How do chromosomes carry genes, and what happens when their inheritance goes wrong?
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.
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What this topic is asking
The College Board (Topic 5.6) wants you to explain the chromosomal basis of inheritance: that genes are carried on chromosomes, how sex is determined, and how errors such as nondisjunction lead to abnormal chromosome numbers (aneuploidy).
The chromosome theory and sex determination
Linkage
This relationship between distance and recombination frequency is the basis of genetic mapping: by measuring how often two linked genes are separated, biologists estimate how far apart they lie on a chromosome. Genes on different chromosomes, by contrast, assort independently and follow Mendel's second law, which is why linkage was the first major exception to independent assortment to be discovered.
Nondisjunction and aneuploidy
Try this
Q1. State where nondisjunction can occur during meiosis. [2 points]
- Cue. Anaphase I (homologous chromosomes fail to separate) or anaphase II (sister chromatids fail to separate).
Q2. Explain why linked genes do not show independent assortment. [2 points]
- Cue. They are on the same chromosome, so they travel together through meiosis unless crossing over separates them.
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 2019 (style)4 marksSection II (long FRQ excerpt). (a) Describe what nondisjunction is and at which stage of meiosis it can occur. (b) Explain how nondisjunction in meiosis I can produce a gamete with an extra chromosome, and predict the chromosome number of a resulting zygote if a normal gamete fertilizes it (assume the species is 2n = 46).Show worked answer →
A 4-point describe-explain-predict FRQ on nondisjunction.
(a) Describe (1 point): nondisjunction is the failure of homologous chromosomes or sister chromatids to separate during meiosis; (1 point) it can occur in anaphase I (homologues fail to separate) or anaphase II (sister chromatids fail to separate).
(b) Explain and predict (2 points): (1 point) if a pair fails to separate in meiosis I, one gamete gets both members of the pair (n + 1 = 24) and another gets neither (n - 1 = 22); (1 point) if the gamete is fertilized by a normal gamete, the zygote has chromosomes (trisomy).
Markers reward defining nondisjunction, naming the stage, and the correct zygote chromosome count of 47.
AP 2017 (style)1 marksSection I (multiple choice). In humans, sex is determined by: (A) the number of X chromosomes only. (B) the presence or absence of the Y chromosome. (C) the total chromosome number. (D) environmental temperature during development.Show worked answer →
The correct answer is (B).
In humans, the presence of a Y chromosome (carrying the sex-determining gene) leads to male development; its absence leads to female development. Individuals are XX (female) or XY (male). Temperature-dependent sex determination (D) occurs in some reptiles, but not in humans.
Related dot points
- Topic 5.1 Meiosis: explain how meiosis produces four haploid cells from one diploid cell, and how it differs from mitosis.
A focused answer to AP Biology Topic 5.1, covering the two divisions of meiosis, homologous chromosomes, the reduction from diploid to haploid, and how meiosis differs from mitosis, with a worked chromosome-count problem.
- 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 5.4 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.
- 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.
- Topic 4.5 Cell Cycle: describe the phases of the cell cycle, including interphase and mitosis, and explain how the events of each phase produce two genetically identical cells.
A focused answer to AP Biology Topic 4.5, covering G1, S, G2, the phases of mitosis, cytokinesis and G0, and how the cycle produces two genetically identical daughter cells, with a worked timing calculation.
Sources & how we know this
- AP Biology Course and Exam Description — College Board (2020)