AHL: Inheritance extensions
IB Biology HLΒ· D2 Inheritance (AHL)Β· 25 min read
1. Allelic Interactionsβ β ββββ± 7 min
Beyond the simple dominant-recessive relationship Mendel described, many genes have more complex interactions between alleles that alter expected phenotypic inheritance patterns.
Codominance
Both alleles are fully and simultaneously expressed in the heterozygote, with no blending of phenotypes.
Example:
Human ABO blood group: I^A I^B individuals have both A and B antigens on red blood cells.
Incomplete dominance is a separate pattern where heterozygotes show a blended phenotype intermediate between the two homozygous parental phenotypes. This is distinct from codominance where both parental phenotypes remain visible.
A red snapdragon (RR) is crossed with a white snapdragon (rr). All F1 offspring are pink. If two F1 pink snapdragons are crossed, what are the expected genotypic and phenotypic ratios of the F2 generation?
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First confirm this is incomplete dominance: heterozygotes (Rr) have an intermediate pink phenotype.
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Each F1 parent is heterozygous (Rr), so each produces two types of gametes:
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Combining gametes in a Punnett square gives the following offspring genotypes:
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Map genotypes to phenotypes for incomplete dominance, leading to a phenotypic ratio of:
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Exam tip:
Always explicitly state the difference between codominance and incomplete dominance if asked in an extended response question to earn full marks.
2. Sex-Linked Inheritanceβ β β βββ± 6 min
Sex-linked genes are located on the X or Y sex chromosomes. Most sex-linked traits are X-linked, because the X chromosome is much larger than the Y and carries far more genes. Males only have one X chromosome, so they are hemizygous for X-linked traits, meaning any recessive allele is always expressed.
Hemizygosity
Having only one copy of a gene in an otherwise diploid cell, which occurs in human males for all X-linked genes.
Red-green colour blindness is an X-linked recessive trait. A carrier mother (heterozygous) has a child with a father who has normal vision. What is the probability that their son is colour blind? What is the probability any child is colour blind?
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Assign alleles: X^B = normal vision (dominant), X^b = colour blindness (recessive). Father genotype = X^B Y, mother genotype = X^B X^b.
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Father produces X^B and Y gametes; mother produces X^B and X^b gametes. Combining these gives four possible offspring genotypes:
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For a son to be colour blind, he must inherit Y from his father and X^b from his mother. There is a 50% chance a son will inherit X^b, so the probability a son is colour blind is 50%.
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Only the X^b Y genotype results in colour blindness out of four possible offspring. The probability any child is colour blind is 25%.
Exam tip:
Always write the sex chromosomes with alleles as superscripts (e.g. X^b Y) when answering sex linkage questions to avoid marker confusion.
3. Autosomal Linkageβ β β β ββ± 6 min
Autosomal linkage occurs when two or more genes are located on the same autosome (non-sex chromosome). Linked genes tend to be inherited together, so they do not assort independently like unlinked genes on different chromosomes. The closer two genes are on a chromosome, the lower the frequency of crossing over between them during meiosis.
Recombination frequency
The proportion of recombinant offspring produced in a cross, used to estimate the distance between two genes on a chromosome.
In sweet peas, flower colour (P = purple, p = red) and pollen shape (L = long, l = round) are linked. A double heterozygous plant (PpLl) with dominant alleles on one chromosome and recessive alleles on the other is test crossed with a homozygous recessive (ppll) plant. Out of 1000 total offspring, 800 are parental types and 200 are recombinant. What is the recombination frequency between the two genes?
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Recombination frequency is calculated by the formula:
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Substitute the values from the question:
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A 20% recombination frequency means the two genes are 20 map units apart on the chromosome.
Exam tip:
Remember that linked genes produce more parental offspring than recombinant offspring; crossing over produces recombinants at a low, non-zero frequency.
