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 × X^h
2X^h × Y
3X^H × X^h
4X^H × Y

2.GeneticsHard

A recessive condition affects 4% of a population in Hardy–Weinberg equilibrium. What fraction f of the population are carriers (heterozygous)?

q² = 0.04

f =

3.Cell biologyMedium

A newly made protein has an N-terminal signal sequence. Where is it directed?

AInto the nucleus
BInto the endoplasmic reticulum
CDirectly to the cytosol for use there
DInto the mitochondrial matrix

4.Cell biologyHard

How does a cell mark a damaged or short-lived protein for destruction by the proteasome?

ABy adding a signal sequence for the ER
BBy removing all its phosphate groups
CBy attaching a chain of ubiquitin molecules
DBy adding mannose-6-phosphate

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

Find the cell

X^h × Y = X^hY

Answer

Cell X^h × Y → X^hY

2.Genetics

Answer: f = 0.32

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 = √0.04 = 0.2

p = 1 − 0.2 = 0.8

Carriers

2pq = 2·0.8·0.2 = 0.32

Answer

f = 0.32

3.Cell biology

Answer: B · Into the endoplasmic reticulum

Why this is the answer

The signal recognition particle (SRP) binds the signal sequence and brings the ribosome to the ER membrane, where the protein enters the ER lumen or membrane.

Answer

B · Into the endoplasmic reticulum

4.Cell biology

Answer: C · By attaching a chain of ubiquitin molecules

Why this is the answer

Enzymes attach a polyubiquitin chain to the target protein. The proteasome recognises this chain, unfolds the protein and breaks it into short peptides.

Answer

C · By attaching a chain of ubiquitin molecules

ExamProblems · examproblems.com/courses/biology