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
Duchenne muscular dystrophy is X-linked recessive (allele d). A carrier mother, XDXd, has children with an unaffected father, XDY. The mother's gametes run down the side. Click the cell for a carrier daughter.
♀ XDXd♂ XDY
- 1Xd × XD
- 2Xd × Y
- 3XD × XD
- 4XD × Y
2.GeneticsHard
In a population of 1000 in Hardy–Weinberg equilibrium, 1% show a recessive trait. How many individuals N are expected to be homozygous dominant?
N = 1000q² = 0.01
N =
3.Cell biologyMedium
How do large proteins such as transcription factors enter the nucleus?
- AThrough nuclear pore complexes, guided by a nuclear localisation signal
- BBy diffusing freely through the nuclear membrane lipids
- CBy exocytosis from the endoplasmic reticulum
- DThey are made inside the nucleus by nuclear ribosomes, so they never enter
4.Cell biologyHard
In eukaryotic mRNA processing, splicing removes:
- AExons
- BIntrons
- CThe 5′ cap
- DThe poly-A tail
3Solutions
1.Genetics
Answer: Xd × XD
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
XdXD, XdY, XDXD, XDY
Find the cell
Xd × XD = XdXD
Answer
Cell Xd × XD → XdXD
2.Genetics
Answer: N = 810
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 = √0.01 = 0.1
p = 0.9
Homozygous dominant
p² = 0.81
N = 1000·0.81 = 810
Answer
N = 810
3.Cell biology
Answer: A · Through nuclear pore complexes, guided by a nuclear localisation signal
Why this is the answer
Proteins with a nuclear localisation signal are recognised by importins and carried through nuclear pore complexes. Translation happens in the cytoplasm.
Answer
A · Through nuclear pore complexes, guided by a nuclear localisation signal
4.Cell biology
Answer: B · Introns
Why this is the answer
The spliceosome removes introns and joins exons; the 5′ cap and poly-A tail are added for stability and export.
Answer
B · Introns
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