, often producing a 3:1 phenotype ratio.
The heterozygote shows an intermediate phenotype, often producing a 1:2:1 phenotype ratio.
Both alleles are expressed in the heterozygote, so the phenotype includes both.
In incomplete dominance, neither allele is fully dominant in the heterozygote. A common example is red-flowered and white-flowered snapdragons producing pink heterozygous offspring.
The key idea is that the alleles still segregate normally in meiosis. They have not permanently mixed into one new allele. The intermediate phenotype appears because of the expression pattern in the heterozygote.
In co-dominance, the heterozygous phenotype shows both allele products. Neither allele hides the other, and the phenotype is not simply halfway between them.
IA and IB alleles are co-dominant. A person with genotype IAIB has blood group AB because both A and B antigens are expressed on the red blood cells.Each individual still has only two alleles for an autosomal gene, one inherited from each parent. However, the gene may exist in more than two forms across the population. This is called multiple alleles.
The ABO blood group has three common alleles: IA, IB and i.
IA and IB are co-dominant with each other, and both are dominant over i.
Phenotypes are A, B, AB and O, depending on which two alleles the individual has.
If the heterozygote shows the dominant phenotype, a monohybrid cross often gives a 3:1 phenotype ratio. If the heterozygote is intermediate, the same cross may give a 1:2:1 phenotype ratio. That is why phenotype data matters in inheritance questions.
In co-dominance, both alleles are fully expressed in the heterozygous phenotype, such as IAIB giving blood group AB.
In incomplete dominance, the heterozygous phenotype is intermediate between the two homozygous phenotypes, such as red and white flowers producing pink offspring.
Multiple alleles means more than two allele forms exist in the population, even though each individual still has only two alleles for the gene.
Non-Mendelian inheritance patterns often produce phenotype ratios other than 3:1 because the heterozygote has its own distinct phenotype.
Look back at what you wrote in the Think First section. What has changed? What did you get right? What surprised you?
In a plant species, red flowers and white flowers show incomplete dominance. Heterozygous plants are pink.
Cross two pink plants. Write the Punnett square, then state the genotype and phenotype ratios.
One parent has genotype IAi and the other has genotype IBi.
List all possible offspring genotypes and identify the possible blood group phenotypes. Explain why this example shows both multiple alleles and co-dominance.
1. What best describes co-dominance?
2. Which statement about incomplete dominance is correct?
3. A gene has three common allele forms in the population. What term describes this?
4. Which genotype gives blood group AB in the ABO system?
iiIAiIAIBIBi5. A cross between two heterozygotes produces a 1:2:1 phenotype ratio. What does this most strongly suggest?
6. Explain the difference between co-dominance and incomplete dominance using one example for each.
3 marks
7. Two pink flowers are crossed in a species showing incomplete dominance. Red flowers are CRCR, white flowers are CWCW, and pink flowers are CRCW.
4 marks
Determine the genotype ratio and phenotype ratio of the offspring.
8. In the ABO blood group system, one parent has genotype IAi and the other has genotype IBi.
5 marks
Use Punnett square reasoning to identify all possible offspring genotypes and phenotypes, and explain why this system demonstrates both co-dominance and multiple alleles.
The heterozygote expresses both allele products, as in blood group AB.
The heterozygote has an intermediate phenotype, as in pink flowers from red and white parents.
A gene can have more than two allele forms in the population even though each individual still carries only two alleles.
Return to the pink-flower claim from the start of the lesson. Rewrite it using the correct inheritance language.
1. B - Co-dominance means both alleles are expressed in the heterozygote.
2. A - In incomplete dominance, the heterozygote is intermediate between the two homozygotes.
3. C - More than two allele forms in the population is called multiple alleles.
4. C - Blood group AB has genotype IAIB.
5. B - A 1:2:1 phenotype ratio suggests the heterozygote has its own distinct phenotype.
Co-dominance means both alleles are fully expressed in the heterozygote, such as IAIB producing blood group AB. Incomplete dominance means the heterozygote shows an intermediate phenotype, such as pink flowers from a red and white flower cross.
The cross is CRCW x CRCW. Each parent produces gametes CR and CW. Offspring genotypes are CRCR, CRCW, CRCW and CWCW. Genotype ratio = 1 CRCR : 2 CRCW : 1 CWCW. Phenotype ratio = 1 red : 2 pink : 1 white.
The first parent produces gametes IA and i. The second parent produces gametes IB and i. Possible offspring genotypes are IAIB, IAi, IBi and ii. Their phenotypes are AB, A, B and O. This shows co-dominance because IA and IB are both expressed together in blood group AB. It shows multiple alleles because the ABO gene exists as three common alleles in the population: IA, IB and i.
Tick this once you can distinguish co-dominance, incomplete dominance and multiple alleles without forcing every question into a simple 3:1 pattern.