4. Polygenic Inheritanceβ β β βββ± 6 min
Polygenic inheritance occurs when a single trait is controlled by multiple independent genes, usually with additive effects. Most continuous traits like human height, skin colour, and seed mass in plants are polygenic. As the number of genes controlling a trait increases, the phenotypic distribution approaches a normal (bell-shaped) curve.
Continuous variation
Variation in phenotype that shows a range of values with no distinct categories, caused by polygenic inheritance combined with environmental factors.
Human skin colour is controlled by three unlinked genes, each with two additive alleles that each contribute to darker skin colour. How many distinct skin colour phenotypes are possible?
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Each person has 2 alleles per gene, so there are 6 total alleles across 3 genes. The number of contributing (dark-skin) alleles can range from 0 to 6.
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Each number of contributing alleles (0, 1, 2, 3, 4, 5, 6) produces a distinct phenotype.
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Total number of distinct phenotypes is 7.
Exam tip:
Be prepared to draw and label a bell curve to show the phenotypic distribution of a polygenic trait in extended response questions.
5. Common Pitfalls
Wrong move:
Confusing codominance with incomplete dominance
Why:
Both are non-Mendelian patterns involving non-complete dominance, but they produce different phenotypic outcomes
Correct move:
Remember the mnemonic: Codominance = Both traits show, Incomplete dominance = Intermediate blend
Wrong move:
Claiming all sex-linked traits are recessive
Why:
X-linked dominant traits exist (though rare), and this error leads to incorrect predictions in pedigree analysis
Correct move:
Always confirm if the trait is described as dominant or recessive, and adjust your predictions accordingly
Wrong move:
Assuming linked genes never produce recombinant offspring
Why:
Crossing over during meiosis can separate linked genes, producing low numbers of recombinant offspring
Correct move:
Linked genes produce more parental offspring than recombinant offspring; recombinant frequency is low, not zero
Wrong move:
Stating polygenic inheritance produces discrete phenotypic classes
Why:
Multiple genes with additive effects produce a continuous range of phenotypes, not discrete ratios
Correct move:
Polygenic traits produce a continuous, bell-shaped distribution of phenotypes, often modified by environmental factors
6. Quick Reference Cheatsheet
Inheritance Pattern | Key Feature | Key Exam Fact |
|---|---|---|
Incomplete dominance | Blended heterozygote phenotype | F2 ratio = 1:2:1 (genotype = phenotype) |
Codominance | Both parental phenotypes expressed | F2 ratio = 1:2:1 |
Multiple alleles |
| ABO blood group is the standard example |
X-linked recessive | More common in males | No male-to-male transmission |
Autosomal linkage | Genes on same autosome | More parental than recombinant offspring |
Polygenic inheritance | Multiple genes, additive effect | Continuous bell-shaped phenotype distribution |
7. Frequently Asked
What is the key difference between codominance and incomplete dominance?
Codominance results in both parental phenotypes being expressed simultaneously in the heterozygote (e.g. AB blood type has both A and B antigens), while incomplete dominance produces a blended intermediate phenotype (e.g. pink snapdragons from red and white parents).
When this came up on past exams
AI-estimated based on syllabus patterns β cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2025 Β· 1
Multiple allele cross ratio question
- 2024 Β· 2
Explain polygenic inheritance
- 2023 Β· 1
Sex linkage Punnett square calculation
Going deeper
What's Next
Understanding inheritance extensions is critical for interpreting genetic data, predicting inheritance patterns in pedigree analysis, and explaining phenotypic variation in natural populations. This foundation prepares you to learn about genetic modification, biotechnological applications, and evolutionary processes, where non-Mendelian patterns explain many observed traits that cannot be accounted for by simple Mendelian inheritance. You will also regularly encounter these concepts in data analysis questions, where you will be asked to identify inheritance patterns from experimental crosses or pedigree data. Mastering these patterns will help you earn full marks in extended response and data-based questions, which account for a large proportion of total marks on IB Biology HL exams.