Non-Mendelian Patterns, Co-dominance, Incomplete Dominance, Multiple Alleles

Q1, Sample Band 6 response (7 marks), annotated

Co-dominance is an inheritance pattern in which both alleles in a heterozygote are fully and independently expressed, so the heterozygote phenotype contains both allele products visibly side-by-side. [1, co-dominance definition] Incomplete dominance is an inheritance pattern in which neither allele is fully dominant in the heterozygote, so the heterozygote shows a phenotype intermediate between the two homozygotes. [1, incomplete dominance definition] Multiple alleles describes a gene that exists in more than two allele forms across the population; each diploid individual still carries only two alleles, but the population gene pool offers more than two possibilities. [1, multiple alleles definition]

Heterozygote × heterozygote crosses under both co-dominance and incomplete dominance typically produce a 1:2:1 phenotype ratio because each of the three genotypes (homozygous A, heterozygous, homozygous B) maps to a visibly different phenotype, in contrast with the 3:1 ratio seen in simple Mendelian dominance where the heterozygote matches one homozygote. [1, ratios with contrast]

Named examples include: co-dominance Shorthorn cattle roan coats (CRCW heterozygotes show red and white hairs co-occurring side-by-side) or blood group AB in humans (IAIB); incomplete dominance snapdragon flower colour (red CRCR × white CWCW → pink heterozygote CRCW); multiple alleles human ABO blood group, with alleles IA, IB and i present in the population. [1, three named examples]

The ABO blood group illustrates two of these patterns simultaneously. At the population level, three allele forms (IA, IB, i) exist, so ABO is a multiple-alleles system. At the heterozygote level, IAIB individuals are blood group AB because both A and B antigens are fully expressed on their red blood cells, which is co-dominance; IA and IB are nonetheless both fully dominant over i. [1, ABO does two patterns at once]

The three patterns therefore share a common diagnostic feature: the heterozygote does not automatically match one homozygote phenotypically, so observed ratios deviating from 3:1 are a signal to consider non-Mendelian inheritance. Recognising whether the heterozygote shows both products (co-dominance), an intermediate (incomplete dominance), or whether more than two alleles exist (multiple alleles), is the key analytical skill when interpreting real-world data. [1, integrated judgement linking back to data interpretation]

Q2, Sample Band 6 response (8 marks), annotated

The mother (IAi) produces gametes carrying either IA or i. The putative father (IAIB) produces gametes carrying either IA or IB. [1, gametes identified]

Combining these in a Punnett square gives four possible offspring genotypes, IAIA, IAIB, IAi, IBi [1, four offspring genotypes]with corresponding phenotypes A, AB, A, B (overall ratio 2 A : 1 AB : 1 B). [1, phenotypes]

The crucial observation is that the father carries no i allele. Every gamete he can produce carries either IA or IB. [1, no i in father's gametes] Because IA and IB are both fully dominant over i, every offspring of this couple inherits at least one A- or B-producing allele and so expresses an A, B or AB phenotype. A genotype of ii necessary for blood-group O, is impossible from this cross, because at least one of the offspring's two alleles must come from the father, and his gametes never carry i. [1, dominance reasoning to rule out O]

The geneticist's conclusion is therefore well supported by standard ABO inheritance rules: the putative father cannot be the biological father of this O-group baby. This is consistent with a hospital mix-up or with non-paternity; the inheritance reasoning alone cannot distinguish the two, so further evidence would be needed before clinical or legal action. [1, conclusion, with appropriate caution]

One important limitation is that the ABO blood-group analysis addresses co-dominance and multiple alleles (as studied in this lesson), but DNA profiling, which compares patterns at many selected marker regions, could provide additional and independent evidence. ABO testing tells us which blood group phenotype is present; DNA profiling compares the full pattern across many DNA regions. These two approaches answer different questions: ABO typing checks one gene's inheritance, while DNA profiling compares many loci simultaneously. A discrepancy between the ABO conclusion and a DNA profile result would require further investigation because neither test alone is infallible evidence of identity. [1, limitation: ABO answers only one gene; DNA profiling compares many markers and answers a different question]

The final evaluation is: under standard ABO inheritance, the putative father is incompatible with the O baby and the geneticist's conclusion is correct. However, any clinical or legal action should not rest on ABO typing alone, because co-dominance and multiple alleles explain the blood-group pattern, but additional DNA evidence from profiling provides a broader and more reliable identity comparison. [1, integrated final evaluation using lesson concepts]

Q3, Sample Band 6 response (6 marks)

The claim is partly correct but largely flawed. [1, judgement]

What is defensible: snapdragons do show incomplete dominance, the heterozygote (CRCW) has an intermediate pink phenotype that is not predicted by a simple 3:1 model. [1, concedes correct element]

What is wrong:

• "Alleles blend in the heterozygote." The alleles do not physically merge, they segregate normally during meiosis. The evidence: a cross between two pink plants regenerates pure red (CRCR) and pure white (CWCW) offspring, which would be impossible if the alleles had blended. [1, refutes blending]
• "Two pink plants must produce all pink offspring." A CRCW × CRCW cross gives a 1 red : 2 pink : 1 white phenotype ratio. Only about half the offspring are pink; a quarter are red and a quarter are white. [1, refutes "all pink"]
• "Mendel's laws only apply when one allele is fully dominant." Mendel's laws of segregation and independent assortment describe how alleles are transmitted through gametes, not how they are expressed. Under incomplete dominance, segregation still operates normally, only the heterozygote phenotype changes. [1, refutes "Mendel doesn't apply"]

Defensible reformulation: "Snapdragons illustrate incomplete dominance, a phenotype-level expression pattern in which the heterozygote shows an intermediate phenotype (pink) rather than matching one homozygote. Underneath, the alleles still segregate independently and normally in meiosis, exactly as Mendel described, that is why crossing two pink plants regenerates a 1:2:1 phenotype ratio (1 red : 2 pink : 1 white) and recovers the pure parental phenotypes. Incomplete dominance modifies how dominance is expressed in the heterozygote, but it does not overturn Mendelian segregation." [1, biologically defensible reformulation]