Chromosomal Inheritance

Chromosomal Inheritance is topic 5.6 of AP Biology, inside Heredity. This page works through three real practice questions on it, with the full reasoning behind each credited answer.

Practise this topic freeThe whole unit
37 questionsStatistical Tests and Data Analysis65% with a figure
Worked examples

Three real chromosomal inheritance questions

From the practice pool, not the mock papers — each with the reasoning that produces the answer.

65% of them come with a figure. Reading the graph or diagram correctly is most of the work here before any content knowledge applies.

Statistical Tests and Data Analysis

A recessive X-linked allele causes a trait. A carrier female and an unaffected male have a son. What is the probability that the son will express the trait?

  1. A1/4; the probability from an autosomal carrier cross
  2. B3/4; the probability of not expressing the trait in some autosomal crosses
  3. C1/2; the son receives one of the mother's two X chromosomescorrect
  4. D0; a carrier mother cannot pass the allele to a son
Why C is correct

A son receives his X chromosome from his mother and Y chromosome from his father. A carrier mother has a 1/2 chance of passing the recessive X-linked allele to a son.

The stem asks for a numerical or quantitative judgment: "A recessive X-linked allele causes a trait. A carrier female and an unaffected male have a son. What is the...". The safe route is to identify the counted event, use the full denominator, and then interpret the number biologically. Here the worked reasoning is A carrier mother has a 1/2 chance of passing the recessive X-linked allele to a son, so choices based on 1/4; the probability from an autosomal carrier cross or 3/4; the probability of not expressing the trait in some autosomal... lose the required setup.

Statistical Tests and Data Analysis · with figure

A rare recessive disorder is autosomal. Two unaffected parents have one affected child. If the parents have another child, what is the probability that the child will be unaffected?

  1. A1/4
  2. B1/3
  3. C3/4correct
  4. D1/2
Why C is correct

Unaffected parents with an affected child must both be carriers, Aa x Aa. Each child has a 1/4 probability of aa and a 3/4 probability of being unaffected.

The stem asks for a numerical or quantitative judgment: "A rare recessive disorder is autosomal. Two unaffected parents have one affected child. If the parents have...". The safe route is to identify the counted event, use the full denominator, and then interpret the number biologically. Here the worked reasoning is Each child has a 1/4 probability of aa and a 3/4 probability of being unaffected, so choices based on 1/4 or 1/3 lose the required setup.

Concept Explanation

During meiosis, a pair of homologous chromosomes fails to separate, producing some gametes with an extra chromosome and some with a missing chromosome. What process is described?

  1. AOsmosis, because water moves across a membrane.
  2. BTranslation, because ribosomes synthesize a polypeptide.
  3. CPolymerase chain reaction, because DNA is amplified in a thermocycler.
  4. DFailure of chromosome separation is nondisjunctioncorrect
Why D is correct

Nondisjunction occurs when homologous chromosomes or sister chromatids fail to separate properly during meiosis, which can produce gametes with abnormal chromosome numbers.

The stem asks students to explain a specific mechanism, not just recall a unit term: "During meiosis, a pair of homologous chromosomes fails to separate, producing some gametes with an extra...". The correct choice is supported by Nondisjunction occurs when homologous chromosomes or sister chromatids fail to separate properly during meiosis, which can produce gametes with abnormal chromosome numbers. The tempting alternatives such as Osmosis, because water moves across a membrane or Translation, because ribosomes synthesize a polypeptide fail because they do not produce the same outcome described in the stem.

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Last reviewed 2026-08-28. Topic and unit names follow the College Board course framework. Question counts describe the PrepScore practice bank, not the exam.
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Work chromosomal inheritance until the reasoning is automatic.

Real AP questions with a full explanation on every answer, and a mistake bank that only clears when you get it right.

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