Page 169 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

5.5 Tetrad Analysis in Fungi 161

Figure 5.23 When genes are linked, PDs exceed NPDs. Figure 5.24 How crossovers between linked genes
generate different tetrads. (a) PDs arise when there is no
P arg3 ura2 ARG3 URA2 crossing-over. (b) Single crossovers between the two genes yield
(a-mating type) ( -mating type) tetratypes (Ts). (c) Double crossovers between linked genes can
generate PD, T, or NPD tetrads, depending on which chromatids
Diploid cell arg3 ura2 / ARG3 URA2 participate in the crossovers.
Meiosis
Duplication Meiosis I Meiosis II
(a) No crossing-over (NCO)
Products of PD NPD T
meiosis arg3 ura2 arg3 URA2 arg3 ura2 arg3 ura2
arg3 ura2 arg3 URA2 arg3 URA2 arg3 ura2 arg3 ura2
ARG3 URA2 ARG3 ura2 ARG3 ura2 arg3 ura2 arg3 ura2 arg3 ura2
ARG3 URA2 ARG3 ura2 ARG3 URA2
ARG3 URA2 ARG3 URA2 ARG3 URA2
Number of 127 3 70
tetrads ARG3 URA2 ARG3 URA2 ARG3 URA2
Parental ditype
(b) Single crossover (SCO)
the 200 tetrads produced had the distribution shown in
Fig. 5.23. As you can see, the 127 PD tetrads far outnumber arg3 ura2 arg3 ura2 arg3 ura2
the 3 NPD tetrads, suggesting that the two genes are linked. arg3 ura2 arg3 URA2 arg3 URA2
Figure 5.24 shows how we can explain the particular ARG3 URA2 ARG3 ura2 ARG3 ura2
kinds of tetrads observed in terms of the various types of ARG3 URA2
meioses possible. If no crossing-over occurs between the ARG3 URA2 ARG3 URA2
two genes, the resulting tetrad will be PD (Fig. 5.24a). A Tetratype
single crossover between ARG3 and URA2 will generate a (c) Double crossover (DCO)
2-strand
T tetrad (Fig. 5.24b). But what about double crossovers?
As you saw earlier (Fig. 5.11), there are actually four dif- arg3 ura2 arg3 ura2 arg3 ura2
ferent possibilities, depending on which chromatids par- arg3 ura2 arg3 ura2 arg3 ura2
ticipate, and each of the four should occur with equal ARG3 URA2 ARG3 URA2 ARG3 URA2
frequency. A double crossover involving only two chro- ARG3 URA2
matids generates a PD tetrad (Fig. 5.24c). Three-strand ARG3 URA2 ARG3 URA2
double crossovers can occur in the two ways depicted in Parental ditype
Fig. 5.24d and e; either way, a T tetrad results. Finally, (d) DCO
if all four chromatids take part in the two crossovers 3-strand
(one crossover involves two strands and the other cross- arg3 ura2 arg3 ura2 arg3 ura2
over, the other two strands), the resulting tetrad is NPD arg3 ura2 arg3 URA2 arg3 URA2
(Fig. 5.24f). Therefore, if two genes are linked, the only ARG3 URA2 ARG3 URA2 ARG3 URA2
way to generate an NPD tetrad is through a four-strand ARG3 ura2
double exchange. When two genes are close together on a ARG3 URA2 ARG3 ura2
chromosome, meioses with one of the four kinds of dou- Tetratype
ble crossovers are much rarer than those with no crossing- (e) DCO
over or single crossovers, which produce PD and T tetrads, 3-strand
respectively. This explains why, if two genes are linked, arg3 ura2 arg3 URA2 arg3 URA2
the number of PDs must greatly exceed the number arg3 ura2 arg3 ura2
of NPDs. arg3 ura2 ARG3 ura2
If we calculate the RF from the data in Fig. 5.23 using ARG3 URA2 ARG3 ura2
the equation RF = [NPD + (1/2)T]/total tetrads, we find ARG3 URA2 ARG3 URA2 ARG3 URA2
that Tetratype

RF = [3 + (1∕2)70]∕200 × 100 = 19 m.u. (f) DCO
4-strand
However, observation of Fig. 5.24 reveals that this arg3 ura2 arg3 URA2 arg3 URA2
equation for RF is not an accurate reflection of the actual arg3 URA2
number of crossover events when two genes are far enough arg3 ura2 arg3 URA2 ARG3 ura2
apart that DCOs occur and NPDs appear. For example, the ARG3 URA2 ARG3 ura2
equation does not count any PDs in the numerator, even ARG3 URA2 ARG3 ura2 ARG3 ura2
though DCO meioses generate some PDs (Fig. 5.24c). Nonparental ditype