← Unit 4: Heredity and continuity of life
Topic 2: Inheritance
Describe and apply non-Mendelian patterns of inheritance including codominance, incomplete dominance, multiple alleles, sex linkage and polygenic inheritance
A focused answer to the QCE Biology Unit 4 dot point on non-Mendelian inheritance. Walks through codominance (ABO blood groups, roan cattle), incomplete dominance (snapdragon flower colour), multiple alleles (ABO, coat colour), X-linked inheritance (haemophilia, colour blindness, Punnett squares with sex chromosomes), and polygenic inheritance (skin colour, height) with continuous variation.
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What this dot point is asking
QCAA wants you to recognise and apply patterns of inheritance that do not produce simple 3:1 or 9:3:3:1 ratios, including codominance, incomplete dominance, multiple alleles, sex linkage and polygenic inheritance. Questions usually present a worded scenario and expect a Punnett square plus an interpretation.
The answer
Mendel's laws describe inheritance for genes with two alleles, complete dominance and independent loci on autosomes. Real inheritance often deviates in five common ways. Each has a defining feature and a characteristic ratio.
Codominance
Definition. Both alleles in a heterozygote are fully expressed in the phenotype. Neither masks the other.
Examples.
- ABO blood groups. I^A I^B individuals (group AB) produce both A and B antigens on red blood cells.
- Roan cattle. A cross between a homozygous red cow (RR) and a homozygous white cow (WW) gives roan offspring (RW) with patches of both red and white hairs.
- MN blood group. L^M L^N individuals express both M and N glycoproteins.
Cross ratio. Heterozygote x heterozygote gives a 1:2:1 phenotype ratio (because all three genotypes are phenotypically distinguishable), not 3:1.
Incomplete dominance
Definition. Neither allele is fully dominant. The heterozygote shows an intermediate (blended) phenotype.
Examples.
- Snapdragon flower colour. RR is red, rr is white, Rr is pink.
- Hypercholesterolaemia. Homozygous affected have very high cholesterol, heterozygotes have moderately raised cholesterol, homozygous normal are unaffected.
Cross ratio. Heterozygote x heterozygote gives a 1:2:1 phenotype ratio (red : pink : white). The pink heterozygote is distinguishable, so the genotype and phenotype ratios match.
Codominance versus incomplete dominance. Codominance shows both alleles at once (patches of red and white). Incomplete dominance shows an intermediate blend (pink).
Multiple alleles
Definition. A gene has more than two possible alleles circulating in the population, although any one individual still carries only two.
Examples.
- ABO blood groups. Three alleles: I^A, I^B and i. I^A and I^B are codominant to each other and both dominant to i. Four phenotypes (A, B, AB, O) arise from six genotypes (I^A I^A, I^A i, I^B I^B, I^B i, I^A I^B, ii).
- Rabbit coat colour. Four alleles in a dominance series: C (full colour) more than c^ch (chinchilla) more than c^h (Himalayan) more than c (albino).
Multiple alleles is a population-level feature. The inheritance in any individual cross still follows the segregation law because each parent only passes one allele.
Sex linkage
Definition. A gene located on a sex chromosome. In humans, X-linked genes are inherited differently from autosomal genes because males (XY) have only one X.
Notation. Show the allele on the chromosome, for example X^H (normal) or X^h (haemophilia). Males are X^H Y or X^h Y. Females are X^H X^H, X^H X^h or X^h X^h.
Examples.
- Haemophilia. Recessive X-linked. Affected males have one X^h. Carrier females are X^H X^h and usually unaffected.
- Red-green colour blindness. Recessive X-linked, about 8 per cent of males and 0.5 per cent of females affected.
- Duchenne muscular dystrophy. X-linked recessive.
Predictions.
- Affected fathers pass the X-linked allele to all daughters and none of their sons (who get the father's Y).
- Carrier mothers pass the allele to half their sons (who express it) and half their daughters (who become carriers).
- An X-linked recessive trait skips a generation and appears more often in males.
Y-linked traits are rare and pass strictly from father to son. X-linked dominant traits (rare, for example some forms of hypophosphataemia) appear in both sexes but are typically more severe in males.
Polygenic inheritance
Definition. A trait controlled by two or more genes. Each gene contributes a small additive effect to the phenotype. The result is continuous variation along a spectrum, not discrete categories.
Examples.
- Human skin colour. At least 4 to 6 genes contribute, each with light and dark alleles. Pooled, they produce a near-continuous range of skin pigmentation.
- Height. Hundreds of loci, each with a small effect, plus large environmental input.
- Eye colour. Mostly polygenic. The simple "brown dominant over blue" model is not accurate.
Pattern. Polygenic traits typically form a bell-shaped (normal) distribution in the population, with most individuals near the mean and fewer at the extremes. Adding the effect of environmental factors (diet for height, sun exposure for skin colour) further smooths the distribution.
Compare to discrete (qualitative) Mendelian traits, which fall into distinct categories such as tall or short, blood group A or B.
Common traps
Confusing codominance and incomplete dominance. Codominance shows both phenotypes simultaneously (AB blood, roan coat). Incomplete dominance shows a blend (pink flower).
