How do inheritance patterns depart from simple Mendelian ratios?
Topic 5.4 Non-Mendelian Genetics: explain inheritance patterns that depart from simple dominance, including incomplete dominance, codominance, sex linkage, polygenic traits and linkage.
A focused answer to AP Biology Topic 5.4, covering incomplete dominance, codominance, multiple alleles, sex-linked traits, polygenic inheritance and gene linkage, with a worked sex-linkage cross.
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What this topic is asking
The College Board (Topic 5.4) wants you to explain inheritance patterns that depart from simple Mendelian dominance: incomplete dominance, codominance, multiple alleles, sex linkage, polygenic inheritance and gene linkage. You should predict outcomes and interpret pedigrees.
Incomplete dominance and codominance
Both give a 1:2:1 phenotype ratio in a heterozygous cross (because every genotype has its own phenotype), unlike the 3:1 of complete dominance.
Multiple alleles and polygenic traits
Sex linkage and gene linkage
A key feature of X-linked inheritance is that a father passes his single X only to his daughters (his sons receive his Y), so an X-linked trait is never passed from father to son. Carrier mothers, who are heterozygous and unaffected, pass the allele to half their children, which produces the characteristic skipping of generations seen in pedigrees. Recognizing these patterns in a pedigree is a common exam task, so it is worth practicing how each inheritance type appears across generations.
Try this
Q1. State the difference between incomplete dominance and codominance. [2 points]
- Cue. Incomplete dominance gives an intermediate blend in the heterozygote; codominance expresses both alleles fully and separately.
Q2. Explain why recessive X-linked traits are more common in males. [2 points]
- Cue. Males have one X, so a single recessive allele is expressed; females need two copies (one on each X) to show the trait.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2021 (style)4 marksSection II (long FRQ excerpt). A recessive X-linked allele causes a trait in humans. A carrier mother (X^A X^a) has children with an unaffected father (X^A Y). (a) Construct a Punnett square for this cross. (b) Calculate the probability that a son will be affected, and explain why affected individuals are more often male.Show worked answer →
A 4-point Punnett-square FRQ on sex linkage.
(a) Construct (2 points): the cross gives offspring , , , (1 point for correct gametes, 1 point for the four offspring).
(b) Calculate and explain (2 points): (1 point) of the two possible sons ( and ), one is affected, so the probability that a son is affected is ; (1 point) males have only one X, so a single recessive allele is expressed, whereas females need two copies, making affected individuals more often male.
Markers reward the correct Punnett square, the probability for sons, and the explanation of why males are more often affected.
AP 2016 (style)1 marksSection I (multiple choice). In a cross between a red (RR) and a white (rr) flower, all offspring are pink. This is an example of: (A) complete dominance. (B) incomplete dominance. (C) codominance. (D) sex linkage.Show worked answer →
The correct answer is (B).
In incomplete dominance the heterozygote shows a blended, intermediate phenotype (pink), because neither allele fully masks the other. In codominance (C) both alleles would be fully and separately expressed (for example red and white patches), not blended.
Related dot points
- Topic 5.3 Mendelian Genetics: apply the laws of segregation and independent assortment to predict genotype and phenotype ratios.
A focused answer to AP Biology Topic 5.3, covering the laws of segregation and independent assortment, Punnett squares, monohybrid and dihybrid crosses, and the chi-square test for goodness of fit, with worked calculations.
- Topic 5.5 Environmental Effects on Phenotype: explain how environmental factors can affect the phenotype produced by a given genotype.
A focused answer to AP Biology Topic 5.5, covering how temperature, nutrients, pH and other environmental factors influence phenotype, the genotype-by-environment interaction, and norms of reaction, with a worked example.
- Topic 5.6 Chromosomal Inheritance: explain the chromosomal basis of inheritance, including sex determination and the consequences of nondisjunction.
A focused answer to AP Biology Topic 5.6, covering the chromosome theory of inheritance, sex determination, linkage, nondisjunction and aneuploidy, with a worked example of nondisjunction.
- Topic 5.2 Meiosis and Genetic Diversity: explain how crossing over, independent assortment and random fertilization produce genetic variation.
A focused answer to AP Biology Topic 5.2, covering crossing over, independent assortment and random fertilization as the three sources of genetic variation, with a worked calculation of gamete combinations.
- Topic 6.7 Mutations: explain the types of mutations and how they affect gene products, phenotype and the variation available to a population.
A focused answer to AP Biology Topic 6.7, covering point mutations (silent, missense, nonsense), frameshift mutations, chromosomal mutations, their effects on proteins and phenotype, and their role as the source of new variation, with a worked example.
Sources & how we know this
- AP Biology Course and Exam Description — College Board (2020)