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
haemophilia is X-linked recessive (allele h). A carrier mother, XHXh, has children with an unaffected father, XHY. The mother's gametes run down the side. Click the cell for an affected son.
♀ XHXh♂ XHY
- 1Xh × XH
- 2Xh × Y
- 3XH × XH
- 4XH × Y
2.GeneticsHard
A recessive condition affects 16% of a population in Hardy–Weinberg equilibrium. What fraction f of the population are carriers (heterozygous)?
q² = 0.16
f =
3.Cell biologyMedium
Which tag added in the Golgi sends enzymes to the lysosomes?
- AUbiquitin
- BA KDEL sequence
- CMannose-6-phosphate
- DA nuclear localisation signal
4.Cell biologyHard
A mutation in the lac repressor stops it binding to the operator. What is the result?
- AThe bacteria can no longer take up glucose
- BThe lac genes are expressed all the time, even without lactose
- CThe lac genes are expressed only when glucose is present in the medium
- DThe lac genes can never be expressed
3Solutions
1.Genetics
Answer: Xh × 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
XhXH, XhY, XHXH, XHY
Find the cell
Xh × Y = XhY
Answer
Cell Xh × Y → XhY
2.Genetics
Answer: f = 0.48
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 = √0.16 = 0.4
p = 1 − 0.4 = 0.6
Carriers
2pq = 2·0.6·0.4 = 0.48
Answer
f = 0.48
3.Cell biology
Answer: C · Mannose-6-phosphate
Why this is the answer
In the Golgi, lysosomal enzymes receive mannose-6-phosphate. Receptors that bind this tag package them into vesicles bound for lysosomes.
Answer
C · Mannose-6-phosphate
4.Cell biology
Answer: B · The lac genes are expressed all the time, even without lactose
Why this is the answer
If the repressor cannot bind the operator, RNA polymerase is never blocked, so the operon is constitutively expressed.
Answer
B · The lac genes are expressed all the time, even without lactose
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