Writing sex-linked alleles without the X. The notation X^h Y, not just h or Y, is required.
Predicting affected female offspring from a single carrier mother and unaffected father. For X-linked recessive, this gives no affected daughters (they all inherit X^H from dad).
Treating multiple alleles as multiple genes. Multiple alleles is many forms of one gene. Polygenic is many genes.
Saying polygenic traits show no Mendelian inheritance. They still obey segregation and independent assortment; their phenotype is just the sum of many genes plus environment.
In one sentence
Non-Mendelian inheritance includes codominance (both alleles expressed in the heterozygote, AB blood), incomplete dominance (blended heterozygote, pink snapdragons), multiple alleles (more than two alleles in the population, ABO), sex linkage (genes on the X or Y chromosome that affect males more than females, haemophilia), and polygenic inheritance (many genes producing continuous variation, skin colour) which gives bell-shaped distributions rather than discrete categories.
Past exam questions, worked
Real questions from past QCAA papers on this dot point, with our answer explainer.
2023 QCAA5 marksIn humans, the ABO blood group is determined by a gene with three alleles: I^A and I^B are codominant and i is recessive. A woman with blood group AB has children with a man with blood group O. Determine the possible blood groups of the children, the genotypic and phenotypic ratios, and explain why this is described as both codominance and multiple alleles.Show worked answer →
A 5-mark answer needs the genotypes, the cross, the ratios and the two non-Mendelian features named.
Parental genotypes. AB woman is I^A I^B. O man is ii.
Punnett square.
| I^A | I^B | |
|---|---|---|
| i | I^A i | I^B i |
| i | I^A i | I^B i |
Offspring.
- 2 I^A i (group A)
- 2 I^B i (group B)
Genotypic ratio. 1 I^A i to 1 I^B i.
Phenotypic ratio. 1 A to 1 B. No children will be AB or O.
Why codominance. In an I^A I^B individual, both alleles are expressed simultaneously. Both A and B antigens appear on the red blood cell surface. Neither allele masks the other.
Why multiple alleles. The gene has three alleles (I^A, I^B, i) circulating in the population. Each diploid individual still carries only two of them, but the population variation is greater than a simple two-allele system. Multiple alleles describes the population; codominance describes the heterozygote relationship.
Markers reward correct genotypes, the 1:1 phenotypic ratio, an explicit statement that AB and O offspring are impossible, and clear definitions of the two non-Mendelian features.
2024 QCAA4 marksHaemophilia is caused by a recessive allele on the X chromosome. A woman whose father had haemophilia marries an unaffected man. Determine the probability of each genotype and phenotype in their children, separated by sex, and explain why haemophilia is more common in males.Show worked answer →
A 4-mark answer needs the maternal genotype reasoning, the cross by sex and the male susceptibility explanation.
Maternal genotype. Her father was X^h Y (haemophilic). All daughters of an affected father receive his X^h, so she must be a carrier X^H X^h.
Paternal genotype. Unaffected, so X^H Y.
Punnett square.
| X^H | Y | |
|---|---|---|
| X^H | X^H X^H | X^H Y |
| X^h | X^H X^h | X^h Y |
Outcomes by sex.
- Daughters (XX). 1/2 X^H X^H (unaffected, not a carrier) and 1/2 X^H X^h (unaffected, carrier).
- Sons (XY). 1/2 X^H Y (unaffected) and 1/2 X^h Y (haemophilic).
So among daughters: 0 affected, 50 per cent carriers. Among sons: 50 per cent affected.
Why more common in males. Males have only one X chromosome. A single recessive X-linked allele expresses because there is no second X to mask it. Females need two copies to express the recessive phenotype, which is much rarer.
Markers reward the father-derived X^h logic, the split by sex, and the hemizygous male explanation.
Related dot points
- Apply Mendel's laws of segregation and independent assortment to predict the outcomes of monohybrid and dihybrid crosses using Punnett squares, and explain the purpose of a test cross
A focused answer to the QCE Biology Unit 4 dot point on Mendelian genetics. Defines genotype, phenotype, allele, homozygous and heterozygous, applies the laws of segregation and independent assortment to monohybrid and dihybrid Punnett squares (3:1 and 9:3:3:1 ratios), and explains how a test cross with a homozygous recessive parent reveals the genotype of an unknown dominant phenotype.
- Interpret pedigrees to deduce patterns of inheritance (autosomal dominant, autosomal recessive, X-linked dominant, X-linked recessive) and calculate the probability of specified offspring genotypes and phenotypes
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- Describe types of mutation (point, frameshift, chromosomal) and the sources of genetic variation including meiosis, fertilisation and mutation, and explain the consequences of mutations for phenotype and population polymorphism
A focused answer to the QCE Biology Unit 4 dot point on mutations and variation. Covers point mutations (silent, missense, nonsense), frameshift indels, chromosomal mutations (deletion, duplication, inversion, translocation, non-disjunction) and the three sources of variation (independent assortment, crossing over, random fertilisation) plus mutation as the ultimate source.