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
♂♀Xb​YXB​Xb​

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 an affected son.

♀ XBXb♂ XbY
  1. 1XB × Xb
  2. 2XB × Y
  3. 3Xb × Xb
  4. 4Xb × Y
2.GeneticsHard
AAAa?Aa?aap (A)q (a)p (A)q (a)q² = 0.0025f = ?

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

The rough endoplasmic reticulum is "rough" because it:

  1. AHas a cell wall
  2. BIs studded with ribosomes
  3. CStores calcium only
  4. DContains DNA
4.Cell biologyHard

Which checkpoint ensures all chromosomes are attached to the spindle before anaphase begins?

  1. AThe spindle assembly (metaphase) checkpoint
  2. BThe G1/S checkpoint
  3. CThe S-phase checkpoint
  4. DThe G2/M checkpoint

3Solutions

1.Genetics

Answer: Xb × 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

XBXb, XBY, XbXb, XbY

Find the cell

Xb × Y = XbY

Answer

Cell Xb × Y → XbY
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 · Is studded with ribosomes

Why this is the answer

Ribosomes bound to the rough ER synthesise proteins that enter the ER lumen for folding and transport.

Answer

B · Is studded with ribosomes
4.Cell biology

Answer: A · The spindle assembly (metaphase) checkpoint

Why this is the answer

The spindle checkpoint delays anaphase until every kinetochore is attached, preventing unequal chromosome distribution.

Answer

A · The spindle assembly (metaphase) checkpoint

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