BIOB 160 Principles of Living Systems · Unit 3: Cell division and inheritance · on Exam 3
3.4 Linked genes and linkage mapping
Study notes for Linked genes and linkage mapping in BIOB 160 Principles of Living Systems at Montana State University: the big ideas, key formulas, a worked example, common mistakes and an exam tip. The same topic has endless practice questions and flashcards in Mathub.
Big ideas
- The chromosome theory of inheritance: genes sit on chromosomes, and chromosome behavior in meiosis explains Mendel’s laws. Thomas Hunt Morgan’s fruit flies showed an eye-color gene on the X chromosome.
- Sex-linked (X-linked) recessive traits, such as red-green color blindness, hemophilia and Duchenne muscular dystrophy, appear far more often in males, who have one X. In female mammals one X is inactivated as a Barr body.
- Linked genes sit close together on one chromosome and tend to be inherited together, so they do not assort independently. Crossing over produces recombinant offspring with new combinations; the rest are parental types.
- Recombination frequency = recombinants ÷ total offspring × 100. It tracks distance: 1% = 1 map unit (centimorgan). It never exceeds 50%, which looks the same as independent assortment.
- A linkage map orders genes using the pairwise distances (the two genes farthest apart are at the ends). Chromosome changes include deletions, duplications, inversions and translocations; nondisjunction causes aneuploidy (trisomy 21, XXY, XO).
Key formulas
Recombination frequency
$$\text{RF} = \dfrac{\text{recombinants}}{\text{total}} \times 100\%$$
Map distance
$$1\% \text{ RF} = 1 \text{ map unit (cM)}$$
Worked example
A testcross gives 420 + 400 parental-type offspring and 90 + 90 recombinants. What is the recombination frequency and the map distance?
Show the solution
Recombinants = 180 of 1,000 total, so RF = 180 / 1,000 × 100 = 18%, which is 18 map units (cM) apart.
Common mistakes
- Dividing recombinants by parentals instead of by the total.
- Expecting recombination frequencies above 50%.
- Forgetting that X-linked recessive traits can skip generations through carrier females.
Exam tipReading: Campbell ch. 15 (pp. 294–299). Identify parentals as the two largest classes and recombinants as the two smallest, then use the formula.
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