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.

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1Formulas and methods

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

2Practice problems

1.GeneticsMedium

Duchenne muscular dystrophy is X-linked recessive (allele d). A carrier mother, X^DX^d, has children with an affected father, X^dY. The mother's gametes run down the side. Click the cell for an affected daughter.

♀ X^DX^d♂ X^dY

1X^D × X^d
2X^D × Y
3X^d × X^d
4X^d × Y

2.GeneticsHard

A recessive condition affects 1 in 400 of a population in Hardy–Weinberg equilibrium. What fraction f of the population are carriers (heterozygous)?

q² = 0.0025

f =

3.Cell biologyMedium

Where does RNA polymerase bind to start transcription of a gene?

AAt the terminator
BAt the promoter
CAt the start codon
DAt the stop codon

4.Cell biologyHard

In the Meselson–Stahl experiment, bacteria grown in ¹⁵N were moved to ¹⁴N. After one round of replication all DNA formed one band of intermediate density. Which model did this rule out?

ABoth semi-conservative and dispersive models
BThe conservative model
CThe semi-conservative model
DThe dispersive model

3Solutions

1.Genetics

Answer: X^d × X^d

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

X^DX^d, X^DY, X^dX^d, X^dY

Find the cell

X^d × X^d = X^dX^d

Answer

Cell X^d × X^d → X^dX^d

2.Genetics

Answer: f = 0.095

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 = √0.0025 = 0.05

p = 1 − 0.05 = 0.95

Carriers

2pq = 2·0.95·0.05 = 0.095

Answer

f = 0.095

3.Cell biology

Answer: B · At the promoter

Why this is the answer

RNA polymerase, helped by transcription factors in eukaryotes, binds the promoter and begins making RNA from the template strand.

Answer

B · At the promoter

4.Cell biology

Answer: B · The conservative model

Why this is the answer

Conservative replication would give separate heavy and light bands. The single hybrid band ruled it out; after the second generation, a light band appeared, which ruled out the dispersive model.

Answer

B · The conservative model

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