1. PD
NPD T AB aB Ab ab
Random assortment
1 : 4
Linkage
>1 :
<1 ?
Centromere linkage
<4

2. Distances between genes can be calculated by counting the different tetrad types;
Formula:
½ T + NPD (recombinants)
X 100
Total tetrads
Distance is expressed as % recombination;
Recombination frequencies can never be > 50%
(= random assortment; genes behave unlinked)

3. Segregation of two unlinked genes
Example: ADE3, ADE2
ADE2
ADE3
ade3
ade2
Haploid, Ade+, white
Haploid, Ade-, white
ADE2
ADE3
ade3
ade2
Diploid, Ade+, white

4. Resulting tetrads after sporulation
Possible distribution of chromosomes during meiosis
ADE2
ade2
ade2
ADE2
ADE3
ade3
ade3
ADE3
Resulting tetrads after sporulation
nonparental ditype
parental ditype
ade3
ADE2
ade2
ade3
ADE3
ade2
ADE3
ade2
ade3
ade3
ADE2
ADE2
ADE3
ADE3
ade2
ADE2

5. Parental Ditype – not informative (two spores carry both mutations)
ade2
ADE2
ade3
ADE3
URA3
Parental Ditype – not informative (two spores carry both mutations)
ADE3
ADE2
ADE3
ADE2
ade3
ade2
ade3
ade2

6. 2 x Ade+, white 2 x Ade-, white ADE3 ADE3 ADE2 ADE2 ade3 ade2 ade3

7. ade2
ADE2
ADE3
URA3
ade3
Non-parental ditype –informative (two spores carry one allele, two spores the other mutant allele)
ADE3
ade2
ADE3
ade2
ade3
ADE2
ade3
ADE2

8. All Ade-, two white, two red!

9. or Tetratype ade2 ADE2 ADE2 ade2 ade3 ade3 ADe3 ADE3 ADE3 ade3 ade2

10. Tetratype Ade+, white Ade-, red Ade-, white Ade-, white ADE3 ADE3 ADE2