1. Linkage
Genes that are physically located on the same chromosome are said to be “linked”.
Linked genes are said to be “mapped” to the same chromosome.
When two genes are mapped to the same chromosome, the genes will not assort independently during meiosis.

2. Linked alleles tend to be inherited together.

3. Crossing over produces new allelic combinations.

4. Chiasmata are the sites of crossing over.

5. Multiple crossovers can include more than two chromatids.

6. Recombinants are produced by crossovers.

7. Independent assortment produces 50% recombinants.

8. For linked genes, recombinant frequencies are less than 50%.

9. Map distances are generally additive.

10. Longer regions have more crossovers and thus higher recombinant frequencies.

11. Three Point Mapping: Forward Analysis
Single Crossovers Region I (SCO I)
jv se+ h+
jv+ se h
6.8 m.u.
0.5 m.u. jv se h
jv+
se+ h+
Region: I II
Single Crossovers Region II (SCO II)
jv se h+
jv+ se+ h
Double Crossovers (DCO)
jv se+ h
jv+ se h+
Prob(XO in Region I) = 0.068
Prob(XO in Region II) = 0.005
Prob(XOs in Region I and II) =
No Crossovers, Parental (NCO)
jv se h
jv+ se+ h+
(0.068) x (0.005) =

12. Three Point Mapping: Forward Analysis
Prob(DCO) = (0.068) x (0.005) =
Prob(SCO I) = =
Prob(SCO II) = =
Prob(NCO) =
1 - Prob(SCO I) - Prob(SCO II) - Prob(DCO) =
6.8 m.u.
0.5 m.u. jv se h
jv+
se+ h+
Region: I II
Double Crossovers (DCO)
jv se+ h
jv+ se h+
Single Crossovers Region I (SCO I)
jv se+ h+
jv+ se h
Single Crossovers Region II (SCO II)
jv se h+
jv+ se+ h
No Crossovers, Parentals (NCO)
jv se h
jv+ se+ h+