AP Biology: Genetic Linkage and Mapping Practice Questions
These original AP Biology practice questions use simplified inheritance data to explore linked genes, recombination frequency, and centimorgan maps.
Free original practice questions for study purposes. Open Exam Prep is an independent study resource and is not affiliated with, endorsed by, or sponsored by the makers of AP Exams.
advertisement
Answer the questions below — you get instant feedback and a full explanation for each.
1. Two genes on the same chromosome are described as linked. Which observation most directly supports that description?
Explanation. Linked genes tend to travel together through meiosis, so nonrecombinant parental combinations are often more common than recombinant combinations. Equal allele frequencies alone do not establish linkage, and linkage does not require new alleles or different parents.
2. An individual has chromosomes AB and ab, where A and B are linked. It is crossed with an individual ab/ab. If no recombination occurs in the first individual, which two offspring genotypes are expected?
Explanation. This is a testcross. Without recombination, the heterozygous parent produces only AB and ab gametes, while the tester always provides ab. The resulting offspring are AB/ab and ab/ab in equal expected numbers.
3. In a testcross, 200 offspring include 18 Ab/ab and 22 aB/ab recombinant offspring. What is the recombination frequency between the two genes?
Explanation. Recombinant offspring total 18 plus 22, or 40. Dividing 40 by 200 gives 0.20, so the recombination frequency is 20%. The calculation uses both reciprocal recombinant classes, not one class alone.
4. A pair of genes has a recombination frequency of 8% in a large testcross. What map distance is the best classroom estimate?
Explanation. For short distances in introductory genetic mapping, a 1% recombination frequency is estimated as 1 centimorgan. An 8% recombinant frequency therefore corresponds to about 8 centimorgans. This is a genetic, not physical base-pair, distance.
5. A heterozygote has linked alleles in the arrangement AB/ab. Which gametes are parental types before considering crossing over?
Explanation. Parental types preserve the allele combinations found on the two homologous chromosomes of the heterozygote. For AB/ab, those combinations are AB and ab. Ab and aB are recombinant combinations produced by a crossover between the loci.
6. Genes A, B, and C have the order A-B-C. Compared with a parental chromosome ABC, what pattern can a double crossover between A-B and B-C produce?
Explanation. A double crossover can restore the parental relationship of the outer markers while switching the middle marker relative to them. This is why three-point mapping uses the least frequent classes to identify double-crossover patterns and gene order.
7. A testcross gives a recombination frequency close to 50% for genes R and S. Which conclusion is most justified?
Explanation. A recombination frequency near 50% is the maximum observable and does not provide evidence of linkage. The genes can be on different chromosomes or so far apart on one chromosome that multiple crossovers make the observed classes approach equal frequencies.
8. A testcross produces 300 offspring, including 12 recombinant offspring in total. What is the estimated map distance between the genes?
Explanation. The recombination frequency is 12 divided by 300, which is 0.04 or 4%. Using the introductory mapping convention, that is about 4 centimorgans. The total recombinant count belongs in the numerator, not one parental class.
9. A heterozygote is written Ab/aB for two linked genes. Which term describes this allele arrangement?
Explanation. In the Ab/aB arrangement, the two dominant alleles are on opposite homologs. This is called repulsion phase. Coupling phase would be AB/ab, and neither arrangement prevents crossing over or proves independent assortment.
10. Why can a genetic map distance not be read as an exact physical DNA distance in base pairs?
Explanation. A centimorgan is based on observed recombination frequency, while base pairs measure physical sequence length. Crossing over is not equally likely in all regions, so the same genetic distance can correspond to different physical distances.
11. A testcross yields 96 AB/ab, 92 ab/ab, 6 Ab/ab, and 6 aB/ab offspring. What is the best estimated distance between A and B?
Explanation. The two recombinant classes total 12 offspring out of 200. That gives a recombination frequency of 6%, so the estimated genetic distance is 6 centimorgans. The large AB and ab classes are parental types and are not added to the numerator.
12. Why are genes that are very close together on the same chromosome usually observed with a low recombination frequency?
Explanation. Recombinant gametes require a crossover in the interval between the loci. A short interval provides less opportunity for such a crossover than a long interval, so nearby genes generally yield fewer recombinant offspring in a testcross.
📘 Want a full structured course and official-style practice tests? Browse top-rated AP Exams prep books and courses. Some links are affiliate links; we may earn a commission at no cost to you.
advertisement
FAQ
How do I identify parental and recombinant offspring in a testcross?
First identify the allele combinations carried by the heterozygous parent. The two most common offspring classes usually preserve those parental combinations. The less common reciprocal classes are recombinant, provided the sample and scoring are reliable.
How is recombination frequency calculated?
Add both recombinant classes, divide by the total offspring, and multiply by 100. For introductory mapping, the resulting percentage is used as an approximate distance in centimorgans for short intervals.
Does a 50 percent recombination frequency prove that genes are on different chromosomes?
No. It means the data do not reveal linkage. Genes on different chromosomes can assort independently, and genes far apart on the same chromosome can also approach 50 percent observed recombination because multiple crossovers can mask one another.