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
red-green colour blindness is X-linked recessive (allele b). A carrier mother, XBXb, has children with an unaffected father, XBY. The mother's gametes run down the side. Click the cell for a carrier daughter.
♀ XBXb♂ XBY
- 1XB × Y
- 2XB × XB
- 3Xb × Y
- 4Xb × XB
2.GeneticsHard
Two AaBb parents are crossed. Gene A gives round seeds (dominant) or wrinkled seeds; gene B gives purple flowers (dominant) or white flowers. Click the one cell for an offspring that is homozygous for round seeds and shows white flowers.
AaBb × AaBb
- 1ab × ab
- 2ab × aB
- 3ab × AB
- 4ab × Ab
- 5aB × ab
- 6aB × aB
- 7aB × AB
- 8aB × Ab
- 9AB × ab
- 10AB × aB
- 11AB × AB
- 12AB × Ab
- 13Ab × ab
- 14Ab × aB
- 15Ab × AB
- 16Ab × Ab
3.Cell biologyMedium
What role does cholesterol play in animal cell membranes?
- AIt helps keep membrane fluidity stable across temperatures
- BIt is the main carrier of glucose
- CIt makes the membrane completely rigid at every temperature
- DIt forms channels for ions
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 attaching a chain of ubiquitin molecules
- CBy removing all its phosphate groups
- DBy adding mannose-6-phosphate
3Solutions
1.Genetics
Answer: Xb × XB
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
XBY, XBXB, XbY, XbXB
Find the cell
Xb × XB = XbXB
Answer
Cell Xb × XB → XbXB
2.Genetics
Answer: Ab × Ab
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.
Genotype asked for
AAbb
Gametes
A homozygous genotype needs the same gamete from both parents, so only one cell in the 16 fits.
Ab × Ab = AAbb
Answer
Cell Ab × Ab → AAbb
3.Cell biology
Answer: A · It helps keep membrane fluidity stable across temperatures
Why this is the answer
Cholesterol reduces fluidity at high temperatures and prevents tight packing at low temperatures, buffering membrane fluidity.
Answer
A · It helps keep membrane fluidity stable across temperatures
4.Cell biology
Answer: B · 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
B · By attaching a chain of ubiquitin molecules
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