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
Duchenne muscular dystrophy is X-linked recessive (allele d). A carrier mother, XDXd, has children with an affected father, XdY. The mother's gametes run down the side. Click the cell for an unaffected son.
♀ XDXd♂ XdY
- 1Xd × Y
- 2Xd × Xd
- 3XD × Y
- 4XD × Xd
2.GeneticsHard
A recessive condition affects 1% of a population in Hardy–Weinberg equilibrium. What fraction f of the population are carriers (heterozygous)?
q² = 0.01
f =
3.Cell biologyMedium
Which motor protein generally carries cargo along microtubules towards the minus end, near the cell centre?
- ADynein
- BActin
- CMyosin
- DKinesin
4.Cell biologyHard
Which statement about the sodium–potassium pump is correct?
- AIt moves 2 Na⁺ out and 3 K⁺ in
- BIt moves Na⁺ and K⁺ in the same direction across the membrane
- CIt needs no energy
- DIt moves 3 Na⁺ out and 2 K⁺ in for each ATP used
3Solutions
1.Genetics
Answer: XD × Y
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
XdY, XdXd, XDY, XDXd
Find the cell
XD × Y = XDY
Answer
Cell XD × Y → XDY
2.Genetics
Answer: f = 0.18
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 = √0.01 = 0.1
p = 1 − 0.1 = 0.9
Carriers
2pq = 2·0.9·0.1 = 0.18
Answer
f = 0.18
3.Cell biology
Answer: A · Dynein
Why this is the answer
Kinesins mostly move towards the plus end (out to the periphery) and dyneins towards the minus end (towards the centrosome). Myosin moves along actin.
Answer
A · Dynein
4.Cell biology
Answer: D · It moves 3 Na⁺ out and 2 K⁺ in for each ATP used
Why this is the answer
The Na⁺/K⁺-ATPase exports 3 Na⁺ and imports 2 K⁺ per ATP, maintaining gradients and contributing to the resting potential.
Answer
D · It moves 3 Na⁺ out and 2 K⁺ in for each ATP used
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