Plasmids/linkage/population genetics

Question 1

Hardy-Weinberg Equilibrium (HWE)

Answer 1

Definition: A principle stating allele and genotype frequencies remain constant across generations in a population if specific conditions are met.

Conditions:
Large population.
Random mating.
No mutation, migration, or natural selection.
Application: Estimates carrier frequencies (e.g., recessive disorders like cystic fibrosis).

Question 2

MN Blood Group Example

Answer 2

Observed Data: 233 M, 385 MN, 129 N.
Calculations:
Totalalleles:1494.LM=8
1494=0.57L M=851/1494=0.57, LN=
643/1494=0.43L N =643/1494=0.43.
HWE Test: χ2=
1.99χ 2
=1.99 < critical value (5.99) → Equilibrium holds.

Question 3

Recombination Frequency (RF)

Answer 3

Formula:
RF=Non-parental phenotypes
Total offspring×100
RF= Total offspring
Non-parental phenotypes×100.
Morgan’s Drosophila Experiment:Body color (yy), eye color (ww), winglength (mm).yy-w
w: 29 recombinants/2205 → 1.3 cM.ww-m
m: 719 recombinants/2205 → 32.6 cM.
Gene Order: yy-ww-m
m (map distances additive: 1.3 + 32.6 ≈ 33.9 cM).

Question 4

Interference

Answer 4

Coefficient of Coincidence (C):
C=Observed double crossovers Expected=1
0.013×0.326×2205=0.107 C= Expected Observed double crossovers =
0.013×0.326×2205
1 =0.107.interference:
I=1−C=0.893
I=1−C=0.893 (89.3% reduction in double crossovers).

Question 5

Linkage vs. Independent Assortment

Answer 5

Parental Bias: Bateson’s sweet pea experiment showed 75.7% parental gametes (vs. 50% expected for independence).

Reason: Physical linkage on the same chromosome.

Question 6

ColE1 Plasmid

Answer 6

Structure:

oriV (replication), rom (copy number control), colE1 (bacteriocin toxin), mob (mobilization gene).

Limitation: Lacks tra genes → cannot self-transfer; requires helper plasmids (e.g., F-plasmid) for conjugation.

Question 7

F-Plasmid Conjugation

Answer 7

Mechanism:
Rolling-circle replication during transfer.

tra genes encode sex pilus.
Integration: Can insert into host chromosome (Hfr cells

Question 8

R-Plasmids

Answer 8

Antibiotic resistance genes (e.g., bla for β-lactamase).

Addiction System: hok/sok toxin-antitoxin.

hok (toxin mRNA) degrades in plasmid-free cells → cell death.

sok RNA (antisense) blocks hok translation in plasmid-containing cells.

Question 9

Ti Plasmid in Biotechnology

Answer 9

Process:

Agrobacterium transfers T-DNA (with inserted gene) to plant cells.

T-DNA integrates into plant genome → tumor formation.

Applications: GM crops (e.g., pest-resistant plants).

Question 10

Degradative Plasmids

Answer 10

TOL Plasmid (pWW0):

Degrades toluene/xylene via upper
Regulated by σ⁵⁴ (Pu promoter) and σ³²/σ³⁸ (Pm promoter).

Question 11

Plasmid Incompatibility

Answer 11

Rule: Plasmids with identical replication/partition systems (same Inc group) cannot coexist.

Example: Two ColE1 plasmids are incompatible; ColE1 and F-plasmid are compatible.