Genetics
- Monohybrid cross (Aa × Aa)
- genotypes 1 : 2 : 1, phenotypes 3 : 1
- Dihybrid cross
- phenotypes 9 : 3 : 3 : 1
- Hardy–Weinberg
- p + q = 1, p² + 2pq + q² = 1
- Independent genes
- multiply the probabilities
Biology: Genetics and Cell Biology: cram pack
Formula sheets, the key method steps for every topic and 4 practice problems, with full solutions at the back.
1Formulas and methods
2Practice problems
1.GeneticsMedium
red-green colour blindness is X-linked recessive (allele b). A carrier mother, XBXb, has children with an affected father, XbY. The mother's gametes run down the side. Click the cell for a carrier daughter.
♀ XBXb♂ XbY
- 1Xb × Y
- 2Xb × Xb
- 3XB × Y
- 4XB × Xb
2.GeneticsHard
In a population of 500 in Hardy–Weinberg equilibrium, 4% show a recessive trait. How many individuals N are expected to be homozygous dominant?
N = 500q² = 0.04
N =
3.Cell biologyMedium
What role does cholesterol play in animal cell membranes?
- AIt is the main carrier of glucose
- BIt makes the membrane completely rigid at every temperature
- CIt helps keep membrane fluidity stable across temperatures
- DIt forms channels for ions
4.Cell biologyHard
In eukaryotic mRNA processing, splicing removes:
- AThe 5′ cap
- BExons
- CThe poly-A tail
- DIntrons
3Solutions
1.Genetics
Answer: XB × Xb
Punnett square: one parent's gametes run along the top, the other's down the side. Each cell joins the gamete above it with the gamete beside it, and every cell is equally likely. A recessive trait only shows when both alleles are recessive; for an X-linked trait, sons get their only X from the mother.
Fill the square
XbY, XbXb, XBY, XBXb
Find the cell
XB × Xb = XBXb
Answer
Cell XB × Xb → XBXb
2.Genetics
Answer: N = 320
Hardy–Weinberg: in a large, randomly mating population with no selection, the allele frequencies p (dominant) and q (recessive), p + q = 1, give genotype frequencies p², 2pq and q².
Allele frequencies
q = √0.04 = 0.2
p = 0.8
Homozygous dominant
p² = 0.64
N = 500·0.64 = 320
Answer
N = 320
3.Cell biology
Answer: C · It helps keep membrane fluidity stable across temperatures
Why this is the answer
Cholesterol reduces fluidity at high temperatures and prevents tight packing at low temperatures, buffering membrane fluidity.
Answer
C · It helps keep membrane fluidity stable across temperatures
4.Cell biology
Answer: D · Introns
Why this is the answer
The spliceosome removes introns and joins exons; the 5′ cap and poly-A tail are added for stability and export.
Answer
D · Introns
ExamProblems · examproblems.com/courses/biology