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

haemophilia is X-linked recessive (allele h). A carrier mother, X^HX^h, has children with an affected father, X^hY. The mother's gametes run down the side. Click the cell for an affected son.

♀ X^HX^h♂ X^hY

1X^H × Y
2X^H × X^h
3X^h × Y
4X^h × X^h

2.GeneticsHard

In a population of 500 in Hardy–Weinberg equilibrium, 1 in 400 show a recessive trait. How many individuals N are expected to be homozygous dominant?

N = 500q² = 0.0025

N =

3.Cell biologyMedium

Which motor protein generally carries cargo along microtubules towards the minus end, near the cell centre?

AKinesin
BDynein
CMyosin
DActin

4.Cell biologyHard

A non-competitive inhibitor binds an enzyme away from the active site. How does it change the kinetics?

AIt has no effect at high substrate levels
BVmax falls and Km stays the same
CVmax stays the same and Km rises
DBoth Vmax and Km rise

3Solutions

1.Genetics

Answer: X^h × 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

X^HY, X^HX^h, X^hY, X^hX^h

Find the cell

X^h × Y = X^hY

Answer

Cell X^h × Y → X^hY

2.Genetics

Answer: N = 451.3

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 = 0.95

Homozygous dominant

p² = 0.9025

N = 500·0.9025 = 451.3

Answer

N = 451.3

3.Cell biology

Answer: B · 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

B · Dynein

4.Cell biology

Answer: B · Vmax falls and Km stays the same

Why this is the answer

A pure non-competitive inhibitor reduces the number of working enzyme molecules, lowering Vmax. Substrate binding is unaffected, so Km stays the same.

Answer

B · Vmax falls and Km stays the same

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